No More Learning

D. Broad, "Note on Achilles and the Tortoise," _Mind_, N. S. ,
Vol. XXII, pp. 318-19. Much valuable work on the interpretation of
Zeno has been done since this article was written. [Note added in
1917. ]

[17] Since the above was written, he has ceased to be used as a
textbook. But I fear many of the books now used are so bad that the
change is no great improvement. [Note added in 1917. ]

[18] The greatest age of Greece was brought to an end by the
Peloponnesian War. [Note added in 1917. ]




VI

ON SCIENTIFIC METHOD IN PHILOSOPHY


When we try to ascertain the motives which have led men to the
investigation of philosophical questions, we find that, broadly
speaking, they can be divided into two groups, often antagonistic, and
leading to very divergent systems. These two groups of motives are, on
the one hand, those derived from religion and ethics, and, on the
other hand, those derived from science. Plato, Spinoza, and Hegel may
be taken as typical of the philosophers whose interests are mainly
religious and ethical, while Leibniz, Locke, and Hume may be taken as
representatives of the scientific wing. In Aristotle, Descartes,
Berkeley, and Kant we find both groups of motives strongly present.

Herbert Spencer, in whose honour we are assembled to-day, would
naturally be classed among scientific philosophers: it was mainly from
science that he drew his data, his formulation of problems, and his
conception of method. But his strong religious sense is obvious in
much of his writing, and his ethical pre-occupations are what make him
value the conception of evolution--that conception in which, as a
whole generation has believed, science and morals are to be united in
fruitful and indissoluble marriage.

It is my belief that the ethical and religious motives in spite of
the splendidly imaginative systems to which they have given rise, have
been on the whole a hindrance to the progress of philosophy, and ought
now to be consciously thrust aside by those who wish to discover
philosophical truth. Science, originally, was entangled in similar
motives, and was thereby hindered in its advances. It is, I maintain,
from science, rather than from ethics and religion, that philosophy
should draw its inspiration.

But there are two different ways in which a philosophy may seek to
base itself upon science. It may emphasise the most general _results_
of science, and seek to give even greater generality and unity to
these results. Or it may study the _methods_ of science, and seek to
apply these methods, with the necessary adaptations, to its own
peculiar province. Much philosophy inspired by science has gone astray
through preoccupation with the _results_ momentarily supposed to have
been achieved. It is not results, but _methods_ that can be
transferred with profit from the sphere of the special sciences to the
sphere of philosophy. What I wish to bring to your notice is the
possibility and importance of applying to philosophical problems
certain broad principles of method which have been found successful in
the study of scientific questions.

The opposition between a philosophy guided by scientific method and a
philosophy dominated by religious and ethical ideas may be illustrated
by two notions which are very prevalent in the works of philosophers,
namely the notion of _the universe_, and the notion of _good and
evil_. A philosopher is expected to tell us something about the nature
of the universe as a whole, and to give grounds for either optimism or
pessimism. Both these expectations seem to me mistaken. I believe the
conception of "the universe" to be, as its etymology indicates, a
mere relic of pre-Copernican astronomy: and I believe the question of
optimism and pessimism to be one which the philosopher will regard as
outside his scope, except, possibly, to the extent of maintaining that
it is insoluble.

In the days before Copernicus, the conception of the "universe" was
defensible on scientific grounds: the diurnal revolution of the
heavenly bodies bound them together as all parts of one system, of
which the earth was the centre. Round this apparent scientific fact,
many human desires rallied: the wish to believe Man important in the
scheme of things, the theoretical desire for a comprehensive
understanding of the Whole, the hope that the course of nature might
be guided by some sympathy with our wishes. In this way, an ethically
inspired system of metaphysics grew up, whose anthropocentrism was
apparently warranted by the geocentrism of astronomy. When Copernicus
swept away the astronomical basis of this system of thought, it had
grown so familiar, and had associated itself so intimately with men's
aspirations, that it survived with scarcely diminished force--survived
even Kant's "Copernican revolution," and is still now the unconscious
premiss of most metaphysical systems.

The oneness of the world is an almost undiscussed postulate of most
metaphysics. "Reality is not merely one and self-consistent, but is a
system of reciprocally determinate parts"[19]--such a statement would
pass almost unnoticed as a mere truism. Yet I believe that it embodies
a failure to effect thoroughly the "Copernican revolution," and that
the apparent oneness of the world is merely the oneness of what is
seen by a single spectator or apprehended by a single mind. The
Critical Philosophy, although it intended to emphasise the subjective
element in many apparent characteristics of the world, yet, by
regarding the world in itself as unknowable, so concentrated attention
upon the subjective representation that its subjectivity was soon
forgotten. Having recognised the categories as the work of the mind,
it was paralysed by its own recognition, and abandoned in despair the
attempt to undo the work of subjective falsification. In part, no
doubt, its despair was well founded, but not, I think, in any absolute
or ultimate sense. Still less was it a ground for rejoicing, or for
supposing that the nescience to which it ought to have given rise
could be legitimately exchanged for a metaphysical dogmatism.


I

As regards our present question, namely, the question of the unity of
the world, the right method, as I think, has been indicated by William
James. [20] "Let us now turn our backs upon ineffable or unintelligible
ways of accounting for the world's oneness, and inquire whether,
instead of being a principle, the 'oneness' affirmed may not merely be
a name like 'substance' descriptive of the fact that certain _specific
and verifiable connections_ are found among the parts of the
experiential flux. . . . We can easily conceive of things that shall have
no connection whatever with each other. We may assume them to inhabit
different times and spaces, as the dreams of different persons do even
now. They may be so unlike and incommensurable, and so inert towards
one another, as never to jostle or interfere. Even now there may
actually be whole universes so disparate from ours that we who know
ours have no means of perceiving that they exist. We conceive their
diversity, however; and by that fact the whole lot of them form what
is known in logic as 'a universe of discourse. ' To form a universe of
discourse argues, as this example shows, no further kind of connexion.
The importance attached by certain monistic writers to the fact that
any chaos may become a universe by merely being named, is to me
incomprehensible. " We are thus left with two kinds of unity in the
experienced world; the one what we may call the epistemological unity,
due merely to the fact that my experienced world is what _one_
experience selects from the sum total of existence: the other that
tentative and partial unity exhibited in the prevalence of scientific
laws in those portions of the world which science has hitherto
mastered. Now a generalisation based upon either of these kinds of
unity would be fallacious. That the things which we experience have
the common property of being experienced by us is a truism from which
obviously nothing of importance can be deducible: it is clearly
fallacious to draw from the fact that whatever we experience is
experienced the conclusion that therefore everything must be
experienced. The generalisation of the second kind of unity, namely,
that derived from scientific laws, would be equally fallacious, though
the fallacy is a trifle less elementary. In order to explain it let us
consider for a moment what is called the reign of law. People often
speak as though it were a remarkable fact that the physical world is
subject to invariable laws. In fact, however, it is not easy to see
how such a world could fail to obey general laws. Taking any arbitrary
set of points in space, there is a function of the time corresponding
to these points, i. e. expressing the motion of a particle which
traverses these points: this function may be regarded as a general law
to which the behaviour of such a particle is subject. Taking all such
functions for all the particles in the universe, there will be
theoretically some one formula embracing them all, and this formula
may be regarded as the single and supreme law of the spatio-temporal
world. Thus what is surprising in physics is not the existence of
general laws, but their extreme simplicity. It is not the uniformity
of nature that should surprise us, for, by sufficient analytic
ingenuity, any conceivable course of nature might be shown to exhibit
uniformity. What should surprise us is the fact that the uniformity is
simple enough for us to be able to discover it. But it is just this
characteristic of simplicity in the laws of nature hitherto discovered
which it would be fallacious to generalise, for it is obvious that
simplicity has been a part cause of their discovery, and can,
therefore, give no ground for the supposition that other undiscovered
laws are equally simple.

