Unity is 'the principle of number,' but is not itself number. The first number, as the Greeks saw, is two. Infinity is no number either. It is not the crowning number of a series of finite quantities, for it can never be got at by counting. As compared with an infinity of a higher order, the infinity of a lower order again is no number: it is (relatively) zero. Neither in lateral extension, then, nor in vertical superimposition, is infinity a number. The numbers are two, three, four, and so on as far as you can count. Each of these terms is called by the schoolmen 'a species of number.'