One can put Caterus's thought this way: from premises about the content of a concept,
only conclusions about the content of a concept can validly follow.
Descartes' reply in a nutshell is that his premises deal in “what belongs to the true and
immutable essence of a thing,” not “what is attributed to it merely by a fiction of the
intellect” (HR II 19)—that is, are not merely about concepts' contents, but about
extramental facts. His criterion for this seems to be that elements of a “merely fictitious”
nature can rightly be separated conceptually: winged horse is “fictitious” because we can
rightly conceive of horses without wings (HR II 20). On the other hand, if elements FG
belong together as part of a “true and immutable nature,” we cannot rightly conceive
them apart: being F entails being G, or conversely (HR II 21). Thus, Descartes goes on to
try to show that
end p.102
existence really does belong to God's “true and immutable nature” without merely
reiterating his Med. V argument, by arguing that the nature of God's power itself entails
His existence (HR II 21). But if one must show that some divine attribute entails God's
existence to show that existence is of God's nature, Descartes has a problem. For if the
Med. V argument really does include a premise about God's true, immutable nature
including existence, it is then an argument for God the defense of whose premises
requires another, independent argument for God's existence. If it is, it is dialectically
useless. For if one can demonstrate God's existence a priori in another way, the Med. V
argument is unneeded: it can't yield any further, independent warrant for belief in God. If
one can't, it has an indefensible premise.
Gassendi wrote:
Existence is a perfection neither in God nor in anything else; it is rather that in the
absence of which there is no perfectionthat which does not exist has neither perfection
nor imperfection, and that which exists (has) its existenceas that by means of which the
thing itself equally with its perfections is in existencenor if the thing lacks existence is it
said to be imperfect, (but rather) to be nothing. (HR II 186)
Descartes' reply is that possible existence is a perfection in the case of a triangle, making
“the idea of a triangle superior to the ideas of chimeras,” and similarly necessary
existence is a perfection in God's case, making the idea of God superior to other ideas
(HR II 228–29). This does not immediately address Gassendi's point about mere
existence; perhaps Descartes means to add that any property a perfection entails is itself a
perfection. This claim would not be implausible, as we see below in discussing Gödel.
Gassendi's second major argument was this:
Although you say that existence quite as much as other perfections is included in the idea
of a being of the highest perfection, you (just) affirm what has to be proved, and assume
your conclusion as a premise. For I might alsosay that in the idea of a perfect Pegasus (is)
contained not only the perfection of having wings but also that of existing. For just as
God is thought to be perfect in every kind of perfection, so is Pegasus thought to be
perfect in its own kind. (HR II 187)
Descartes offers no reply to the parody. Perhaps he would treat “existing Pegasus” as he
did Caterus's “existing lion”: the “complex” captures no “true, immutable nature”—since
it's not the case that the attribute of being Pegasus is such that necessarily, if it exists, it