undertook a
reconstruction, he found it necessary to throw over a vast amount of
what had passed as truth, if only with a view to building up again upon
a firmer foundation. It appeared to him that much was uncritically
accepted as true in philosophy and in the sciences which a little
reflection revealed to be either false or highly doubtful.
Accordingly, he decided to clear the ground by a sweeping doubt, and to
begin his task quite independently.

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