It has often been a primary argument of some substance dualists that mere bodily identity is insufficient to ground our unified personal experience over time. Our identity must therefore, they argue, be grounded in something more. Eliminativists like the great Daniel Dennett argued that our apprehension of a unified Self enduring through time is illusory. The substance dualist theory is that humans are composed of essentially two parts, a physical body and a non-physical soul. Personal identity is grounded primarily in a soul, in the strictly literal sense, the Cartesian dualist says that humans are souls, not merely that they have souls.
“I thence concluded that I was a substance whose whole essence or nature consists only in thinking, and which, that it may exist, has need of no place, nor is dependent on any material thing.” (Descartes, Discourse on Method, IV.)
Descartes regarded the soul as his only essential part, so he says that his existence requires “no place” and is not “dependent on any material thing.” In sharp contrast, monist theories must ground identity within the material realm, to regard personal identity as a kind of bodily identity. This is most strongly represented in the identity theory, where the mind just is identical with the brain, and mental states are identified with particular neuro-physical states. Smart famously expressed disbelief that the entire world could be explained in physical and material terms with the exception of conscious states.
“So sensations, states of consciousness, do seem to be the one sort of thing left outside the physicalist picture, and for various reasons I just cannot believe that this can be so. That everything should be explicable in terms of physics (together of course with descriptions of the ways in which the parts are put together-roughly, biology is to physics as radio-engineering is to electromagnetism) except the occurrence of sensations seems to me to be frankly unbelievable.” (J. J. C. Smart, The Philosophical Review, Vol. 68, No. 2 Apr., 1959, p. 142.)
If humans are identical with their bodies upon monist theories and identical with their souls upon the substance dualist theories, one can make arguments from personal identity to favor one theory over the other. Consider the following kind of argument.
It seems every presently existing adult should concede r ∧ e ∧ ~b, that our present bodies did not exist in 2010, our bodies are constantly undergoing gradual replacement. Billions of cells die and are placed constantly, parts of our bodies are gradually replaced with the matter from food and drink we consume. The present set of atoms that compose this body were not in this form in 2010 and our bodies are constantly undergoing change. Yes, it seems that ~b is a quite well supported claim. But our identity and conscious experience remains unified and persistent across time, regardless of these radical changes with our body. Our existence is constituted in our conscious existence across time.
The most controversial premise seems to be the second premise. To avoid the force of the argument, it seems to me, the best approach would be to try and deny the entailment specified. The physicalist may claim that when he says he is identical with his body, he does not mean any particular body at a point in time, but rather that he is identical with the general form of the body in question. The body in 2010 and his present body share many resemblances, they have similar parts, memories, and ways of living. I would say that this kind of claim seems insufficient. There seems to be an enduring subject of phenomenally conscious experiences which is more than the mere form of the body in question.
It is possible to construct two complete individuals out of the physical bits of matter that constitute my body. Substituting the necessary parts so that the two resultant individuals each have 50% of the matter of the original person. But they would not both be me because they have different phenomenally conscious experiences (i.e. if you poked one with a needle the other would not feel it.) Even if they both resembled me very well and had similar memories and beliefs, and ways of living they would not make them both me in this sense.
One might claim that they are both me, but this seems false because the only kind of identity that matters here is the kind of identity that Descartes mentioned, “thinking,” or conscious experiencing. If you poked the body that existed in 2010 with a needle, a particular subject would feel pain, if you poked any body which presently exists with a needle would the same subject feel pain? Or does some does the new consciousness subject inhabit the body after a sufficient amount of matter has been replaced? Hence, the entailment in premise 2 seems well founded to me on the basis of such considerations. If I were identical with the body in 2010, (which no longer exists due to gradual replacement of parts,) then I, who presently exist, did not exist in 2010. Composition is not always identity, but that depends upon the kind of identity claims being made (as with the Ship of Theseus).
When considering the third premise, and assessing whether or not e is true, it is necessary to make clear that the claim is whether or not the same conscious individual that now exists also existed in 2010. The claim of e is not necessarily a claim about the body, or about something other than the body, it is a claim about whether or not the same consciousness that now persists one existed in 2010 as well. To my mind, the rest of the argument seems to follow with clarity from accepting the first three premises, though premise 8 may require some Cartesian assumptions.
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