The fallacies to which these two kinds of unity have given rise
suggest a caution as regards all use in philosophy of general
_results_ that science is supposed to have achieved. In the first
place, in generalising these results beyond past experience, it is
necessary to examine very carefully whether there is not some reason
making it more probable that these results should hold of all that has
been experienced than that they should hold of things universally. The
sum total of what is experienced by mankind is a selection from the
sum total of what exists, and any general character exhibited by this
selection may be due to the manner of selecting rather than to the
general character of that from which experience selects. In the second
place, the most general results of science are the least certain and
the most liable to be upset by subsequent research. In utilizing these
results as the basis of a philosophy, we sacrifice the most valuable
and remarkable characteristic of scientific method, namely, that,
although almost everything in science is found sooner or later to
require some correction, yet this correction is almost always such as
to leave untouched, or only slightly modified, the greater part of the
results which have been deduced from the premiss subsequently
discovered to be faulty. The prudent man of science acquires a certain
instinct as to the kind of uses which may be made of present
scientific beliefs without incurring the danger of complete and utter
refutation from the modifications likely to be introduced by
subsequent discoveries. Unfortunately the use of scientific
generalisations of a sweeping kind as the basis of philosophy is just
that kind of use which an instinct of scientific caution would avoid,
since, as a rule, it would only lead to true results if the
generalisation upon which it is based stood in _no_ need of
correction.

We may illustrate these general considerations by means of two
examples, namely, the conservation of energy and the principle of
evolution.

(1) Let us begin with the conservation of energy, or, as Herbert
Spencer used to call it, the persistence of force. He says:[21]

"Before taking a first step in the rational interpretation of
Evolution, it is needful to recognise, not only the facts that
Matter is indestructible and Motion continuous, but also the fact
that Force persists. An attempt to assign the _causes_ of
Evolution would manifestly be absurd if that agency to which the
metamorphosis in general and in detail is due, could either come
into existence or cease to exist. The succession of phenomena
would in such case be altogether arbitrary, and deductive Science
impossible. "

This paragraph illustrates the kind of way in which the philosopher is
tempted to give an air of absoluteness and necessity to empirical
generalisations, of which only the approximate truth in the regions
hitherto investigated can be guaranteed by the unaided methods of
science. It is very often said that the persistence of something or
other is a necessary presupposition of all scientific investigation,
and this presupposition is then thought to be exemplified in some
quantity which physics declares to be constant. There are here, as it
seems to me, three distinct errors. First, the detailed scientific
investigation of nature does not _presuppose_ any such general laws as
its results are found to verify. Apart from particular observations,
science need presuppose nothing except the general principles of
logic, and these principles are not laws of nature, for they are
merely hypothetical, and apply not only to the actual world but to
whatever is _possible_. The second error consists in the
identification of a constant quantity with a persistent entity. Energy
is a certain function of a physical system, but is not a thing or
substance persisting throughout the changes of the system. The same is
true of mass, in spite of the fact that mass has often been defined as
_quantity of matter_. The whole conception of quantity, involving, as
it does, numerical measurement based largely upon conventions, is far
more artificial, far more an embodiment of mathematical convenience,
than is commonly believed by those who philosophise on physics. Thus
even if (which I cannot for a moment admit) the persistence of some
entity were among the necessary postulates of science, it would be a
sheer error to infer from this the constancy of any physical quantity,
or the _a priori_ necessity of any such constancy which may be
empirically discovered. In the third place, it has become more and
more evident with the progress of physics that large generalisations,
such as the conservation of energy or mass, are far from certain and
are very likely only approximate. Mass, which used to be regarded as
the most indubitable of physical quantities, is now generally believed
to vary according to velocity, and to be, in fact, a vector quantity
which at a given moment is different in different directions. The
detailed conclusions deduced from the supposed constancy of mass for
such motions as used to be studied in physics will remain very nearly
exact, and therefore over the field of the older investigations very
little modification of the older results is required. But as soon as
such a principle as the conservation of mass or of energy is erected
into a universal _a priori_ law, the slightest failure in absolute
exactness is fatal, and the whole philosophic structure raised upon
this foundation is necessarily ruined. The prudent philosopher,
therefore, though he may with advantage study the methods of physics,
will be very chary of basing anything upon what happen at the moment
to be the most general results apparently obtained by those methods.

