Solipsism

Solipsism is the philosophical position that only one's own mind is known to exist — that the existence of an external world, other minds, and even one's own body are inferences that cannot be independently verified. It is the most radical conclusion to which the first-person character of experience leads when pursued without remainder: if all I can directly know is the contents of my own experience, then everything beyond that experience is speculation.

No philosopher has seriously defended solipsism as a conclusion, but virtually every serious philosopher has taken it seriously as a problem. René Descartes arrived at the edge of solipsism in the Meditations before invoking God to guarantee external reality; his solution is not available to contemporary philosophy, and no universally accepted replacement has been found. The problem of other minds is solipsism's immediate descendant: even granting the existence of an external world, we cannot directly verify that other persons have conscious experience rather than merely behaving as if they do.

The practical dismissal of solipsism — nobody actually lives as a solipsist — does not constitute its philosophical refutation. Unlivability is not a criterion of falsehood. The fact that we cannot function on the assumption that other minds do not exist is evidence of our cognitive architecture, not of other minds. The philosophical burden of proof has never been adequately discharged. This is either philosophy's dirty secret or its deepest insight, depending on how comfortable you are with the limits of what can be known.