Academic journal article Philosophy Today

Cartesian Dualism, and the Universe as Turing Machine

Academic journal article Philosophy Today

Cartesian Dualism, and the Universe as Turing Machine

Article excerpt

In the field of computability and algorithmicity, there have recently been two essays that are, in my opinion, of great interest: Peter Slezak's "Descartes's Diagonal Deduction," and David Deutsch's "Quantum Theory, the Church-Turing Principle and the Universal Quantum Computer." Briefly, the former shows that Descartes's Cogito argument is, in crucial places, structurally similar to Godel's proof that there are statements true but unprovable within a formal system such as Principia Mathematica, while the latter provides strong arguments for believing that the universe can be represented as a Turing machine. But suggestive though these essays are, still more suggestive, I think, are the implications of their conjoining. What I propose to do in this essay is to sketch the arguments of Slezak and of Deutsch, and then to discuss some of these implications, particularly for the establishment of a scientific theology.

Slezak and Descartes

Slezak's argument is quite simple. He begins by reviewing various accounts of Descartes's Cogito argument, and suggests that these accounts lack an essential ingredient: a plausible, textually faithful rational reconstruction of the argument. With a view to filling this lack, he observes that the representational character of ideas in Descartes makes them semantically analogous to pictures or sentences, the object of which is the physical world. Slezak's concern is to demonstrate that the reasoning employed by Descartes can be shown to be compatible with a physicalist account of the mind and, indeed, can be seen to follow from certain kinds of physical or information-processing arrangements-even though, he cautions, there are "notoriously troublesome features of subjective, introspective experience which seem to be intractable to any physicalistic reduction" (Slezak, 1983, 15).

Slezak sets the stage by presenting pertinent observations by such thinkers as Wittgenstein and Gunderson. He quotes, for example, the following words of Gunderson:

If a thoroughgoing physicalism . . . is true, why should it even seem so difficult for me to view my mind or self as an item wholly in the world? (Slezak, 1983, 17)

Slezak suggests that perplexity such as that cited by Gunderson is of a particular type-one concerning the place of the entity itself in relation to the rest of the world-and using an example of Gunderson's, he points to a parallel between the apparent irreducibility of the mind's relation to the rest of the world and the fact (for instance) that the one thing a periscope cannot locate is its own cross-hairs. Yet, as Slezak implies, the parallel is of limited application; for although the periscope cannot see its own cross-hairs, both periscope and cross-hairs are both expressible in physical terms-whereas, of course, the mind and the body seem irreducible to each other.

Slezak's next step is to adopt Gunderson's method of looking at the world W from the point of view of a person M. According to this method, M will have an "internal model" W* of the world. Seen this way, it is logically impossible for M itself to appear "directly as a physical object in W* among the other physical objects represented in W*, since W* is itself part of M" (Slezak, 1983, 19). Slezak is quick to point out, however, that for many "ordinary purposes" there is no contradiction in representing the self as a physical object. There are problems only when we try to push that representation too far.

Next, Slezak seeks to rephrase Descartes's argument in Gunderson's terms. Descartes, of course, started by doubting the existence of the world; rephrased, this doubt can be expressed as the possibility that everything in W* fails to correspond to W. Slezak follows Descartes's own recommendation that thoughts must be "arranged in an order like the natural order of the numbers" (Slezak, 1983, 24) to achieve an enumeration of propositions to which assent would be given. Such an enumeration might look like the following:1

(1) Grass is green

(2) Roses are red

(3) Snow is white

etc. …

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