(2) The philosophy of evolution, which was to be our second example,
illustrates the same tendency to hasty generalisation, and also
another sort, namely, the undue preoccupation with ethical notions.
There are two kinds of evolutionist philosophy, of which both Hegel
and Spencer represent the older and less radical kind, while
Pragmatism and Bergson represent the more modern and revolutionary
variety. But both these sorts of evolutionism have in common the
emphasis on _progress_, that is, upon a continual change from the
worse to the better, or from the simpler to the more complex. It
would be unfair to attribute to Hegel any scientific motive or
foundation, but all the other evolutionists, including Hegel's modern
disciples, have derived their impetus very largely from the history of
biological development. To a philosophy which derives a law of
universal progress from this history there are two objections. First,
that this history itself is concerned with a very small selection of
facts confined to an infinitesimal fragment of space and time, and
even on scientific grounds probably not an average sample of events in
the world at large. For we know that decay as well as growth is a
normal occurrence in the world. An extra-terrestrial philosopher, who
had watched a single youth up to the age of twenty-one and had never
come across any other human being, might conclude that it is the
nature of human beings to grow continually taller and wiser in an
indefinite progress towards perfection; and this generalisation would
be just as well founded as the generalisation which evolutionists base
upon the previous history of this planet. Apart, however, from this
scientific objection to evolutionism, there is another, derived from
the undue admixture of ethical notions in the very idea of progress
from which evolutionism derives its charm. Organic life, we are told,
has developed gradually from the protozoon to the philosopher, and
this development, we are assured, is indubitably an advance.
Unfortunately it is the philosopher, not the protozoon, who gives us
this assurance, and we can have no security that the impartial
outsider would agree with the philosopher's self-complacent
assumption. This point has been illustrated by the philosopher Chuang
Tz? in the following instructive anecdote:

"The Grand Augur, in his ceremonial robes, approached the shambles
and thus addressed the pigs: 'How can you object to die? I shall
fatten you for three months. I shall discipline myself for ten
days and fast for three. I shall strew fine grass, and place you
bodily upon a carved sacrificial dish. Does not this satisfy you? '

Then, speaking from the pigs' point of view, he continued: 'It is
better, perhaps, after all, to live on bran and escape the
shambles. . . . '

'But then,' added he, speaking from his own point of view, 'to
enjoy honour when alive one would readily die on a war-shield or
in the headsman's basket. '

So he rejected the pigs' point of view and adopted his own point
of view. In what sense, then, was he different from the pigs? "

I much fear that the evolutionists too often resemble the Grand Augur
and the pigs.

The ethical element which has been prominent in many of the most
famous systems of philosophy is, in my opinion, one of the most
serious obstacles to the victory of scientific method in the
investigation of philosophical questions. Human ethical notions, as
Chuang Tz? perceived, are essentially anthropocentric, and involve,
when used in metaphysics, an attempt, however veiled, to legislate for
the universe on the basis of the present desires of men. In this way
they interfere with that receptivity to fact which is the essence of
the scientific attitude towards the world. To regard ethical notions
as a key to the understanding of the world is essentially
pre-Copernican. It is to make man, with the hopes and ideals which he
happens to have at the present moment, the centre of the universe and
the interpreter of its supposed aims and purposes. Ethical metaphysics
is fundamentally an attempt, however disguised, to give legislative
force to our own wishes. This may, of course, be questioned, but I
think that it is confirmed by a consideration of the way in which
ethical notions arise. Ethics is essentially a product of the
gregarious instinct, that is to say, of the instinct to co-operate
with those who are to form our own group against those who belong to
other groups. Those who belong to our own group are good; those who
belong to hostile groups are wicked. The ends which are pursued by our
own group are desirable ends, the ends pursued by hostile groups are
nefarious. The subjectivity of this situation is not apparent to the
gregarious animal, which feels that the general principles of justice
are on the side of its own herd. When the animal has arrived at the
dignity of the metaphysician, it invents ethics as the embodiment of
its belief in the justice of its own herd. So the Grand Augur invokes
ethics as the justification of Augurs in their conflicts with pigs.
But, it may be said, this view of ethics takes no account of such
truly ethical notions as that of self-sacrifice. This, however, would
be a mistake. The success of gregarious animals in the struggle for
existence depends upon co-operation within the herd, and co-operation
requires sacrifice, to some extent, of what would otherwise be the
interest of the individual. Hence arises a conflict of desires and
instincts, since both self-preservation and the preservation of the
herd are biological ends to the individual. Ethics is in origin the
art of recommending to others the sacrifices required for co-operation
with oneself. Hence, by reflexion, it comes, through the operation of
social justice, to recommend sacrifices by oneself, but all ethics,
however refined, remains more or less subjective. Even vegetarians do
not hesitate, for example, to save the life of a man in a fever,
although in doing so they destroy the lives of many millions of
microbes. The view of the world taken by the philosophy derived from
ethical notions is thus never impartial and therefore never fully
scientific. As compared with science, it fails to achieve the
imaginative liberation from self which is necessary to such
understanding of the world as man can hope to achieve, and the
philosophy which it inspires is always more or less parochial, more or
less infected with the prejudices of a time and a place.

I do not deny the importance or value, within its own sphere, of the
kind of philosophy which is inspired by ethical notions. The ethical
work of Spinoza, for example, appears to me of the very highest
significance, but what is valuable in such work is not any
metaphysical theory as to the nature of the world to which it may give
rise, nor indeed anything which can be proved or disproved by
argument. What is valuable is the indication of some new way of
feeling towards life and the world, some way of feeling by which our
own existence can acquire more of the characteristics which we must
deeply desire. The value of such work, however immeasurable it is,
belongs with practice and not with theory. Such theoretic importance
as it may possess is only in relation to human nature, not in relation
to the world at large. The scientific philosophy, therefore, which
aims only at understanding the world and not directly at any other
improvement of human life, cannot take account of ethical notions
without being turned aside from that submission to fact which is the
essence of the scientific temper.


II

If the notion of the universe and the notion of good and evil are
extruded from scientific philosophy, it may be asked what specific
problems remain for the philosopher as opposed to the man of science?
It would be difficult to give a precise answer to this question, but
certain characteristics may be noted as distinguishing the province of
philosophy from that of the special sciences.

In the first place a philosophical proposition must be general. It
must not deal specially with things on the surface of the earth, or
with the solar system, or with any other portion of space and time. It
is this need of generality which has led to the belief that philosophy
deals with the universe as a whole. I do not believe that this belief
is justified, but I do believe that a philosophical proposition must
be applicable to everything that exists or may exist. It might be
supposed that this admission would be scarcely distinguishable from
the view which I wish to reject. This, however, would be an error, and
an important one. The traditional view would make the universe itself
the subject of various predicates which could not be applied to any
particular thing in the universe, and the ascription of such peculiar
predicates to the universe would be the special business of
philosophy. I maintain, on the contrary, that there are no
propositions of which the "universe" is the subject; in other words,
that there is no such thing as the "universe. " What I do maintain is
that there are general propositions which may be asserted of each
individual thing, such as the propositions of logic. This does not
involve that all the things there are form a whole which could be
regarded as another thing and be made the subject of predicates. It
involves only the assertion that there are properties which belong to
each separate thing, not that there are properties belonging to the
whole of things collectively. The philosophy which I wish to advocate
may be called logical atomism or absolute pluralism, because, while
maintaining that there are many things, it denies that there is a
whole composed of those things. We shall see, therefore, that
philosophical propositions, instead of being concerned with the whole
of things collectively, are concerned with all things distributively;
and not only must they be concerned with all things, but they must be
concerned with such properties of all things as do not depend upon the
accidental nature of the things that there happen to be, but are true
of any possible world, independently of such facts as can only be
discovered by our senses.

