Algebra and arithmetic the only sciences in which we can carry on a chain of reasoning to any degree of intricacy, and yet preserve a perfect exactness and certainty. We are possessed of a precise standard, by which we can judge of the equality and proportion of numbers; and according as they correspond or not to that standard, we determine their relations, without any possibility of error. "When two numbers are so combined, as that the one has always a unit answering to every unit of the other, we pronounce them equal"; and it is for want of such a standard of equality in [spatial extension, that geometry can scarce be esteemed a perfect and infallible science. (I. III. I.)