No More Learning

Now the thing that indicates most clearly the essence of logic is the assertoric force with which a sentence is uttered. But no word, or part of a sentence, corresponds to this; the same series of words may be uttered with assertoric force at one time, and not at another. In language assertoric force is bound up with the predicate.
1 A different version of the manuscript has 'to be trying to make' in place of 'to make' (ed. ).
2 This sentence and the one following are crossed out in the manuscript (ed. ).
? ? [Notes for Ludwig DarmstaedterP [July 1919]
I started out from mathematics. The most pressing need, it seemed to me, was to provide this science with a better foundation. I soon realized that number is not a heap, a series of things, nor a property of a heap either, but that in stating a number which we have arrived at as the result of counting we are making a statement about a concept. (Plato, The Greater Hippias. )
The logical imperfections of language stood in the way of such investigations. I tried to overcome these obstacles with my concept-script. In this way I was led from mathematics to logic.
What is distinctive about my conception of logic is that I begin by giving pride of place to the content of the word 'true', and then immediately go on to introduce a thought as that to which the question 'Is it true? ' is in principle applicable. So I do not begin with concepts and put them together to form a thought or judgement; I come by the parts of a thought by analysing the thought. This marks off my concept-script from the similar inventions of Leibniz and his successors, despite what the name suggests; perhaps it was not a very happy choice on my part.
Truth is not part of a thought. We can grasp a thought without at the same time recognizing it as true-without making a judgement. Both grasping a thought and making a judgement are acts of a knowing subject, and are to be assigned to psychology. But both acts involve something that does not belong to psychology, namely the thought.
False thoughts must be recognized too, not of course as true, but as indispensable aids to knowledge, for we sometimes arrive at the truth by way of false thoughts and doubts. There can be no questions if it is essential to the content of any question that that content should be true.
Negation does not belong to the act of judging, but is a constituent of a thought. The division of thoughts (judgements) into affirmative and negative is of no use to logic, and I doubt if it can be carried through.
Where we have a compound sentence consisting of an antecedent and a consequent, there are two main cases to distinguish. The antecedent and consequent may each have a complete thought as its sense. Then, over and
1 This piece is dated at the end by Frege himself. It is one of the few manuscripts of Frege which have come down to us in the original and is in the possession of the Staatsbibliothek der Stiftung Prezij)ischer Ku/turbesitz where it forms part of the collection of the historian of science Ludwig Darmstaedter, ref. number 1919-95 (ed. ).
? ? 254
[Notes for Ludwig Darmstaedter]
above these, we have the thought expressed by the whole compound sentence. By recognizing this thought as true, we recognize neither the thought in the antecedent as true, nor that in the consequent as true. A second case is where neither antecedent nor consequent has a sense in itself, but where nevertheless the whole compound sentence does express a thought-a thought which is general in character. In such a case we have a relation, not between judgements or thoughts, but between concepts, the relation, namely, of subordination. The antecedent and consequent are here sentences only in the grammatical, not in the logical, sense. The first thing that strikes us here is that a thought is made up out of parts that are not themselves thoughts. The simplest case of this kind is where one of the two parts is in need of supplementation and is completed by the other part, which is saturated: that is to say, it is not in need of supplementation. The former part then corresponds to a concept, the latter to an object (subsumption of an object under a concept). However, the object and concept are not constituents of the thought expressed. The constituents of the thought do refer to the object and concept, but in a special way. There is also the case where a part doubly in need of supplementation is completed by two saturated parts. The former part corresponds to a relation. -An object stands in a relation to an object. -Where logic is concerned, it seems that every combination of parts results from completing something that is in need of supplementation; where logic is concerned, no whole can consist of saturated parts alone. The sharp separation of what is in need of supplementation from what is saturated is very important. When all is said and done, people have long been familiar with the former in mathematics (+, :, . . ;-, sin, =, >). In this connection they speak of functions, and yet it would seem that in most cases they have only a vague notion of what is at stake.
