There are a number of importantly different views associated with the term solipsism. Its Latin roots—solus, meaning "alone," and ipse, meaning "self"—suggest the rough idea that a solipsistic doctrine is going to put some sort of emphasis on the self standing alone, but there are radically different ways in which a philosopher might develop that emphasis. In particular, we must distinguish an extreme metaphysical thesis, a view about the nature of mental states (sometimes misleadingly referred to as methodological solipsism), an epistemological/methodological thesis, and an ethical thesis.

Metaphysical Solipsism

The simplest and most radical of doctrines associated with solipsism is the puzzling doctrine that only the self exists. Stated in these terms, the doctrine is scarcely intelligible. The obvious question concerns whose self precisely it is that is supposed to be the only existing thing. It is easiest to state the doctrine from the first-person...