Suppose that I see a bank being robbed. As a result, I believe that the masked man robbed the bank. Later, a detective tells me that their suspect is my father. I say that the masked man has a property that my father doesn’t: he’s someone I believe to have robbed the bank.
Here’s the formal argument:
1. The masked man has the property of being someone I believe robbed the bank
2. My father lacks the property of being someone I believe robbed the bank
3. Therefore, my father is not identical with the masked man.
This is a bad argument because it could still be the case that my father is the masked man, despite both premises being true. What this illustrates is that arguing along this style is invalid when the property in question involves someone’s psychological attitude.
Now is Descartes’ argument similar?
1. My body possesses the property of being something the existence of which I doubt
2. I don’t posses the property of being something the existence of which I doubt
3. Therefore, I am not identical with my body