A general statement can be negated. In this way we arrive at what logicians call existential judgements and particular judgements. The existential thoughts I have in mind here are such as are expressed in German by 'es gibt'. 1 This phrase is never followed immediately by a proper name in the singular, or by a word accompanied by the definite article, but always by a concept-word (nomen appellativum) without a definite article. In existential sentences of this kind we are making a statement about a concept. Here we have an instance of how a concept can be related to a second level concept in a way analogous to that in which an object is related to a concept under which it falls. Closely akin to these existential thoughts are thoughts that are particular: indeed they may be included among them. But we can also say that what is expressed by a sentence of the particular form is that a concept stands in a certain second level relation to a concept. The distinction between first and second level concepts can only be grasped clearly by one who has clearly grasped the distinction between what is in
1 i. e. judgements that are expressed in English by 'there is' or 'there are' (trans. ).
? [Notes for Ludwig Darmstaedter] 255
need of supplementation and what is saturated. A saturated part obtained by analysing a thought can sometimes itself be split up in the same way into a part in need of supplementation and a saturated part. The sentence 'The capital of Sweden is situated at the mouth of Lake Malar' can be split up into a part in need of supplementation and the saturated part 'the capital of Sweden'. This can further be split up into the part 'the capital of, which stands in need of supplementation, and the saturated part 'Sweden'. Splitting up the thought expressed by a sentence corresponds to such a splitting up of the sentence. The functions of Analysis correspond to parts of thoughts
that are thus in need of supplementation, without however being such.
A distinction has to be drawn between the sense and meaning of a sign (word, expression). If an astronomer makes a statement about the moon, the moon itself is not part of the thought expressed. The moon itself is the meaning of the expression 'the moon'. Therefore in addition to its meaning this expression must also have a sense, which can be a constituent of a thought. We can regard a sentence as a mapping of a thought: corresponding to the whole-part relation of a thought and its parts we have, by and large, the same relation for the sentence and its parts. Things are different in the domain of meaning. We cannot say that Sweden is a part of the capital of Sweden. The same object can be the meaning of different expressions, and any one of them can have a sense different from any other. Identity of meaning can go hand in hand with difference of sense. This is what makes it possible for a sentence o f the form ' A = B ' to express a thought with more content than one which merely exemplifies the law of identity. A statement in which something is recognized as the same again can be of far greater cognitive value than a particular case of the law of
identity.
Even a part of a thought, or a part of a part of a thought, that is in need of
supplementation, has something corresponding to it in the realm of meaning. Yet it is wrong to call this a concept, say, or a relation, or a function, although we can hardly avoid doing so. Grammatically, 'the concept of God' has the form of a saturated expression. Accordingly its sense cannot be anything in need of supplementation. When we use the words 'concept', 'relation', 'function' (as this is understood in Analysis), our words fail of their intended target. In this case even the expression 'the meaning', with the
definite article, should really be avoided.
It is not, however, only parts of sentences that have meaning; even a
whole sentence, whose sense is a thought, has one. All sentences that express a true thought have the same meaning, and all sentences that express a false thought have the same meaning (the True and the False). Sentences and parts of sentences with different meanings also have different senses. If in a sentence or part of a sentence one constituent is replaced by another with a different meaning, the different sentence or part that results does not have to have a different meaning from the original; on the other hand, it always has a different sense. If in a sentence or part of a sentence
? ? 256 [Notes for Ludwig Darmstaedter]
one constituent is replaced by another with the same meaning but not with the same sense, the different sentence or part that results has the same meaning as the original, but not the same sense. All this holds for direct, not for indirect, speech.
A thought can also be the meaning of a sentence (indirect speech, the subjunctive mood)1? The sentence does not then express this thought, but can be regarded as its proper name. Where we have a clause in indirect speech
occurring within direct speech, and we replace a constituent of this clause by another which has the same meaning in direct speech, then the whole which results from this transformation does not necessarily have the same meaning as the original.
