What is solipsism? A solipsist advocates solipsism.
When thinking about the sources of our understanding, it’s important to distinguish understanding that is based on introspection (“first person” evidence) from understanding that is based on the observation of the behavior of others (“third person” evidence). A solipsist denies any analogy between first person and third person evidence.
In other words, for a solipsist nothing else is sufficiently like what is understood introspectively (mind considered “internally” rather than “externally”).
An epistemological solipsist denies that he (or she) can know that other minds exist because no inference from a behavioral state to a mental state satisfies any of the canons of validity (valid reasoning). His appeal is not merely to the fact that another mind would be other. His appeal is a rational one: there is an epistemological difference between knowing that I myself am in a certain state of mind and knowing that another is in a certain state of mind.
For example, suppose that I have a headache; the proposition that I have a headache is self-evident. On the other hand, as Butchvarov points out in Skepticism in Ethics (pp. 129-130), I do need to perform an inference if I were to know that you have a headache, and, according to the epistemological solipsist, no such inference is valid.
By way of contrast, a metaphysical solipsist believes that it is false that other minds exist, that no other minds are real. As usual, the burden of proof is on whoever makes the positive existential claim. (A positive existential claim is one that judges that something exists or is real, whereas a negative existential claim denies that.) In this case, the positive existential claim is that other minds are real. The burden of proof here is quite heavy! On the other hand, with the possible exception of the early Wittgenstein, no major philosopher has ever advocated metaphysical solipsism, although, of course, some may have actually been metaphysical solipsists. Why, though, since there are no others, would a metaphysical solipsist try to convince anyone else of anything?
OBJECTION: These issues are irrelevant, abstract, or unimportant. Why should I think about them?
REPLY: Well, axiology, the study of values, is important. There is an axiological implication here, namely, that ethical solipsism follows from metaphysical solipsism. Ethical solipsism is the view that no state of anyone else is a good. Obviously, if no others exist, then no state of another exists to be a good. Therefore, one should never promote the good of another—since there is no good of another! In other words, for an ethical solipsist, nothing is a good unless it is one’s own good.
(Don’t confuse ethical solipsism with ethical egoism. An ethical egoist thinks that one ought not to promote anyone else’s good or that one ought only to promote one’s own good. Judging from the way many people behave, ethical egoism is quite a popular view.)