There are two basic tacks a situationist can take to undo virtue ethics:
1. Prove empirically that moral agency is disunified. Machery admits that this hasn't been demonstrated, but is a "live possibility." Experiments show that, in many cases, context overwhelms moral judgment; furthermore, the moral I, the self that decides, seems to be cobbled together from moods, biases, unconscious demands, and desires that are contradictory, uncontrollable, or easily diverted. To rip Juliet's words from context, morally speaking, I am not I, if there be such an I.
2. Deconstruct moral intuitions. Here Machery's critique is more pointed. How do we know which virtues are in fact virtuous? Sometimes we must look to moral intuition as a guide. Yet experiments show that moral intuitions can be foiled by a mere slight rewording of a hypothetical, or, on the other hand, raising or lowering the moral stakes can sometimes create no difference in people's choices. Machery sums up:
We know that some, and maybe many, moral intuitions are spurious (as is shown by the Asian flu case), but, save a few cases, we do not know how to identify the intuitions that ought to be jettisoned.Let me warn that this blog post is a summary of a paper critiquing a book, and is therefore too far removed from the discussion for anything but thought-provocation. If you are a student of moral philosophy, you should check out Machery's article and read Appiah's book. (Whether it will make you a better person is an open question.)