The miracle of number. The adjectival use of number-words is misleading. In arithmetic a number-word makes its appearance in the singular as a proper name of an object of this science; it is not accompanied by the indefinite article, but is saturated. Subsumption: 'Two is a prime,' not subordination. The combinations 'each two', 'all twos' do not occur.
Yet amongst mathematicians we find a great lack of clarity and little agreement. Is number an object that arithmetic investigates or is it a counter in a game? Is arithmetic a game or a science? According to one man, we are to understand by 'number' a series of objects of the same kind; according to another, a spatial, material structure produced by writing. A third denies that number is spatial at all. Perhaps there are times when arithmeticians merely delude themselves into thinking that they understand by 'number' what they say they do. If this is not so, then they are attaching different senses to sentences with same wording; and if they still believe that they are working within one and the same discipline, then they are just deluding themselves. A definition in arithmetic that is never adduced in the course of a proof, fails of its purpose. With almost every technical term in arithmetic ('infinite series', 'determinant', 'expression', 'equation') the same questions keep cropping up: Are the things we see the subject-matter of arithmetic? Or are they only signs for, means by which we may recognize, the objects to be investigated, not those objects themselves? Is what is designated a number? and, if it isn't, what else is? Until arithmeticians have agreed on answers to these questions and in their ways of talking remain in conformity with these answers, there will be no science of arithmetic in the true sense of the word-or else a science is made up of series of words where it doesn't matter what sense they have or whether they have any sense at all. Since a statement of number based on counting contains an assertion about a concept, in a logically perfect language a sentence used to make such a statement must contain two parts, first a sign for the concept about which the statement is made, and secondly a sign for a second level concept. These second level concepts form a series and there is a rule in accordance with
1 In German clauses in indirect speech are often put into the subjunctive mood (Trans. ).
? ? [Notes for Ludwig Darmstaedter] 257
which, if one of these concepts is given, we can specify the next. But still we do not have in them the numbers of arithmetic; we do not have objects, but concepts. How can we get from these concepts to the numbers of arithmetic in a way that cannot be faulted? Or are there simply no numbers in arithmetic? Could the numerals help to form signs for these second level concepts, and yet not be signs in their own right?
Bad Kleinen, 26th July, 1919. Dr. Gottlob Frege, formerly Professor at Jena.
? ? Logical Generality1 [Not before 1923]
I published an article in this journal on compound thoughts, in which some space was devoted to hypothetical compound thoughts. It is natural to look for a way of making a transition from these to what in physics, in mathematics and in logic are called laws. We surely very often express a law in the form of a hypothetical compound sentence composed of one or more antecedents and a consequent. Yet, right at the outset, there is an obstacle in our path. The hypothetical compound thoughts I discussed do not count as laws, since they lack the generality which distinguishes laws from particular facts, such as, for instance, those we are accustomed to encounter in history. In point of fact the distinction between law and particular fact cuts very deep. It is what creates the fundamental difference between the activity of the physicist and of the historian. The former seeks to establish laws; history tries to establish particular facts. Of course history tries to understand the causes of things too, and to do that it must at least presuppose that events conform to laws.
This may be enough by way of preliminary to show the necessity of a closer study of generality.
The value a law has for our knowledge rests on the fact that it comprises many-indeed, infinitely many-particular facts as special cases. We profit from our knowledge of a law by gathering from it a wealth of particular pieces of information, using the inference from the general to the particular, for which of course a mental act-that of inferring-is still always required. Anyone who knows how to draw such an inference has also grasped what is meant by generality in the sense of the word intended here. By inferences of a different sort, we may derive new laws from ones we already acknowledge.
What, now, is the essence of generality? Since we are here concerned with laws, and laws are thoughts, the only thing that can be at issue in the present context is the generality of a thought. Every science progresses by recognizing a succession of thoughts as true; but here it is seldom thoughts that are the object of our investigation, what we make statements about. What figure as such are for the most part the objects of sense perception. In
1 Frege in the first sentence refers to the article Gedankengefiige that appeared in 1923, as the third part of the Logischen Untersuchungen published in the journal Beitriige zur Rhilosophie des deutschen ldealismus. This establishes the date. We may accept further that Frege intended to develop this fragment into a fourth part of this series of essays (ed. ).
