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Zombies and Penetrability

Moody's overall argument, as presented above, is obviously formally valid, since it's a simple modus tollens. And, as indicated above, I think his sub-argument for (1), though vague, can be victoriously specified via routes I myself have mapped [4]. The problem is the extraordinarily vague premise (2). The first source of obscurity concerns Moody's underlying modal notions. In order to analyze these notions, let us use g as a variable ranging over groups of zombies visiting us from another world.gif And let us deploy the traditional symbol `' from modal logic to say that some proposition or state of affairs p is logically possible (and to say that some proposition p is logically necessary, where iff ). Finally, use the predicate P to denote ``penetrability," where the idea is that if zombies can be found out, if they can be revealed as zombies, they are said to be penetrable. One possible contrual of (2), given this (humble) machinery, and the fact that Moody's gedankenexperiments (as is common) are designed to show that certain propositions are coherent, and hence logically possible, is

(2)
But of course this can't be right, for in accordance with modus tollens (2) must be the negation of (1)'s consequent, and that consequent is surely not the proposition that it's not logically possible that a group of visiting zombies is penetrated. It seems clear that Moody needs (and wants) to make a claim about the penetrability of all visiting groups of zombies. What good would it do him to claim that some groups are penetrable? Or, worse, would good would it do Moody to claim that it's logically possible that some groups are penetrable? Such claims are truisms. After all, some groups of visiting zombies might carry signs with them declaring that they are zombies. Or, if God is logically possible, then some groups could show up and be thunderously deemed by the Almighty to be empty-headed. A much better bet for a more plausible sharpening of (2) is
(2)
But the problem is that the concept of penetrability itself hides within it a notion of possibility: intuitively, some group is penetrable if it's possible for that group to be penetrated. Here I think Moody must have in mind a concept of humanly possible and impossible. Specifically, I take him to be saying, in (2), that it's logically necessary that a group of zombie visitors is such that it's humanly possible to unmask them, that is (with the subscript to indicate `humanly penetrable'):
(2)
And now, given that (1) is replaced by
(1)
(CI)
which would be an encapsulation of the claim that ``since consciousness is superfluous, humans can't necessarily unmask a group of visiting zombies," we have constructed, on Moody's behalf, quite a respectable case against (CI).

Or have we? Isn't it perfectly easy to imagine that a group---call it the `A Team,' or simply, later, `a'---of zombie visitors perform always and only those behaviors in keeping with the presence of consciousness? After all, there is certainly some set of utterances, and some set of bodily movements, which preclude any inferences to the proposition that the bearers of these utterances and movements are not conscious.gif Whatever clever questions humans pose to the A Team, they come back with the ``right answers." For example, with respect to the inverted spectrum problem, there is certainly some set of sets of responses that precludes any suspicion that those giving these responses are zombies. (That there is such a set of sets follows immediately from the undeniable fact that we don't suspect, when hearing about ISP from human philosophers, that they may be zombies.) Surely it is conceivable that the A Team invariably responds with a set of behaviors from this set.

But now the A Team thought-experiment gives us

(3)
From which follows by existential quantifier introduction. From this, by quantifier shift, we have , which is equivalent to , that is, Moody's (2) is false---and so his argument fails. It's as easy as that.



next up previous
Next: Moody's Fallacy Up: IN DEFENSE OF IMPENETRABLE Previous: Moody's Argument



Selmer Bringsjord
Tue Apr 2 12:59:33 EST 1996