Thursday, April 3, 2008

Twin Earth and Descriptivism

Some people have expressed interest in the "Twin Earth" objection to descriptivism and Searle's response. It's hard to discern how the objection to descripitivism is supposed to go on the basis of the passage from Searle, however. So I thought I would include this excerpt from Nathan Salmon's book Frege's Puzzle to help clarify how the objection to descriptivism might go:

One compelling argument against identifying the (semantic content) of a (proper) name with its purely conceptual (descriptive) content can be extracted from Hilary Putnam's twin-earth thought experiment. The argument depends on two plausible assumptions. First, it assumes that one's (purely psychological) state of consciousness determines which (purely conceptual or purely qualitative (descriptive)) concepts one is grasping, in the sense that if person A is in the very same (purely psychological) state of consciousness as person B, then, for any (purely conceptual or purely qualitative (descriptive)) concept c, A grasps c if and only if B grasps c. Second, the argument assumes that the information component that corresponds to the individual that a given piece of information is about determines that individual, in the sense that, if a piece of information p is information about an individual x and the component of p corresponding to x is also (appropriately) a component of the piece of information q, then q is also information concerning x. For example, on this assumption, if the (semantic content) of the name 'Socrates' is appropriately part of a piece of information p, then p is information concerning Socrates. Now, suppose that in a far corner of the universe there is a planet on which there is a perfect duplicate of a particular earthly woman. Each lives a life on her own planet qualitatively identical to the others'. Even their mental streams of consciousness are qualitatively identical. Moreover, each has a husband named 'Hubert', and the two Huberts are dead ringers for one another except that the earthly Hubert weighs exactly 165 pounds whereas his alien counterpart weighs exactly 165.000000001 pounds. Now, suppose that both wives simultaneously utter, assertively and sincerely, the string of symbols 'Hubert weighs exactly 165 pounds' in conversation, each talking about her own husband. The speakers are in exactly the same (purely psychological0 state of consciousness. In fact, their very brain matter is in exactly the same configuration, molecule for molecule. Hence, by the first assumption, the purely conceptual (descriptive) content that each associates with her use of the name 'Hubert' is exactly the same. but the information (proposition) encoded by the sentence uttered, as used on the two occasions, is different. This is evident because the information asserted by the earthly woman concerns her husband and is true whereas the information asserted by the alien woman concerns her husband and is strictly false. Hence, by the second assumption, the (semantic content) of the name 'Hubert' as used by the two women is different. The purely conceptual (descriptive) content is the same, but the (semantic content) is different. It follows that the (semantic content) of a (proper) name cannot be simply its purely conceptual (descriptive) content.

Salmon, Nathan. 1986. Frege's Puzzle. Atascadero, CA: Ridgeview Publishing Company. Pp. 66-67.

Please comment on this post with thoughts or questions.