1. I can conceive of a world in which my mind exists without a body.
2. If I can conceive of a world, then that world is metaphysically possible.
3. So, it is metaphysically possible for my mind to exist without a body.
4. If it's metaphysically possible for my mind to exist without a body, then my mind is not the same as a body.
5. So my mind is not the same as a body - i.e., Cartesian Dualism is true.