This brings us to a second characteristic of philosophical
propositions, namely, that they must be _a priori_. A philosophical
proposition must be such as can be neither proved nor disproved by
empirical evidence. Too often we find in philosophical books arguments
based upon the course of history, or the convolutions of the brain, or
the eyes of shell-fish. Special and accidental facts of this kind are
irrelevant to philosophy, which must make only such assertions as
would be equally true however the actual world were constituted.

We may sum up these two characteristics of philosophical propositions
by saying that _philosophy is the science of the possible_. But this
statement unexplained is liable to be misleading, since it may be
thought that the possible is something other than the general, whereas
in fact the two are indistinguishable.

Philosophy, if what has been said is correct, becomes indistinguishable
from logic as that word has now come to be used. The study of logic
consists, broadly speaking, of two not very sharply distinguished
portions. On the one hand it is concerned with those general statements
which can be made concerning everything without mentioning any one
thing or predicate or relation, such for example as "if _x_ is a member
of the class ? and every member of ? is a member of ? , then _x_ is a
member of the class ? , whatever _x_, ? , and ? may be. " On the other
hand, it is concerned with the analysis and enumeration of logical
_forms_, i. e. with the kinds of propositions that may occur, with the
various types of facts, and with the classification of the constituents
of facts. In this way logic provides an inventory of possibilities, a
repertory of abstractly tenable hypotheses.

It might be thought that such a study would be too vague and too
general to be of any very great importance, and that, if its problems
became at any point sufficiently definite, they would be merged in the
problems of some special science. It appears, however, that this is
not the case. In some problems, for example, the analysis of space and
time, the nature of perception, or the theory of judgment, the
discovery of the logical form of the facts involved is the hardest
part of the work and the part whose performance has been most lacking
hitherto. It is chiefly for want of the right logical hypothesis that
such problems have hitherto been treated in such an unsatisfactory
manner, and have given rise to those contradictions or antinomies in
which the enemies of reason among philosophers have at all times
delighted.

By concentrating attention upon the investigation of logical forms, it
becomes possible at last for philosophy to deal with its problems
piecemeal, and to obtain, as the sciences do, such partial and
probably not wholly correct results as subsequent investigation can
utilise even while it supplements and improves them. Most
philosophies hitherto have been constructed all in one block, in such
a way that, if they were not wholly correct, they were wholly
incorrect, and could not be used as a basis for further
investigations. It is chiefly owing to this fact that philosophy,
unlike science, has hitherto been unprogressive, because each original
philosopher has had to begin the work again from the beginning,
without being able to accept anything definite from the work of his
predecessors. A scientific philosophy such as I wish to recommend will
be piecemeal and tentative like other sciences; above all, it will be
able to invent hypotheses which, even if they are not wholly true,
will yet remain fruitful after the necessary corrections have been
made. This possibility of successive approximations to the truth is,
more than anything else, the source of the triumphs of science, and to
transfer this possibility to philosophy is to ensure a progress in
method whose importance it would be almost impossible to exaggerate.

The essence of philosophy as thus conceived is analysis, not
synthesis. To build up systems of the world, like Heine's German
professor who knit together fragments of life and made an intelligible
system out of them, is not, I believe, any more feasible than the
discovery of the philosopher's stone. What is feasible is the
understanding of general forms, and the division of traditional
problems into a number of separate and less baffling questions.
"Divide and conquer" is the maxim of success here as elsewhere.

Let us illustrate these somewhat general maxims by examining their
application to the philosophy of space, for it is only in application
that the meaning or importance of a method can be understood. Suppose
we are confronted with the problem of space as presented in Kant's
Transcendental AEsthetic, and suppose we wish to discover what are the
elements of the problem and what hope there is of obtaining a solution
of them.
It will soon appear that three entirely distinct problems,
belonging to different studies, and requiring different methods for
their solution, have been confusedly combined in the supposed single
problem with which Kant is concerned. There is a problem of logic, a
problem of physics, and a problem of theory of knowledge. Of these
three, the problem of logic can be solved exactly and perfectly; the
problem of physics can probably be solved with as great a degree of
certainty and as great an approach to exactness as can be hoped in an
empirical region; the problem of theory of knowledge, however, remains
very obscure and very difficult to deal with. Let us see how these
three problems arise.

(1) The logical problem has arisen through the suggestions of
non-Euclidean geometry. Given a body of geometrical propositions, it
is not difficult to find a minimum statement of the axioms from which
this body of propositions can be deduced. It is also not difficult, by
dropping or altering some of these axioms, to obtain a more general or
a different geometry, having, from the point of view of pure
mathematics, the same logical coherence and the same title to respect
as the more familiar Euclidean geometry. The Euclidean geometry itself
is true perhaps of actual space (though this is doubtful), but
certainly of an infinite number of purely arithmetical systems, each
of which, from the point of view of abstract logic, has an equal and
indefeasible right to be called a Euclidean space. Thus space as an
object of logical or mathematical study loses its uniqueness; not only
are there many kinds of spaces, but there are an infinity of examples
of each kind, though it is difficult to find any kind of which the
space of physics may be an example, and it is impossible to find any
kind of which the space of physics is certainly an example. As an
illustration of one possible logical system of geometry we may
consider all relations of three terms which are analogous in certain
formal respects to the relation "between" as it appears to be in
actual space. A space is then defined by means of one such three-term
relation. The points of the space are all the terms which have this
relation to something or other, and their order in the space in
question is determined by this relation. The points of one space are
necessarily also points of other spaces, since there are necessarily
other three-term relations having those same points for their field.
The space in fact is not determined by the class of its points, but by
the ordering three-term relation. When enough abstract logical
properties of such relations have been enumerated to determine the
resulting kind of geometry, say, for example, Euclidean geometry, it
becomes unnecessary for the pure geometer in his abstract capacity to
distinguish between the various relations which have all these
properties. He considers the whole class of such relations, not any
single one among them. Thus in studying a given kind of geometry the
pure mathematician is studying a certain class of relations defined by
means of certain abstract logical properties which take the place of
what used to be called axioms. The nature of geometrical _reasoning_
therefore is purely deductive and purely logical; if any special
epistemological peculiarities are to be found in geometry, it must not
be in the reasoning, but in our knowledge concerning the axioms in
some given space.

