An argument for platonic realism regarding universals (that abstract entities exist and can do so without dependence on their instances) takes the following form:
(1) There is a possible world where there are no particular things colored red.
(2) Despite the fact of (1), the following propositions are true, and necessarily so; that ‘necessarily, red is a color (there is no possible world where red is not a color)’, and ‘necessarily, red resembles orange more than it does blue, (there is no possible world where red doesn’t resemble orange more than it does blue’, and also that ‘necessarily, no single indivisible entity can be both red and green at the same time (there is no possible world where a single thing can be both red and green).
(3) Since the propositions expressed in (2) are necessary truths, they are also true in the possible world expressed in premise (1). But (1) stipulated that that there is a world where no particular thing is colored red. So the question is, if propositions are true or false in virtue of the content they express, how can it be the case in the possible world where there are no instances of red that propositions expressed in premise (2) are true? The way it out is to assert (4).
(4) In a possible world where nothing in particular is colored red the necessary propositions expressed in (2) are true because they refer to the form of red, which is an uninstantiated universal. Therefore,
(5) Platonism in true.
The only way out of this consequence, it seems, is the denial of the last part of (3); namely, that propositions can only be true or false in virtue of the content they express and refer too. To say it another way, one must deny that propositions need not have intention to be true or false. But this seems crazy, and I cannot even begin to conceive of what this might look like, so pending further enlightenment by some sneaky Aristotelians, I must declare (5).
Well, assuming everything I’ve articulated adds up, consider the following proposition:
(6) Necessarily, to be a person is to have rational faculties (either dormant or otherwise).
If (6) is true, then so is
(7) There is a possible world where there are no instances of persons, but since (6) is necessarily true, it’s also true in such a world. But if (6) is true (and necessarily so),
(8) (6) is (necessarily) true in virtue of the fact that it expresses, namely, that a person necessarily has rational faculties (dormant or otherwise). If this is a fact, then there is at least one person, and He must necessarily exist, since (6) could only be true in virtue of this fact. And if there is a person that exists necessarily, then he can be nothing other than what men call God.
... No, I presume I may say that we
more certainly know that there is a
God than that there is anything else without
us. When I say we know I mean there
is such knowledge within our reach which
we cannot miss, if we will but apply our
minds to that, as we do to several other
inquiries.
John Locke,
Chapter X of Essay Concerning Human Understanding