? Logical Generality 259
predicating something of these, we utter thoughts. And this is the usual way in which thoughts figure in science. Here, in predicating generality of thoughts, we are making them the objects of our investigation and they take over the place normally occupied by the objects of sense perception. These latter, which elsewhere, and in particular in the natural sciences, are the objects of enquiry, are essentially different from thoughts. For thoughts cannot be perceived by the senses. To be sure, signs which express thoughts can be audible or visible, but not the thoughts themselves. Sense impressions can lead us to recognize the truth of a thought; but we can also grasp a thought without recognizing it as true. False thoughts are thoughts too.
If a thought cannot be perceived by the senses, it is not to be expected that its generality can be. I am not in a position to produce a thought in the way a mineralogist presents a sample of a mineral so as to draw attention to its characteristic lustre. It may be impossible to use a definition to fix what is meant by generality.
Language may appear to offer a way out, for, on the one hand, its sentences can be perceived by the senses, and, on the other, they express thoughts. As a vehicle for the expression of thoughts, language must model itself upon what happens at the level of thought. So we may hope that we can use it as a bridge from the perceptible to the imperceptible. Once we have come to an understanding about what happens at the linguistic level, we may find it easier to go on and apply what we have understood to what holds at the level of thought-to what is mirrored in language. Here it isn't a
question of the day to day understanding of language, of grasping the thoughts expressed in it: it's a question of grasping the property of thoughts that I call logical generality. Of course for this we have to reckon upon a meeting of minds between ourselves and others, and here we may be disappointed. Also, the use of language requires caution. We should not overlook the deep gulf that yet separates the level of language from that of the thought, and which imposes certain limits on the mutual correspondence of the two levels.
Now, in what form does generality make its appearance in language? We have various expressions for the same general thought:
'All men are mortal'
'Every man is mortal'
'If something is a man, it is mortal'.
The differences in the expression do not affect the thought itself. It is advisable for us to confine ourselves to using one mode of expression, so that we do not take incidental differences, in the tone of the thought, say, to be differences of thought. The expressions involving 'all' and 'every' are not suitable for use wherever generality is present, since not every law can be cast in this form. In the last mode of expression we have the form of the hypothetical compound sentence-a form we can hardly avoid using in other cases too-together with the indefinitely indicating parts of the
? ? 260 Logical Generality
sentence, 'something' and 'it'; and these really contain the expression of generality. From this mode of expression we may easily make the transition to the particular, by replacing the indefinitely indicating parts of the sentence by one that designates definitely:
'If Napoleon is a man, Napoleon is mortal. '
In view of this possibility of the transition from the general to the particular, expressions of generality with indefinitely indicating parts of the sentence are alone of use to us, but if we were restricted to 'something' and 'it', we would only be able to deal with the very simplest cases. Now it is natural to copy the methods of arithmetic by selecting letters for indefinitely indicating parts of a sentence:
'If a is a man, a is a mortal. '
Here the equiform letters cross-refer to one another. Instead of letters equiform with 'a' we could just as well take ones equiform with 'b' or 'c'. But it is essential that they should be equiform. However, taken strictly, we are stepping outside the confines of a spoken language designed to be heard and moving into the region of a written or printed language designed for the eye. A sentence which an author writes down is primarily a direction for forming a spoken sentence in a language whose sequences of sounds serve as signs for expressing a sense. So at first there is only a mediated connection set up between written signs and a sense that is expressed. But once this connection is established, we may also regard the written or printed sentence as an immediate expression of a thought, and so as a sentence in the strict sense of the word. In this way we obtain a language dependent on the sense of sight, which, if need be, can even be learnt by a deaf man. In this, individual letters can be adopted as indefinitely indicating parts of a sentence. The language we have just indicated, which I will call the object-language,1 is to serve for us as a bridge from the perceptible to the imperceptible. It contains two different constituents: those with the form of words and the individual letters. The former correspond to words of the spoken language, the latter have an indefinitely indicating role.