(2) The physical problem of space is both more interesting and more
difficult than the logical problem. The physical problem may be
stated as follows: to find in the physical world, or to construct from
physical materials, a space of one of the kinds enumerated by the
logical treatment of geometry. This problem derives its difficulty
from the attempt to accommodate to the roughness and vagueness of the
real world some system possessing the logical clearness and exactitude
of pure mathematics. That this can be done with a certain degree of
approximation is fairly evident If I see three people _A_, _B_, and
_C_ sitting in a row, I become aware of the fact which may be
expressed by saying that _B_ is between _A_ and _C_ rather than that
_A_ is between _B_ and _C_, or _C_ is between _A_ and _B_. This
relation of "between" which is thus perceived to hold has some of the
abstract logical properties of those three-term relations which, we
saw, give rise to a geometry, but its properties fail to be exact, and
are not, as empirically given, amenable to the kind of treatment at
which geometry aims. In abstract geometry we deal with points,
straight lines, and planes; but the three people _A_, _B_, and _C_
whom I see sitting in a row are not exactly points, nor is the row
exactly a straight line. Nevertheless physics, which formally assumes
a space containing points, straight lines, and planes, is found
empirically to give results applicable to the sensible world. It must
therefore be possible to find an interpretation of the points,
straight lines, and planes of physics in terms of physical data, or at
any rate in terms of data together with such hypothetical additions as
seem least open to question. Since all data suffer from a lack of
mathematical precision through being of a certain size and somewhat
vague in outline, it is plain that if such a notion as that of a point
is to find any application to empirical material, the point must be
neither a datum nor a hypothetical addition to data, but a
_construction_ by means of data with their hypothetical additions. It
is obvious that any hypothetical filling out of data is less dubious
and unsatisfactory when the additions are closely analogous to data
than when they are of a radically different sort. To assume, for
example, that objects which we see continue, after we have turned away
our eyes, to be more or less analogous to what they were while we were
looking, is a less violent assumption than to assume that such objects
are composed of an infinite number of mathematical points. Hence in
the physical study of the geometry of physical space, points must not
be assumed _ab initio_ as they are in the logical treatment of
geometry, but must be constructed as systems composed of data and
hypothetical analogues of data. We are thus led naturally to define a
physical point as a certain class of those objects which are the
ultimate constituents of the physical world. It will be the class of
all those objects which, as one would naturally say, _contain_ the
point. To secure a definition giving this result, without previously
assuming that physical objects are composed of points, is an agreeable
problem in mathematical logic. The solution of this problem and the
perception of its importance are due to my friend Dr. Whitehead. The
oddity of regarding a point as a class of physical entities wears off
with familiarity, and ought in any case not to be felt by those who
maintain, as practically every one does, that points are mathematical
fictions. The word "fiction" is used glibly in such connexions by many
men who seem not to feel the necessity of explaining how it can come
about that a fiction can be so useful in the study of the actual world
as the points of mathematical physics have been found to be. By our
definition, which regards a point as a class of physical objects, it
is explained both how the use of points can lead to important
physical results, and how we can nevertheless avoid the assumption
that points are themselves entities in the physical world.

Many of the mathematically convenient properties of abstract logical
spaces cannot be either known to belong or known not to belong to the
space of physics. Such are all the properties connected with continuity.
For to know that actual space has these properties would require an
infinite exactness of sense-perception. If actual space is continuous,
there are nevertheless many possible non-continuous spaces which will be
empirically indistinguishable from it; and, conversely, actual space may
be non-continuous and yet empirically indistinguishable from a possible
continuous space. Continuity, therefore, though obtainable in the _a
priori_ region of arithmetic, is not with certainty obtainable in the
space or time of the physical world: whether these are continuous or not
would seem to be a question not only unanswered but for ever
unanswerable. From the point of view of philosophy, however, the
discovery that a question is unanswerable is as complete an answer as
any that could possibly be obtained. And from the point of view of
physics, where no empirical means of distinction can be found, there can
be no empirical objection to the mathematically simplest assumption,
which is that of continuity.

The subject of the physical theory of space is a very large one,
hitherto little explored. It is associated with a similar theory of
time, and both have been forced upon the attention of philosophically
minded physicists by the discussions which have raged concerning the
theory of relativity.

(3) The problem with which Kant is concerned in the Transcendental
AEsthetic is primarily the epistemological problem: "How do we come to
have knowledge of geometry _a priori_? " By the distinction between the
logical and physical problems of geometry, the bearing and scope of
this question are greatly altered. Our knowledge of pure geometry is
_a priori_ but is wholly logical. Our knowledge of physical geometry
is synthetic, but is not _a priori_. Our knowledge of pure geometry is
hypothetical, and does not enable us to assert, for example, that the
axiom of parallels is true in the physical world. Our knowledge of
physical geometry, while it does enable us to assert that this axiom
is approximately verified, does not, owing to the inevitable
inexactitude of observation, enable us to assert that it is verified
_exactly_. Thus, with the separation which we have made between pure
geometry and the geometry of physics, the Kantian problem collapses.
To the question, "How is synthetic _a priori_ knowledge possible? " we
can now reply, at any rate so far as geometry is concerned, "It is not
possible," if "synthetic" means "not deducible from logic alone. " Our
knowledge of geometry, like the rest of our knowledge, is derived
partly from logic, partly from sense, and the peculiar position which
in Kant's day geometry appeared to occupy is seen now to be a
delusion. There are still some philosophers, it is true, who maintain
that our knowledge that the axiom of parallels, for example, is true
of actual space, is not to be accounted for empirically, but is as
Kant maintained derived from an _a priori_ intuition. This position is
not logically refutable, but I think it loses all plausibility as soon
as we realise how complicated and derivative is the notion of physical
space. As we have seen, the application of geometry to the physical
world in no way demands that there should really be points and
straight lines among physical entities. The principle of economy,
therefore, demands that we should abstain from assuming the existence
of points and straight lines. As soon, however, as we accept the view
that points and straight lines are complicated constructions by means
of classes of physical entities, the hypothesis that we have an _a
priori_ intuition enabling us to know what happens to straight lines
when they are produced indefinitely becomes extremely strained and
harsh; nor do I think that such an hypothesis would ever have arisen
in the mind of a philosopher who had grasped the nature of physical
space. Kant, under the influence of Newton, adopted, though with some
vacillation, the hypothesis of absolute space, and this hypothesis,
though logically unobjectionable, is removed by Occam's razor, since
absolute space is an unnecessary entity in the explanation of the
physical world. Although, therefore, we cannot refute the Kantian
theory of an _a priori_ intuition, we can remove its grounds one by
one through an analysis of the problem. Thus, here as in many other
philosophical questions, the analytic method, while not capable of
arriving at a demonstrative result, is nevertheless capable of showing
that all the positive grounds in favour of a certain theory are
fallacious and that a less unnatural theory is capable of accounting
for the facts.

