In accordance with this principle, he rejected the testimony of the senses touching the existence of a world of external things.  Do not the senses sometimes deceive us?  And, since men seem to be liable to error in their reasonings, even in a field so secure as that of mathematical demonstration, he resolved further to repudiate all the reasonings he had heretofore accepted.  He would not even assume himself to be in his right mind and awake; might he not be the victim of a diseased fancy, or a man deluded by dreams?

Could anything whatever escape this all-devouring doubt?  One truth seemed unshakable:  his own existence, at least, emerged from this sea of uncertainties.  I may be deceived in thinking that there is an external world, and that I am awake and really perceive things; but I surely cannot be deceived unless I exist. Cogito, ergo sum—­I think, hence I exist; this truth Descartes accepted as the first principle of the new and sounder philosophy which he sought.

As we read farther in Descartes we discover that he takes back again a great many of those things that he had at the outset rejected as uncertain.  Thus, he accepts an external world of material things.  How does he establish its existence?  He cannot do it as the empiricist does it, by a reference to experienced fact, for he does not believe that the external world is directly given in our experience.  He thinks we are directly conscious only of our ideas of it, and must somehow prove that it exists over against our ideas.

By his principles, Descartes is compelled to fall back upon a curious roundabout argument to prove that there is a world.  He must first prove that God exists, and then argue that God would not deceive us into thinking that it exists when it does not.

Now, when we come to examine Descartes’ reasonings in detail we find what appear to us some very uncritical assumptions.  Thus, he proves the existence of God by the following argument:—­

I exist, and I find in me the idea of God; of this idea I cannot be the author, for it represents something much greater than I, and its cause must be as great as the reality it represents.  In other words, nothing less than God can be the cause of the idea of God which I find in me, and, hence, I may infer that God exists.

Where did Descartes get this notion that every idea must have a cause which contains as much external reality as the idea does represented reality?  How does he prove his assumption?  He simply appeals to what he calls “the natural light,” which is for him a source of all sorts of information which cannot be derived from experience.  This “natural light” furnishes him with a vast number of “eternal truths”, these he has not brought under the sickle of his sweeping doubt, and these help him to build up again the world he has overthrown, beginning with the one indubitable fact discussed above.

To the men of a later time many of Descartes’ eternal truths are simply inherited philosophical prejudices, the results of the reflections of earlier thinkers, and in sad need of revision.  I shall not criticise them in detail.  The important point for us to notice is that we have here a type of philosophy which depends upon truths revealed by the reason, independently of experience, to carry one beyond the sphere of experience.