This object-language is to be distinguished from the language in which I conduct my train of thought. That is the usual written or printed German, my meta-language. But the sentences of the object-language are the objects to be talked about in my meta-language. And so I must be able to designate them in my meta- language, just as in an article on astronomy the planets are designated by
1 The German editors see the distinction Frege here draws between a 'Hilfssprache' and a 'Darlegungssprache' as anticipating the distinction sub- sequently drawn by Tarski between an 'object-language' and a 'meta-language', and certainly the resemblance is close: close enough for us to have decided to avail ourselves of the expedient of using Tarski's way of speaking to translate Frege's two
notions: the reader is not to suppose that Frege anticipates this actual terminology and may judge for himself how far he anticipates Tarski's thought (trans. ).
? ? ? Logical Generality 261
their proper names 'Venus', 'Mars', etc. As such proper names of the sentences o f the object-language I use these very sentences, but enclosed in quotation marks. Moreover it follows from this that the sentences of the object-language are never given assertoric force. 'If a is a man, a is mortal' is a sentence of the object-language in which a general thought is expressed. We move from the general to the particular by substituting for the equiform indefinitely indicating letters equiform proper names. It belongs to the essence of our object-language that equiform proper names designate the same object (man). Here empty signs (names) are not proper names at all. * By substituting for the indefinitely indicating letters equiform with 'a', proper names of the form 'Napoleon', we obtain
'IfNapoleon is a man, Napoleon is mortal. '
This sentence is not however to be regarded as a conclusion, since the sentence 'If a is a man, a is mortal' is not given assertoric force, and so the thought expressed in it is not presented as one recognized to be true, for only a thought recognized as true can be made the premise o f an iriference. But an inference can be made of this, by freeing the two sentences of our object-
language from quotation marks, thus making it possible to put them forward with assertoric force.
The compound sentence ' I f Napoleon be a man, then is Napoleon mortal'1 expresses a hypothetical compound thought, composed of one condition and one consequence. The former is expressed in the sentence 'Napoleon is a man', the latter in 'Napoleon is mortal'. Strictly, however, our compound sentence contains neither a sentence equiform with 'Napoleon is a man' nor one equiform with 'Napoleon is mortal'. In this divergence between what holds at the linguistic level and what holds at the level of the thought, there emerges a defect in our object-language which is still to be remedied. I wish now to dress the thought that I expressed above in the
*I call proper names of our object-language equiform, if they are intended to be so by the writer and are meant to be of the same size, if we can recognize this to be the writer's intention even if it is imperfectly realized.
1 In German, unlike English, the order of the words in a sentence is altered when it is made into the antecedent or consequent of a hypothetical. The compound sentence with antecedent 'Napoleon is a man' and consequent 'Napoleon is mortal' transliterates 'If Napoleon a man is, is Napoleon mortal'. Throughout the preceding discussion we have translated such hypothetical sentences into natural English.
Here, where Frege is concerned with the deviation between the vernacular (German) and a language which reflects more accurately 'the level of thought', it has proved necessary to resort to a clumsy English rendering of the hypothetical sentence. The fact that English happens not to have what Frege here argues is a defective correspondence between the level of language and that of the thought, does not, of course, detract from the force or interest of the point he is here making (trans. ).
? ? 262 Logical Generality
sentence 'IfNapoleon be a man, then is Napoleon mortal', in the sentence 'If Napoleon is a man, Napoleon is mortal'; which, in what follows, I wish to call the second sentence. Similar cases are to be treated in the same way. So I want also to transform the sentence 'Ifa be a man, then is a mortal' into 'If a is a man, a is mortal', which in what follows I will call the sentence. * In the first sentence I distinguish the two individual letters equiform with 'a' from the remaining part.
