III
Project Gutenberg EPUB3 edition
Poincaré, Henri and Halsted, George Bruce
Let us, therefore, see the geometer at work and seek to catch his process.
The task is not without difficulty; it does not suffice to open a work at random and analyze any demonstration in it.
We must first exclude geometry, where the question is complicated by arduous problems relative to the rôle of the postulates, to the nature and the origin of the notion of space. For analogous reasons we can not turn to the infinitesimal analysis. We must seek mathematical thought where it has remained pure, that is, in arithmetic.
A choice still is necessary; in the higher parts of the theory of numbers, the primitive mathematical notions have already undergone an elaboration so profound that it becomes difficult to analyze them.
It is, therefore, at the beginning of arithmetic that we must expect to find the explanation we seek, but it happens that precisely in the demonstration of the most elementary theorems the authors of the classic treatises have shown the least precision and rigor. We must not impute this to them as a crime; they have yielded to a necessity; beginners are not prepared for real mathematical rigor; they would see in it only useless and irksome subtleties; it would be a waste of time to try prematurely to make them more exacting; they must pass over rapidly, but without skipping stations, the road traversed slowly by the founders of the science.
Why is so long a preparation necessary to become habituated to this perfect rigor, which, it would seem, should naturally impress itself upon all good minds? This is a logical and psychological problem well worthy of study.
But we shall not take it up; it is foreign to our purpose; all I wish to insist on is that, not to fail of our purpose, we must recast the demonstrations of the most elementary theorems and give them, not the crude form in which they are left, so as not to harass beginners, but the form that will satisfy a skilled geometer.
Definition of Addition.—I suppose already defined the operation x + 1, which consists in adding the number 1 to a given number x.
This definition, whatever it be, does not enter into our subsequent reasoning.
We now have to define the operation x + a, which consists in adding the number a to a given number x.
Supposing we have defined the operation
x + (a − 1),
the operation x + a will be defined by the equality
(1)x + a = [x + (a − 1)] + 1.
We shall know then what x + a is when we know what x + (a − 1) is, and as I have supposed that to start with we knew what x + 1 is, we can define successively and 'by recurrence' the operations x + 2, x + 3, etc.
This definition deserves a moment's attention; it is of a particular nature which already distinguishes it from the purely logical definition; the equality (1) contains an infinity of distinct definitions, each having a meaning only when one knows the preceding.
Properties of Addition.—Associativity.—I say that
a + (b + c) = (a + b) + c.
In fact the theorem is true for c = 1; it is then written
a + (b + 1) = (a + b) + 1,
which, apart from the difference of notation, is nothing but the equality (1), by which I have just defined addition.
Supposing the theorem true for c = γ, I say it will be true for c = γ + 1.
In fact, supposing
(a + b) + γ = a + (b + γ),
it follows that
[(a + b) + γ] + 1 = [a + (b + γ)] + 1
or by definition (1)
(a + b) + (γ + 1) = a + (b + γ + 1) = a + [b + (γ + 1)],
which shows, by a series of purely analytic deductions, that the theorem is true for γ + 1.
Being true for c = 1, we thus see successively that so it is for c = 2, for c = 3, etc.
Commutativity.—1º I say that
a + 1 = 1 + a.
The theorem is evidently true for a = 1; we can verify by purely analytic reasoning that if it is true for a = γ it will be true for a = γ + 1; for then
(γ + 1) + 1 = (1 + γ) + 1 = 1 + (γ + 1);
now it is true for a = 1, therefore it will be true for a = 2, for a = 3, etc., which is expressed by saying that the enunciated proposition is demonstrated by recurrence.
2º I say that
a + b = b + a.
The theorem has just been demonstrated for b = 1; it can be verified analytically that if it is true for b = β, it will be true for b = β + 1.
The proposition is therefore established by recurrence.
Definition of Multiplication.—We shall define multiplication by the equalities.