Another question by which the capacity of the analytic method can be
shown is the question of realism. Both those who advocate and those
who combat realism seem to me to be far from clear as to the nature of
the problem which they are discussing. If we ask: "Are our objects of
perception _real_ and are they _independent_ of the percipient? " it
must be supposed that we attach some meaning to the words "real" and
"independent," and yet, if either side in the controversy of realism
is asked to define these two words, their answer is pretty sure to
embody confusions such as logical analysis will reveal.

Let us begin with the word "real. " There certainly are objects of
perception, and therefore, if the question whether these objects are
real is to be a substantial question, there must be in the world two
sorts of objects, namely, the real and the unreal, and yet the unreal
is supposed to be essentially what there is not. The question what
properties must belong to an object in order to make it real is one to
which an adequate answer is seldom if ever forthcoming. There is of
course the Hegelian answer, that the real is the self-consistent and
that nothing is self-consistent except the Whole; but this answer,
true or false, is not relevant in our present discussion, which moves
on a lower plane and is concerned with the status of objects of
perception among other objects of equal fragmentariness. Objects of
perception are contrasted, in the discussions concerning realism,
rather with psychical states on the one hand and matter on the other
hand than with the all-inclusive whole of things. The question we have
therefore to consider is the question as to what can be meant by
assigning "reality" to some but not all of the entities that make up
the world. Two elements, I think, make up what is felt rather than
thought when the word "reality" is used in this sense. A thing is real
if it persists at times when it is not perceived; or again, a thing is
real when it is correlated with other things in a way which experience
has led us to expect. It will be seen that reality in either of these
senses is by no means necessary to a thing, and that in fact there
might be a whole world in which nothing was real in either of these
senses. It might turn out that the objects of perception failed of
reality in one or both of these respects, without its being in any way
deducible that they are not parts of the external world with which
physics deals. Similar remarks will apply to the word "independent. "
Most of the associations of this word are bound up with ideas as to
causation which it is not now possible to maintain. _A_ is independent
of _B_ when _B_ is not an indispensable part of the _cause_ of _A_.
But when it is recognised that causation is nothing more than
correlation, and that there are correlations of simultaneity as well
as of succession, it becomes evident that there is no uniqueness in a
series of casual antecedents of a given event, but that, at any point
where there is a correlation of simultaneity, we can pass from one
line of antecedents to another in order to obtain a new series of
causal antecedents. It will be necessary to specify the causal law
according to which the antecedents are to be considered. I received a
letter the other day from a correspondent who had been puzzled by
various philosophical questions. After enumerating them he says:
"These questions led me from Bonn to Strassburg, where I found
Professor Simmel. " Now, it would be absurd to deny that these
questions caused his body to move from Bonn to Strassburg, and yet it
must be supposed that a set of purely mechanical antecedents could
also be found which would account for this transfer of matter from one
place to another. Owing to this plurality of causal series antecedent
to a given event, the notion of _the_ cause becomes indefinite, and
the question of independence becomes correspondingly ambiguous. Thus,
instead of asking simply whether _A_ is independent of _B_, we ought
to ask whether there is a series determined by such and such causal
laws leading from _B_ to _A_. This point is important in connexion
with the particular question of objects of perception. It may be that
no objects quite like those which we perceive ever exist unperceived;
in this case there will be a causal law according to which objects of
perception are not independent of being perceived. But even if this be
the case, it may nevertheless also happen that there are purely
physical causal laws determining the occurrence of objects which are
perceived by means of other objects which perhaps are not perceived.
In that case, in regard to such causal laws objects of perception will
be independent of being perceived. Thus the question whether objects
of perception are independent of being perceived is, as it stands,
indeterminate, and the answer will be yes or no according to the
method adopted of making it determinate. I believe that this confusion
has borne a very large part in prolonging the controversies on this
subject, which might well have seemed capable of remaining for ever
undecided. The view which I should wish to advocate is that objects of
perception do not persist unchanged at times when they are not
perceived, although probably objects more or less resembling them do
exist at such times; that objects of perception are part, and the only
empirically knowable part, of the actual subject-matter of physics,
and are themselves properly to be called physical; that purely
physical laws exist determining the character and duration of objects
of perception without any reference to the fact that they are
perceived; and that in the establishment of such laws the propositions
of physics do not presuppose any propositions of psychology or even
the existence of mind. I do not know whether realists would recognise
such a view as realism. All that I should claim for it is, that it
avoids difficulties which seem to me to beset both realism and
idealism as hitherto advocated, and that it avoids the appeal which
they have made to ideas which logical analysis shows to be ambiguous.
A further defence and elaboration of the positions which I advocate,
but for which time is lacking now, will be found indicated in my book
on _Our Knowledge of the External World_. [22]

The adoption of scientific method in philosophy, if I am not mistaken,
compels us to abandon the hope of solving many of the more ambitious
and humanly interesting problems of traditional philosophy. Some of
these it relegates, though with little expectation of a successful
solution, to special sciences, others it shows to be such as our
capacities are essentially incapable of solving. But there remain a
large number of the recognised problems of philosophy in regard to
which the method advocated gives all those advantages of division into
distinct questions, of tentative, partial, and progressive advance,
and of appeal to principles with which, independently of temperament,
all competent students must agree. The failure of philosophy hitherto
has been due in the main to haste and ambition: patience and modesty,
here as in other sciences, will open the road to solid and durable
progress.

FOOTNOTES:

[19] Bosanquet, _Logic_, ii, p. 211.

[20] _Some Problems of Philosophy_, p 124.

[21] _First Principles_ (1862), Part II, beginning of chap. viii.

[22] Open Court Company, 1914.