*The first sentence, unlike the second, does not express a compound thought, since neither 'a is a man' nor 'a is mortal' expresses a thought. We have here really only parts of a sentence, not sentences.
? ? ? [Diary Entries on the Concept ofNumbersP [23. 3. 1924-25. 3. 1924]
23. 3. 1924 My efforts to become clear about what is meant by number have resulted in failure. We are only too easily misled by language and in this particular case the way we are misled is little short of disastrous. The sentences 'Six is an even number', 'Four is a square number', 'Five is a prime number' appear analogous to the sentences 'Sirius is a fixed star', 'Europe is a continent'-sentences whose function is to represent an object as falling under a concept. Thus the words 'six', 'four', and 'five' look like proper names of objects and 'even number', 'square number', and 'prime number', along with 'number' itself, look like concept-words. So the problem appears to be to work out more clearly the nature of the concept designated by the word 'number' and to exhibit the objects that, as it seems, are designated by number-words and numerals.
24. 3. 1924 From our earliest education onwards we are so accustomed to using the word 'number' and the number-words that we do not regard our way of using them as something that stands in need of a justification. To the mathematicians it appears beneath their dignity to concern themselves with such childish matters. But one finds amongst them the most different and contradictory statements about number and numbers. Indeed, when one has been occupied with these questions for a long time one comes to suspect that our way of using language is misleading, that number-words are not proper names of objects at all and words like 'number', 'square number' and the rest are not concept-words; and that consequently a sentence like 'Four is a square number' simply does not express that an object is subsumed under a concept and so just cannot be construed like the sentence 'Sirius is a fixed star'. But how then is it to be construed?
25. 3. 1924 At first, however, I was still captive to the error that language gives rise to. It is easy to see that a number is not a collection of things. What is a collection? A collection is a thing, a thing that is made up of things. The mathematician couples the word 'one' with the definite article
1 These remarks were taken from a diary that Frege kept from 10. 3. 24 to 9. 5. 24; the other entries are simply statements of political attitudes which cannot be counted as part of the scientific NachlajJ (ed. ). For further details concerning the nature and content of this Diary, readers are referred to Michael Dummett, Frege: Philosophy of Language, London 1973, p. xii. See also the article on Frege in the Encyclopaedia Briltanica (fifteenth edition) Trans.
? 264 [Diary Entries on the Concept ofNumber]
and speaks of 'the number one'. If now a number is a thing, we must produce the thing that is called the number one. Many people even want to call numerals numbers. If this were true, we should have to be able to specify which of these numerals was the number one. What a calamity it would then be if this One were ever to be destroyed by fire! It is already a step forward when a number is seen not as a thing but as something belonging to a thing, where the view is that different things, in spite of their differences, can possess the same One, as different leaves can, say, all possess the colour green. Now which things possess the number one? Does not the number one belong to each and every thing?
? ? ? Number 1 [September 1924]
My efforts to throw light on the questions surrounding the word 'number' and the words and signs for individual numbers seem to have ended in complete failure. Still these efforts have not been wholly in vain. Precisely because they have failed, we can learn something from them. The difficulties of these investigations are often greatly underrated. Anyone who can regard a number of a series of objects of the same kind shows that he is so far from having any real understanding that he does not yet have even an inkling of the task involved.
First of all we need to come to an understanding about the words 'number', 'numeral', 'number-word'. In everyday life one often calls the numerals numbers. In the present context we must exclude this way of speaking. The simplest thing seems to be to understand by a numeral a sign that designates a number and by a number-word a word which designates a number. Then numerals and number-words will have the same role, namely that of designating a number; only the vehicle used will be different.
