1.
Descartes
now uses the principle that whatever he can clearly and distinctly conceive independently of sensory experience must
be true. He thus recovers all of his general ideas: extension, number, shape,
duration and so on. Of course, it’s just his crescendo of invalid inferences in
the Third Meditation that has made all of this possible.
2.
He
now declares that the idea of God is among his general ideas.
3.
Remember
that at this point Descartes has still not recovered the existence of a single
aspect of the external world.Now he
goes for God as the first such object. He argues as follows:
(i)
Premise:
It’s part of his general idea of God that God is perfect.
(ii)
Premise:
Non-existence is a kind of imperfection.
(iii)
Conclusion
(from (i) and (ii)): Therefore, if God is perfect God must exist.
(iv)
Conclusion
(from (i) and (iii): Therefore, God exists.
On this argument: I’ve already questioned his basis
for premise (i) in discussing Meditation 3. And premise (ii) seems to invoke
little more than a word game. Far from being an obviously true claim, it’s not
at all clear to me that it even means anything.
4.
Descartes
then goes on to argue that all his
knowledge depends on his knowledge of God. This is the trickiest point in the
whole Meditations. Let me spell it
out as an argument:
(i)
Premise:
All Descartes could establish before he
invoked God was that he was thinking at
the moment of his thought.
(ii)
Premise:
To count as knowledge, a belief must have some stability over time.
(iii)
Premise:
All error results from letting the will run ahead of the understanding. This
means: from not concentrating exclusively on clear and distinct ideas.
(iv)
Premise:
But it’s too hard to concentrate on a whole body of clear and distinct ideas at
once.
(v)
Conclusion
(from (ii), (iii) and (iv)): Therefore, if Descartes has to keep all his clear
and distinct ideas in focus at the same time, he’ll cease to know anything as
soon as he relaxes his concentration.
(vi)
Premise:
Descartes can keep one clear and
distinct idea steadily in his mind.
(vii)
Premise:
If God exists, then Descartes can’t be systematically deceived.
(viii)
Premise:
Descartes has a clear and distinct idea of a perfect God.
(ix)
Conclusion
(from previous argument and (viii):
Therefore, God exists.
(x)
Conclusion
(from (vi) , (vii) and (viii)): Because
he knows God exists, Descartes can’t be systematically deceived.
(xi)
Premise:
If none of Descartes’s beliefs had stability over time, he’d be systematically
deceived.
(xii)
Conclusion
(from (ix) and (x)): Descartes’s beliefs have stability over time.
(xiii)
Conclusion
(from (ii), (ix), and (xi)): Because he knows God exists, Descartes can know
other things.