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The Starting Argument in the Zombie Attack

Selmer Bringsjord


Searle wants to draw a certain conclusion from V2:

In [V2] we imagined that the mediating relationship between the mind and the behavior patterns was broken. In this case, the silicon chips did not duplicate the causal powers of the brain to produce conscious mental states, they only duplicated certain input-output functions of the brain. The underlying conscious mental life was left out ([3], 68).

And here is Dennett's reaction:

But that is only one of the logically possible interpretations of his second variation tex2html_wrap_inline55 The other is the crucial one: while you tex2html_wrap_inline55 are dying, another consciousness is taking over your body. The speech acts you faintly hear your body uttering are not yours, but they are also not nobody's! tex2html_wrap_inline55 I cannot see how Searle could simply have overlooked this gaping loophole in his thought-experiment. But there it is tex2html_wrap_inline55 I am baffled ([2], 198-9).

But what exactly does Searle want from V2? He tells us explicitly on page 69 of The Rediscovery of the Mind that he wants to establish via V2 and V3 that a certain trio of propositions is inconsistent. The trio, reproduced verbatim (p. 69):

Brains cause conscious mental phenomena.
There is some sort of conceptual or logical connection between conscious mental phenomena and external behavior.
The capacity of the brain to cause consciousness is conceptually distinct from its capacity to cause motor behavior. A system could have consciousness without behavior and behavior without consciousness.

Represent the three propositions using elementary logical machinery: Bx iff x is a brain; Mx iff x causes (a full range of) mental phenomena; and Ex iff x causes (a full range of) external behavior. Then the trio, with Searle's underlying modal notions brought to the surface, and a denoting the brain of the character in our thought-experiments, becomes

(1 tex2html_wrap_inline77 )
(2 tex2html_wrap_inline77 )
(3 tex2html_wrap_inline77 )

The set {(1 tex2html_wrap_inline77 ), (2 tex2html_wrap_inline77 ), (3 tex2html_wrap_inline77 )} is provably inconsistent, in garden variety contexts; the proof is trivial, for example, in quantificational S5 (which I happen to like) and the weaker T.gif

Proof. Proposition (1 tex2html_wrap_inline77 ) is superfluous. Then, e.g., instantiate appropriately on axiom- schema T to get, with (2 tex2html_wrap_inline77 ), by modus ponens, tex2html_wrap_inline105 ; instantiate to tex2html_wrap_inline107 , derive by propositional logic that tex2html_wrap_inline109 , rewrite this by the rule known as necessitation to tex2html_wrap_inline111 , and in turn rewrite this as tex2html_wrap_inline113 , and then, by double negation, as tex2html_wrap_inline115 , which of course contradicts (3 tex2html_wrap_inline77 )'s first conjunct.

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Next: References

Selmer Bringsjord
Tue Oct 7 08:27:53 EDT 1997