What, then, are numbers themselves? What sort of thing is it that one means to designate by a number-word or numeral? Obviously it is not anything that can be perceived by the senses. A number can neither be smelt, nor tasted, nor seen, nor felt. It jars to speak of warm or yellow or bitter numbers, just as it would jar if one chose to speak of yellow or warm or bitter points in geometry. We may seek to discover something about numbers themselves from the use we make of numerals and number-words. Numerals and number-words are used, like names of objects, as proper names. The sentence 'Five is a prime number' is comparable with the sentence 'Sirius is a fixed star'. In these sentences an object (five, Sirius) is presented as falling under a concept (prime number, fixed star) (a case of an object's being subsumed under a concept). By a number, then, we are to understand an object that cannot be perceived by the senses. Even the
1 According to a remark made by the previous editors on the transcripts on which this edition is based, these comments stem 'from two drafts of which the first is dated: Sept. 1924 to 23. 111. 1924'. The date 23. 111. 1924 apparently refers to the remarks which Frege made in his diary on that day (cf. p. 263). It is also clear from remarks made by the previous editors that the first three paragraphs come from the second draft, and the last two paragraphs from the first (ed. ).
The curious dating may mean that Frege added, on Sept. 1924, the last two paragraphs to a re-working of the diary entry of 23. 111. 1924 (trans. ).
? 266 Number
objects of geometry, points, straight lines, surfaces etc. cannot really be perceived by the senses.
These investigations are especially difficult because in the very act of conducting them we are easily misled by language: by language which is, after all, an indispensable tool for carrying them out. Indeed one might think that language would first have to be freed from all logical imperfections before it was employed in such investigations. But of course the work necessary to do this can itself only be done by using this tool, for all its imperfections. Fortunately as a result of our logical work we have acquired a yardstick by which we are apprised of these defects. Such a yardstick is at work even in language, obstructed though it may be by the many illogical features that are also at work in language.
In pre-scientific discourse what are called numbers are often no more than numerals. These signs are ordered in a series beginning with the sign '1'. In this series each sign, with the exception of the last, has an immediate successor, and each sign, with the exception of the first (the sign '1') an immediate predecessor. For example 1? 2? 3? 4? 5, in which I have separated the individual numerals by asterisks.
? ? Sources of Knowledge of Mathematics and the mathematical natural Sciences1
[1924/5]
When someone comes to know something it is by his recognizing a thought to be true. For that he has first to grasp the thought. Yet I do not count the grasping of the thought as knowledge, but only the recognition of its truth, the judgement proper. What I regard as a source of knowledge is what justifies the recognition of truth, the judgement.
I distinguish the following sources of knowledge:
1. Senseperception
2. Thelogicalsourceofknowledge
3. The geometrical and temporal sources of knowledge.
Each of these is subject to its own disturbances, which detract from its value.
A. Sense Illusions
A sense impression is not in itself a judgement, but becomes important in that it is the occasion for our making a judgement. When this happens, we may be misled: in such cases we talk of 'sense illusions'. Since for the mathematical natural sciences sight is the most important sense, we will examine it in greater detail. To our consciousness, the line of vision from the eye to the object is straight. In the majority of cases, the corresponding light
1 From the correspondence between Frege and Richard Honigswald (cf. the letter from Honigswald to Frege of 24/4/1925), it emerges that Frege submitted this article to Bruno Bauch for the series edited by Honigswald and Bauch, Wissen- schaftliche Grundfragen. In the letter mentioned Honigswald gave a very favourable judgement on Frege's article, but suggested that it be essentially enlarged, 'Our concern in our venture isn't to produce a philosophical journal, in which in each issue many articles are put together, but a series of parts, each comprising between 4 to 6 sheets, and appearing on their own in bookform'. He asked Frege to resubmit the article for the Wissenschaftliche Grundfragen affer enlarging it, and listed a number of points where further reworking appeared to him to be desirable. Because of his death three months later, Frege was obviously only able to do some of the preliminary work required for this. In the NachlajJ three sheets, now lost, with notes pointing in this direction were to be found, as may be learnt from a note of the earlier editors to the Frege-Honigswald correspondence. Cf. further, Frege's letters to Honigswald of 26/4/1925 und Inter (cd. ).
