On the other hand the master was emphatic enough on the distinction between the reason and the senses. It might seem to us moderns that the old saying “seeing is believing,” with its implied prohibition against believing aught that might not be seen, erred, if at all, from a very excess of reasonableness. To Descartes, seeing with the body’s eye was still but flimsy evidence. This organ had too often deceived him to be implicitly trusted, and to back its testimony with that of the other senses,--touch, taste, smell, hearing--was but to multiply unreliable witnesses. Their combined voices might give a certain presumption in favor of their opinion but never an assurance amounting to certainty. Not indeed with the eye of the body but with the eye of the mind could we see truth in its nakedness. But what is this organ, this eye of the mind, and who is to teach us to use it? The mathematicians, Descartes replies, have long possessed it and long used it. It is to them we must turn for instruction. If we do--if we turn to Euclid, say--we find that the whole complex and difficult body of truth that we call geometry is made to follow from a few simple truths so certain that we call them self-evident. Why should not all truths be susceptible of the same kind of proof? Why should we not be able to find an axiom of axioms whose certainty was no mere matter of observation, and why, if we find it, should we not be able to draw from it all possible truth, as we deduce the theorems of geometry from its axioms? It was this idea that Descartes followed and it was this idea that Spinoza accepted at his hands: To construct a theory of life that should be no mere summing up of various peoples’ experiences, but should, after the manner of geometry, draw from an indubitable source the certainties of morals and religion no less than the truths of science. But when we learn what is this axiom of axioms that Spinoza received from Descartes and made the fountain head of all truth, we find ourselves spectators of one of those curious tricks of the human mind that make its history always diverting. It may well seem to us as though rationalism were not so much standing on its reason as standing on its head. For that axiom which is to be the simplest and most certain of all truths, more elementary than that 2 and 2 make 4, or that a straight line is the shortest distance between two points,--that axiom is the very proposition before which most of us find our reason staggering, our faith panting and breathless. That axiom consists of two words: “_God is_.”