Philosophy

always asserts merely the whole or part of the premisses, though usually in new language. The difficulty of mathematics consists in seeing that this is so in particular cases. In practice, the mathematician has a set of rules according to which his symbols can be manipulated, and he acquires technical skill in working according to the rules in the same sort of way as a billiard-player does. But there is a difference between mathematics and billiards: the rules of billiards are arbitrary, whereas in mathematics some at least are in some sense “true”. A man cannot be said to understand mathematics unless he has “seen” that these rules are right. Now what does this consist of? I think it is only a more complicated example of the process of understanding that “Napoleon” and “Bonaparte” refer to the same person. To explain this, however, we must revert to what was said, in the chapter on “Language”, about the understanding of form. Human beings possess the power of reacting to form. No doubt some of the higher animals also possess it, though to nothing like the same extent as men do; and all animals, except a few of the most intelligent species, appear to be nearly devoid of it. Among human beings, it differs greatly from one individual to another, and increases, as a rule, up to adolescence. I should take it as what chiefly characterises “intellect”. But let us see, first, in what the power consists. When a child is being taught to read, he learns to recognise a given letter, say H, whether it is large or small, black or white or red. However it may vary in these respects his reaction is the same: he says “H”. That is to say, the essential feature in the stimulus is its form. When my boy, at the age of just under three, was about to eat a three-cornered piece of bread and butter, I told him it was a triangle. (His slices were generally rectangular.) Next day, unprompted, he pointed to triangular bits in the pavement of the Albert Memorial, and called them “triangles”. Thus the form of the bread and butter, as opposed to its edibility, its softness, its colour, etc., was what had impressed him. This sort of thing constitutes the most elementary kind of reaction to form. Now “matter” and “form” can be placed, as in the Aristotelian philosophy, in a hierarchy. From a triangle we can advance to a polygon, thence to a figure, thence to a manifold of points. Then we can go on and turn “point” into a formal concept, meaning “something that has relations which resemble spatial relations in certain formal respects”. Each of these is a step away from “matter” and further into the region of “form”. At each stage the difficulty increases. The difficulty consists in having a uniform reaction (other than boredom) to a stimulus of this kind. When we “understand” a mathematical expression, that means that we can react to it in an appropriate manner, in fact, that it has “meaning” for us. This is also what we mean by “understanding” the word “cat”. But it is easier to understand the word “cat”, because the resemblances between different cats are of a sort which causes even animals to have a uniform reaction to all cats. When we come to algebra, and have to operate with x and y, there is a natural desire to know what x and y really are. That, at least, was my feeling: I always thought the teacher knew what they really were, but would not tell me. To “understand” even the simplest formula in algebra, say (x + y)² = x² + 2xy + y², is to be able to react to two sets of symbols in virtue of the form which they express, and to perceive that the form is the same in both cases. This is a very elaborate business, and it is no wonder that boys and girls find algebra a bugbear. But there is no novelty in principle after the first elementary perceptions of form. And perception of form consists merely in reacting alike to two stimuli which are alike in form but very different in other respects. For, when we can do that, we can say, on the appropriate occasion, “that is a triangle”; and this is enough to satisfy the examiner that we know what a triangle is, unless he is so old-fashioned as to expect us to reproduce the verbal definition, which is of course a far easier matter, in which, with patience, we might teach even a parrot to succeed. The meanings of complex mathematical symbols are always fixed by rules in relation to the meaning of simpler symbols; thus their meanings are analogous to those of sentences, not to those of single words. What was said earlier about the understanding of sentences applies, therefore, to any group of symbols which, in mathematics, will be declared to have the same meaning as another group, or part of that meaning. We may sum up this discussion by saying that mathematical inference consists in attaching the same reactions to two different groups of signs, whose meanings are fixed by convention in relation to their constituent parts, whereas induction consists, first, in taking something as a sign of something else, and later, when we have learned to take A as a sign of B, in taking A as also a sign of C. Thus the usual cases of induction and deduction are distinguished by the fact that, in the former, the inference consists in taking one sign as a sign of two different things, while in the latter the inference consists in taking two different signs as signs of the same thing. This statement is a little too antithetical to be taken as an exact expression of the whole truth in the matter. What is true, however, is that both kinds of inferences are concerned with the relation of a sign to what it signifies, and therefore come within the scope of the law of association.