Sid asked:

Could I prove to a solipsist that I am not a figment of his imagination if he were to just stop imagining that I exist?

Answer by Helier Robinson

Yes, in principle.

First, let’s get the meaning of solipsism clear. If I define an imperceptible, for me, now, as anything not in my consciousness now, then solipsism results from the premise that no such imperceptibles exist. This means that (i) time does not exist because past and future are imperceptible hence (ii) change does not exist and (iii) my experience of passage of time and of change are illusions, and all my memories and expectations are false; also (iv) all my beliefs are false, since a belief is a perception substitute, a belief in the existence of an imperceptible; and (v) all explanations are false, since explanations are descriptions of imperceptible causes. Note that all explanations, beliefs, memories, illusions, etc. are only those that I am conscious of now, since all others are imperceptible and so do not exist. In particular, any explanation of why solipsism should be true is a false explanation.

So how can the you in my imagination prove to me that you are not just a figment of my imagination? (Notice, by the way, that if I am imagining you then I cannot stop doing so, since there is no time.) One way is to prove that the premise that no imperceptibles exist leads to a contradiction, in which case at least one imperceptible exists; and another is to prove in some other way that at least one imperceptible exists, such as by the ontological argument. Neither of these prove that you are more that a figment in my imagination, but they prove that you could be real, and probably are so.