1.
Descartes
now reasons that because he knows God to be perfect, he knows that God can’t
aim to systematically deceive him. This might
follow if we granted Descartes
knowledge that his idea of perfection is `complete’ (clear and distinct). But
as there’s no reason to grant Descartes such knowledge. Note that most theology
doesn’t claim such knowledge to be possible; many theologians maintain that God
isn’t completely knowable. Can Descartes be sure that God doesn’t have some larger
plan, unknown to Descartes, that requires him to deceive Descartes? Indeed,
just a bit later, Descartes admits that
he doesn’t have full knowledge of God’s plan. So he seems to take back his own
premise.
2.
Objection:
Doesn’t Descartes’s argument against deception prove too much? Namely, that he
could never make any errors at all?
3.
Descartes’s
answer: He’s already argued, in Meditation 3, that there can be no errors in
his understanding. Error can arise because God has given him an absolutely free
will. This permits his will to fail to
follow his understanding. This is the source of his errors. Thus, he, not God, makes his errors.
4.
Again,
we must ask: how can Descartes know that his will is absolutely free. He says
that he feels no constraints on its operation. So? This could show only that
Descartes hasn’t yet found the limits of his will; is can’t show that there aren’t any limits.
5.
As
a result of this (invalid) argument, Descartes makes a key move in his main
project to establish certainty. He argues that he can avoid all error if he
never lets his will go where his understanding doesn’t lead. What does he mean
by this? He means that he should never believe anything on the basis of sensory
evidence or imagination alone; he should require of himself that he able to
construct a logical argument for anything he’s considering believing (except
the first fact, that he thinks, which he discovers by pure inner perception
[apperception]).
6.
Thus
Descartes makes pure logical argument the
proper basis for science.