VII

THE ULTIMATE CONSTITUENTS OF MATTER[23]


I wish to discuss in this article no less a question than the ancient
metaphysical query, "What is matter? " The question, "What is matter? "
in so far as it concerns philosophy, is, I think, already capable of
an answer which in principle will be as complete as an answer can hope
to be; that is to say, we can separate the problem into an essentially
soluble and an essentially insoluble portion, and we can now see how
to solve the essentially soluble portion, at least as regards its main
outlines. It is these outlines which I wish to suggest in the present
article. My main position, which is realistic, is, I hope and believe,
not remote from that of Professor Alexander, by whose writings on this
subject I have profited greatly. [24] It is also in close accord with
that of Dr. Nunn. [25]

Common sense is accustomed to the division of the world into mind and
matter. It is supposed by all who have never studied philosophy that
the distinction between mind and matter is perfectly clear and easy,
that the two do not at any point overlap, and that only a fool or a
philosopher could be in doubt as to whether any given entity is mental
or material. This simple faith survives in Descartes and in a
somewhat modified form in Spinoza, but with Leibniz it begins to
disappear, and from his day to our own almost every philosopher of
note has criticised and rejected the dualism of common sense. It is my
intention in this article to defend this dualism; but before defending
it we must spend a few moments on the reasons which have prompted its
rejection.

Our knowledge of the material world is obtained by means of the
senses, of sight and touch and so on. At first it is supposed that
things are just as they seem, but two opposite sophistications soon
destroy this naive belief. On the one hand the physicists cut up
matter into molecules, atoms, corpuscles, and as many more such
subdivisions as their future needs may make them postulate, and the
units at which they arrive are uncommonly different from the visible,
tangible objects of daily life. A unit of matter tends more and more
to be something like an electromagnetic field filling all space,
though having its greatest intensity in a small region. Matter
consisting of such elements is as remote from daily life as any
metaphysical theory. It differs from the theories of metaphysicians
only in the fact that its practical efficacy proves that it contains
some measure of truth and induces business men to invest money on the
strength of it; but, in spite of its connection with the money market,
it remains a metaphysical theory none the less.

The second kind of sophistication to which the world of common sense
has been subjected is derived from the psychologists and
physiologists. The physiologists point out that what we see depends
upon the eye, that what we hear depends upon the ear, and that all our
senses are liable to be affected by anything which affects the brain,
like alcohol or hasheesh. Psychologists point out how much of what we
think we see is supplied by association or unconscious inference, how
much is mental interpretation, and how doubtful is the residuum which
can be regarded as crude datum. From these facts it is argued by the
psychologists that the notion of a datum passively received by the
mind is a delusion, and it is argued by the physiologists that even if
a pure datum of sense could be obtained by the analysis of experience,
still this datum could not belong, as common sense supposes, to the
outer world, since its whole nature is conditioned by our nerves and
sense organs, changing as they change in ways which it is thought
impossible to connect with any change in the matter supposed to be
perceived. This physiologist's argument is exposed to the rejoinder,
more specious than solid, that our knowledge of the existence of the
sense organs and nerves is obtained by that very process which the
physiologist has been engaged in discrediting, since the existence of
the nerves and sense organs is only known through the evidence of the
senses themselves. This argument may prove that some reinterpretation
of the results of physiology is necessary before they can acquire
metaphysical validity. But it does not upset the physiological
argument in so far as this constitutes merely a _reductio ad absurdum_
of naive realism.

These various lines of argument prove, I think, that some part of the
beliefs of common sense must be abandoned. They prove that, if we take
these beliefs as a whole, we are forced into conclusions which are in
part self-contradictory; but such arguments cannot of themselves
decide what portion of our common-sense beliefs is in need of
correction. Common sense believes that what we see is physical,
outside the mind, and continuing to exist if we shut our eyes or turn
them in another direction. I believe that common sense is right in
regarding what we see as physical and (in one of several possible
senses) outside the mind, but is probably wrong in supposing that it
continues to exist when we are no longer looking at it. It seems to me
that the whole discussion of matter has been obscured by two errors
which support each other. The first of these is the error that what we
see, or perceive through any of our other senses, is subjective: the
second is the belief that what is physical must be persistent.
Whatever physics may regard as the ultimate constituents of matter, it
always supposes these constituents to be indestructible. Since the
immediate data of sense are not indestructible but in a state of
perpetual flux, it is argued that these data themselves cannot be
among the ultimate constituents of matter. I believe this to be a
sheer mistake. The persistent particles of mathematical physics I
regard as logical constructions, symbolic fictions enabling us to
express compendiously very complicated assemblages of facts; and, on
the other hand, I believe that the actual data in sensation, the
immediate objects of sight or touch or hearing, are extra-mental,
purely physical, and among the ultimate constituents of matter.

My meaning in regard to the impermanence of physical entities may
perhaps be made clearer by the use of Bergson's favourite illustration
of the cinematograph. When I first read Bergson's statement that the
mathematician conceives the world after the analogy of a
cinematograph, I had never seen a cinematograph, and my first visit to
one was determined by the desire to verify Bergson's statement, which
I found to be completely true, at least so far as I am concerned.
When, in a picture palace, we see a man rolling down hill, or running
away from the police, or falling into a river, or doing any of those
other things to which men in such places are addicted, we know that
there is not really only one man moving, but a succession of films,
each with a different momentary man. The illusion of persistence
arises only through the approach to continuity in the series of
momentary men. Now what I wish to suggest is that in this respect the
cinema is a better metaphysician than common sense, physics, or
philosophy. The real man too, I believe, however the police may swear
to his identity, is really a series of momentary men, each different
one from the other, and bound together, not by a numerical identity,
but by continuity and certain intrinsic causal laws. And what applies
to men applies equally to tables and chairs, the sun, moon and stars.
Each of these is to be regarded, not as one single persistent entity,
but as a series of entities succeeding each other in time, each
lasting for a very brief period, though probably not for a mere
mathematical instant. In saying this I am only urging the same kind of
division in time as we are accustomed to acknowledge in the case of
space. A body which fills a cubic foot will be admitted to consist of
many smaller bodies, each occupying only a very tiny volume; similarly
a thing which persists for an hour is to be regarded as composed of
many things of less duration. A true theory of matter requires a
division of things into time-corpuscles as well as into
space-corpuscles.

