Irwin in a sense does deal with the first objection I raised yesterday against his account of Compresence of Opposites, but through what seems to me a kind of unfortunate verbal slide.
The objection was that his construal is too weak and captures too much. Any instance in which one property is indeterminate relative to some other concurrent property would count as "Compresence of Opposites", and that can't be what Plato was driving at (viz. in those passages which make us want to ascribe that sort of doctrine to him).
I gave as examples yesterday a circle which could be green or not, or a green patch which could be circular or not. Yet actually the problem for Irwin's account is worse than that. Because Irwin characterizes "Compresence" in terms of types only, his characterization carries with it no notion that those tokens need even be instantiated in the same subject. Thus, that the core of an apple is white yet its skin can be either red or not red would also count as "Compresence of Opposites".
Admittedly, for someone in debate to point out even that sort of weak "compresence" would suffice to rebut the claim that a thing of the one sort makes it so that the other thing is of that other sort. We could rebut the claim that the white core of the apple made the skin red by pointing out that there are apples with white cores and not-red skin. Or we could rebut the claim that the circularity of a green circle is responsible for the greenness of the circle by pointing out that there are circles that are not green. Which is all to say that we might serviceably take what Irwin calls "Compresence of Opposites" as shorthand for a certain argumentative move one makes, in response to a proposed claim about what sorts of things "make it so that" other things are the way they are. That is, we can take "Compresence of Opposites" to be a fancy way of saying that one should use Moore's Open Question Test to examine a proposed definition. And I suppose in that way we might in some sense "save" this otherwise much too weak characterization of Compresence.
But to understand the doctrine in that way would be, so to speak, to change it from a metaphysical or physical thesis (which it is for Plato) into an elenchic strategy for dealing with claims about what "makes" something to be what it is--claims which will clearly be relative to the interests and theories of those who are proposing the definitions to be tested.
Now if you look carefully at Irwin's exposition in Chapter 10 of Plato's Ethics, you'll see that changing the point of the doctrine is exactly what he does. He accomplishes this by the purely verbal maneuver of restating the doctrine until it takes a form which might pass muster. Here's the relevant paragraph (p. 157):
The discussion of explanation in the Phaedo refers primarily to properties: having a head, being taller by a head, and so on. When he contrasts the 'safe' explanation referring to Forms with the defective explanations he has illustrated, Plato insists that we should say that beautiful things are beautiful because of the Beautiful Itself, not 'by having a bright colour or shape or anything else of that kind' (Phd. 100c9-d2). He seems to mean the same by saying (1) that bright-coloured things, say, are both beautiful and ugly, (2) that bright colour is both beautiful and ugly, or (3) that bright colour makes things beautiful and ugly. The third formula conveys his main point most accurately. [Emphasis mine.]In one of the next sections of the book Irwin goes on to discuss Plato's commitment to the doctrine of flux. And I almost think that that transition from (1) to (3) was meant to foreshadow it playfully, as an instance.