What is a line? A series of points. The line, then, is an intellectual construction, and involves only the successive fluxions of a point. What is a triangle? An intellectual construction wherein the extremities of three lines are united. What is a circle? Also an intellectual construction; the space enclosed by a circumference formed by the extremity of a line revolved around a point. What are all other curves? Lines described by the movement of a point governed by a certain law of inflexion. What is a surface? Is not its idea generated by the motion of a line, just as that of a solid is generated by the motion of a surface? And what are all the objects of geometry but lines, surfaces, and solids of various kinds, combined in various ways? Universal arithmetic, whether arithmetic properly so called, or algebra, is a creation of the understanding. Number is a collection of units, and it is the understanding that collects them. Two is only one and one, and three only two and one; and thus with all numerical values. The ideas expressing these values consequently contain a creation of our mind, are its work, and include nothing not placed there by it. We have already observed that algebra is a kind of language. Its rules are partly conventional, and its most complicated formulas may be reduced to a conventional principle. Take one of the simplest: a^0 = 1: but why is it? Because a^0 = a^{n-n}; why? Because there is a conventional usage to mark division by the remainder of the exponents; and consequently a^n/a^n, which is evidently equal to one, may be expressed a^n/a^n = a^{n-n} = 1. 305. These observations seem to prove Vico's system to be really true, so far as pure mathematics, that is, science of the purely ideal order, is concerned. Possibly also the same may be said of it in relation to other science, as for example, metaphysics; but we shall not follow it farther, since it is not easy to find a ground free from conflicting opinions. Moreover, having shown how far Vico's system is admissible in mathematics, we have thereby given a solution to difficulties to which it is subject in its other branches. 306. That in a purely ideal order the understanding constructs is undeniable, and the schools agree in this. There is no doubt that reason supposes, combines, compares, deduces; operations which are inconceivable without some kind of intellectual construction. The understanding in this case knows what it makes, because its work is present to it: when it combines it knows that it combines; when it compares or deduces, it knows that it compares or deduces; when it builds upon certain suppositions, which it has itself established, it knows in what they consist, since it rests upon them.