? 268 Sources ofKnowledge ofMathematics and natural Sciences
ray from the object to our eye is also straight, or the deviation from a straight line is too slight to matter to us. If we notice the deviation, we speak of an optical illusion. We encounter such cases with the reflection of the light in a mirror, with diffraction or refraction of the light. Because of these illusions we might from the very outset regard visual perception as a source of knowledge that is unreliable and hence of little value, and yet it is precisely sense perception that is regarded by many as the most reliable, if not the only, source of knowledge. Of course a mirror appears to be an opening in the wall giving us a view of a neighbouring room; of course a calm stretch of water gives us the illusion of a sun which seems to be looking up at us; but people are no longer deceived by such cases, because there are at our disposal a diversity of means for correcting the judgement gained from the first impression. Of course, if there were no laws governing events, or if the laws governing events in the physical world were unknowable for us, we would lack the means for recognizing illusions for what they are and thus for rendering them harmless. The laws of nature we already know enable us to avoid being misled by sense illusions. Thus knowledge about the refraction of light tells us that many of the images we see through the microscope are wholly unreliable. In order to know the laws of nature we need perceptions that are free from illusion. And so, on its own, sense perception can be of little use to us, since to know the laws of nature we also need the other sources of knowledge: the logical and the geometrical. Thus we can only advance step by step--each extension in our knowledge of the laws of nature providing us with a further safeguard against being deceived by the senses and the purification of our perceptions helping us to a better knowledge of the laws of nature. We must be careful not to overestimate the value of sense perception, for without the other sources of knowledge, which protect us from being deceived, we could hardly get anywhere with it. We need the perceptions, but to make use of them, we also need the other sources of knowledge. Only all taken in conjunction make it possible for us to
penetrate ever deeper into mathematical physics.
For mathematics on its own, we don't need sense perception as a source
of knowledge: for it the logical and geometrical sources suffice.
At times sense perception has been an outright hindrance to the advancement of knowledge. For a long time the idea of the spherical shape of the earth was almost universally held to be completely absurd, since if it were true, on the bottom side of the earth men's heads would dangle downwards from their feet. People had been led by their senses to regard the upwards direction as everywhere the same; and even today there are difficulties in such considerations which children find difficult to surmount. It was a long time before the idea of the spherical shape of the earth gained sufficient ground for Columbus to put his trust in it and be able to embark
on his famous voyage. Its success and the subsequent circumnavigations of the earth were the victory of scientific reflection over the old view: a view
? Sources ofKnowledge ofMathematics and natural Sciences 269 that was almost overwhelmingly suggested by, and apparently unassailably
grounded in, sense perception.
B. The logical Source of Knowledge and Language
The senses present us with something external and because of this it is easier to comprehend how mistakes can occur than it is in the case of the logical source of knowledge which is wholly inside us and thus appears to be more proof against contamination. But appearances are deceptive. For our thinking is closely bound up with language and thereby with the world of the senses. Perhaps our thinking is at first a form of speaking which then becomes an imaging of speech. Silent thinking would in that case be speech which has become noiseless, taking place in the imagination. Now we may of course also think in mathematical signs; yet even then thinking is tied up with what is perceptible to the senses. To be sure, we distinguish the sentence as the expression of a thought from the thought itself. We know we can have various expressions for the same thought. The connection of a thought with one particular sentence is not a necessary one; but that a thought of which we are conscious is connected in our mind with some sentence or other is for us men necessary. But that does not lie in the nature of the thought but in our own nature. There is no contradiction in supposing there to exist beings that can grasp the same thought as we do without needing to clad it in a form that can be perceived by the senses. But still, for us men there is this necessity. Language is a human creation; and so man had, it would appear, the capacity to shape it in conformity with the logical disposition alive in him. Certainly the logical disposition of man was at work in the formation of language but equally alongside this many other dispositions-such as the poetic disposition. And so language is not constructed from a logical blueprint.
One feature of language that threatens to undermine the reliability of thinking is its tendency to form proper names to which no objects correspond.