Flanagan on Descartes’ Modal Argument for Dualism

I’ve just started reading through Owen Flanagan’s book The Science of the Mind and he presents Descartes’ Modal Argument as follows:

1. I cannot possibly doubt that I exist as a thinking thing. (This was established as we tried to doubt our existence and found ourselves, therefore, affirming it.)
2. I can doubt, however, that I have a body, and thus that I exist as a physical thing.
3. Therefore, thinking is essential to what I am. My body is not. Furthermore, I know my mind more easily than I know my body. “From this I knew that I was a substance the whole essence or nature of which is to think, and that for its existence there is no need of any place, nor does it depend on any material thing so that this ‘me,’ that is to say, the soul by which I am what I am, is entirely distinct from body, and is even more easy to know than the latter; and even if body were not, the soul would not cease to be what it is.”

Flanagan goes on to praise the intuitive merit of the argument based on the epistemological principles laid down in earlier passages of the book. He does believe it has a problem, however, and claims that it has a missing premise. I claimed as much in an earlier post, but his missing premise ends up looking much different than mine based on the alternative construction of the argument. He calls this missing premise “alpha”:

Alpha: when an entity is known for certain to have property x, but not known for certain to have property y, then x is essential to the entity and y is not.

Obviously, this assumption is false. He gives the example of a geometry student who comes to believe- with certainty- that a set of three Cartesian coordinates makes a triangle, but fails to see that the internal angles of this triangle add up to two right angles. The problem is an epistemic problem.

I see this objection as a cousin to the host of objections relating to conceivability-entails-possibility. Likewise, the best answer to this objection is fairly weak: either you simply see that it is possible to exist without your body or you don’t. I do see that it is possible (I am not simply failing to see that it is impossible). I am then warranted in my belief that I am not essentially a body.

Edited to Add: While I think Alpha is wrong as a fast and tight principle, it seems that if something fits the criteria laid out in the principle, one has reasons (and probably good reasons) for suspecting that it describes essential properties. If all we had was Descartes’ argument -AND- you were convinced that the second premise (I can doubt, however, that I have a body, and thus that I exist as a physical thing) is true then we would be epistemically in the clear for believing that we are not essentially body.

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