1. There is a possible world at which my mind exists apart from my body. (Premise)
2. Whatever is possible is necessarily possible, e.g. possible in all possible worlds. (Premise, S5)
3. Hence, my mind possibly exists apart from my body in the actual world. (From 1 and 2)
4. Therefore, my mind is distinct from my body. (Implied by 3)
A skeptic's best bet to avoid the conclusion that the mind is immaterial is to reject premise (1). However, one would be hard-pressed to find any inconsistency in the conception of the mind existing apart from the body. Premise (2) is based on the relatively uncontroversial S5 axiom of modal logic, and (3) simply follows from (1) and (2). (4) is implied by (3) because unless two things - A and B - are distinct, then A cannot exist apart from B. Therefore, unless the mind is actually distinct from the body, then it would not be possible for the mind to exist apart from the body, which contradicts (1).