Oddly, Feser doesn't specifically respond to my critcism. Instead, he refers back to his American Catholic Philosophical Quarterly article. But in that article, he doesn't specifically respond to the epistemology objection, either. Here's what he wrote:
Dillard also suggests that Kripke’s point is epistemological rather than metaphysical—that his argument shows at most only that the claim that someone is thinking in accordance with a certain function (such as addition) is underdetermined by the physical evidence, and not that the physical facts are themselves indeterminate. This is odd given that both Kripke and Ross explicitly insist that the points they are respectively making are metaphysical rather than merely epistemological. Indeed, Kripke says that “not even what an omniscient God would know . . . could establish whether I meant plus or quus,” because for the reasons given above, everything about my past behavior, sensations, and the like is compatible (not just compatible as far as we know, but compatible full stop) with my meaning either plus or quus. Nor does Dillard say anything to show otherwise.That is, Feser merely states that Ross says that his point is metaphysical, not epistemological. But Feser doesn't give any additional reasons for us to believe that Ross has actually established this. Well, I agree that Ross says that - but I don't think he has established it.
Here's why. Note that Ross's argument is just as valid when talking about what another person is doing when (say) adding. That is, when I am trying to determine whether Hilda is actually adding, or merely simulating adding, all I can do is investigate her physical actions and responses. If Ross's argument is correct, then from a finite amount of data such as these I cannot determine whether Hilda is adding or not. So (if Ross is right) I can never know whether another person is capable of addition.
But note that from the above it doesn't follow that Hilda is not adding. It may be that Hilda is in fact doing something perfectly determinate. I just can't know whether she is or not. So it is clear that Ross's argument doesn't get us past the epistemological.
This point ties in with my second complaint about Ross: the double standard. If I can't say for sure that another person is not adding, then by the same token I cannot say for sure that a machine is not adding.*
In his article, Feser almost makes the same point. Kripke's original point (if I understand it correctly) was, not only can I not be sure what someone else means when they say something, I cannot even be sure what I mean when I say something. That is, even my own thoughts are indeterminate in meaning. Ross obviously doesn't want this conclusion - his own argument relies on one's own thoughts being determinate. Feser points out that (using Frege's conception of meaning) we cannot infer from the external indeterminacy that there is no internal meaning. He writes:
Frege emphasized that the sense of an expression is not a private psychological entity such as a sensation or mental image, any more than it is something material. Thus he would hardly take an argument to the effect that meaning cannot be fixed either by sensations and mental images or by bodily behavior to establish that there is no determinate meaning at all.
But establishing that there is "no determinate meaning at all" is precisely what Ross needs for his argument. So the argument fails.
* Though it is not directly relevant to the argument, I want to point out that the situation is actually worse with respect to the machine than it is with respect to another person. We can open up the machine, trace its circuits or it mechanism or whatever, and deduce what it will do for a given input. With another person, we can only investigate the physical outputs: we can't open up Hilda's brain and trace its circuitry. Well, not yet, at any rate.