The world may be conceived as consisting of a multitude of entities
arranged in a certain pattern. The entities which are arranged I shall
call "particulars. " The arrangement or pattern results from relations
among particulars. Classes or series of particulars, collected
together on account of some property which makes it convenient to be
able to speak of them as wholes, are what I call logical constructions
or symbolic fictions. The particulars are to be conceived, not on the
analogy of bricks in a building, but rather on the analogy of notes
in a symphony. The ultimate constituents of a symphony (apart from
relations) are the notes, each of which lasts only for a very short
time. We may collect together all the notes played by one instrument:
these may be regarded as the analogues of the successive particulars
which common sense would regard as successive states of one "thing. "
But the "thing" ought to be regarded as no more "real" or
"substantial" than, for example, the role of the trombone. As soon as
"things" are conceived in this manner it will be found that the
difficulties in the way of regarding immediate objects of sense as
physical have largely disappeared.

When people ask, "Is the object of sense mental or physical? " they
seldom have any clear idea either what is meant by "mental" or
"physical," or what criteria are to be applied for deciding whether a
given entity belongs to one class or the other. I do not know how to
give a sharp definition of the word "mental," but something may be
done by enumerating occurrences which are indubitably mental:
believing, doubting, wishing, willing, being pleased or pained, are
certainly mental occurrences; so are what we may call experiences,
seeing, hearing, smelling, perceiving generally. But it does not
follow from this that what is seen, what is heard, what is smelt, what
is perceived, must be mental. When I see a flash of lightning, my
seeing of it is mental, but what I see, although it is not quite the
same as what anybody else sees at the same moment, and although it
seems very unlike what the physicist would describe as a flash of
lightning, is not mental. I maintain, in fact, that if the physicist
could describe truly and fully all that occurs in the physical world
when there is a flash of lightning, it would contain as a constituent
what I see, and also what is seen by anybody else who would commonly
be said to see the same flash. What I mean may perhaps be made plainer
by saying that if my body could remain in exactly the same state in
which it is, although my mind had ceased to exist, precisely that
object which I now see when I see the flash would exist, although of
course I should not see it, since my seeing is mental. The principal
reasons which have led people to reject this view have, I think, been
two: first, that they did not adequately distinguish between my seeing
and what I see; secondly, that the causal dependence of what I see
upon my body has made people suppose that what I see cannot be
"outside" me. The first of these reasons need not detain us, since the
confusion only needs to be pointed out in order to be obviated; but
the second requires some discussion, since it can only be answered by
removing current misconceptions, on the one hand as to the nature of
space, and on the other, as to the meaning of causal dependence.

When people ask whether colours, for example, or other secondary
qualities are inside or outside the mind, they seem to suppose that
their meaning must be clear, and that it ought to be possible to say
yes or no without any further discussion of the terms involved. In
fact, however, such terms as "inside" or "outside" are very ambiguous.
What is meant by asking whether this or that is "in" the mind? The
mind is not like a bag or a pie; it does not occupy a certain region
in space, or, if (in a sense) it does, what is in that region is
presumably part of the brain, which would not be said to be in the
mind. When people say that sensible qualities are in the mind, they do
not mean "spatially contained in" in the sense in which the blackbirds
were in the pie. We might regard the mind as an assemblage of
particulars, namely, what would be called "states of mind," which
would belong together in virtue of some specific common quality. The
common quality of all states of mind would be the quality designated
by the word "mental"; and besides this we should have to suppose that
each separate person's states of mind have some common characteristic
distinguishing them from the states of mind of other people. Ignoring
this latter point, let us ask ourselves whether the quality designated
by the word "mental" does, as a matter of observation, actually belong
to objects of sense, such as colours or noises. I think any candid
person must reply that, however difficult it may be to know what we
mean by "mental," it is not difficult to see that colours and noises
are not mental in the sense of having that intrinsic peculiarity which
belongs to beliefs and wishes and volitions, but not to the physical
world. Berkeley advances on this subject a plausible argument[26]
which seems to me to rest upon an ambiguity in the word "pain. " He
argues that the realist supposes the heat which he feels in
approaching a fire to be something outside his mind, but that as he
approaches nearer and nearer to the fire the sensation of heat passes
imperceptibly into pain, and that no one could regard pain as
something outside the mind. In reply to this argument, it should be
observed in the first place that the heat of which we are immediately
aware is not in the fire but in our own body. It is only by inference
that the fire is judged to be the cause of the heat which we feel in
our body. In the second place (and this is the more important point),
when we speak of pain we may mean one of two things: we may mean the
object of the sensation or other experience which has the quality of
being painful, or we may mean the quality of painfulness itself. When
a man says he has a pain in his great toe, what he means is that he
has a sensation associated with his great toe and having the quality
of painfulness. The sensation itself, like every sensation, consists
in experiencing a sensible object, and the experiencing has that
quality of painfulness which only mental occurrences can have, but
which may belong to thoughts or desires, as well as to sensations. But
in common language we speak of the sensible object experienced in a
painful sensation as a pain, and it is this way of speaking which
causes the confusion upon which the plausibility of Berkeley's
argument depends. It would be absurd to attribute the quality of
painfulness to anything non-mental, and hence it comes to be thought
that what we call a pain in the toe must be mental. In fact, however,
it is not the sensible object in such a case which is painful, but the
sensation, that is to say, the experience of the sensible object. As
the heat which we experience from the fire grows greater, the
experience passes gradually from being pleasant to being painful, but
neither the pleasure nor the pain is a quality of the object
experienced as opposed to the experience, and it is therefore a
fallacy to argue that this object must be mental on the ground that
painfulness can only be attributed to what is mental.

If, then, when we say that something is in the mind we mean that it
has a certain recognisable intrinsic characteristic such as belongs to
thoughts and desires, it must be maintained on grounds of immediate
inspection that objects of sense are not in any mind.

A different meaning of "in the mind" is, however, to be inferred from
the arguments advanced by those who regard sensible objects as being
in the mind. The arguments used are, in the main, such as would prove
the causal dependence of objects of sense upon the percipient. Now
the notion of causal dependence is very obscure and difficult, much
more so in fact than is generally realised by philosophers. I shall
return to this point in a moment. For the present, however, accepting
the notion of causal dependence without criticism, I wish to urge that
the dependence in question is rather upon our bodies than upon our
minds. The visual appearance of an object is altered if we shut one
eye, or squint, or look previously at something dazzling; but all
these are bodily acts, and the alterations which they effect are to be
explained by physiology and optics, not by psychology. [27] They are in
fact of exactly the same kind as the alterations effected by
spectacles or a microscope. They belong therefore to the theory of the
physical world, and can have no bearing upon the question whether what
we see is causally dependent upon the mind.