One can read Descartes' Meditation III argument about the concept of God as an attempt
to warrant (21a). It is, in effect, an argument that the concept of God has contents such
that nobody has this concept unless it has an instance—that the causal story behind
anyone's having that concept must include a God. If recent externalists are right, there are
many such concepts, for example, water. And if the concept of a sort of item is externally
determined in the right way, then something like (21a) will hold for it. Suppose that an
appropriate externalist story about natural kind concepts is correct, and that water is a
natural kind. Then because the concept of water is determined by the real external nature
of water, if being H 2 O is part of that concept, it follows that water is H 2 O. It's not clear
a priori why God or perfect being could not be an externally determined concept. And
that Descartes was in general the patron saint of anti-externalism hardly precludes his
claiming that there is one exception to it, which the argument from perfection reveals. On
the other hand, any argument that externalism holds for the concept of God is ipso facto
one that God really exists. If to back a premise in an argument for God, one needs a
second, discrete argument for God, then the first argument cannot be stronger than the
second and is not independent of it. So if it took such an argument to back (21a), an
argument resting on (21a) would be useless.
Meditation V: Third Try
Our third reading of Meditation V begins by noting again its talk of God's essence and
what it includes. Descartes later claimed that the Meditation V argument is:
That which we clearly and distinctly understand to belong to the true and immutable
nature of anything, its essence, can be truly affirmed of that thingto exist belongs to
[God]'s true and immutable nature; thereforeHe exists. (HR II 19)
In accord with this, we might render the Med. V argument as
- If the “true and immutable nature” of x includes being F, then Fx.
- The “true and immutable nature” of God includes existence. So
- God exists.
To respect Descartes' claim that this somehow encapsulates Med. V, we might expand the
argument by deriving (27) from - The “true and immutable nature” of God includes having all perfections, and
- Existence is a perfection.
Perhaps Descartes did not see (21)–(25) and (26)–(30) as distinct. He distinguishes ideas
that grasp “true and immutable natures” from ideas that are just “fictitiousdue to a mental
synthesis” (HR II 20). If an idea does not have its content simply due to a mental
operation, it grasps a mind-independent truth. That is, it has its content by grasping
something that is somehow also extramentally the case. Descartes' thought, then, seems
to be that some ideas grasp “natures” that have some status beyond them, the idea of God
being one; for these ideas, the “nature” is just the idea's content, and so we can switch
indifferently between nature-talk and talk of concepts (ideas' contents).
Descartes' talk of “true and immutable natures” has two functions in (26)–(30). One is
trying to lend credibility to (29). If it's part of a thing's nature that it is F, says Descartes,
we did not simply dream this up, and so we can trust our impression that such a thing