Solipsism (/ˈsɒlɪpsɪzəm/; from Latin solus, meaning ‘alone’, and ipse, meaning ‘self’) is the philosophical idea that only one’s own mind is sure to exist.
While no great philosopher has explicitly espoused solipsism, this can be attributed to the inconsistency of much philosophical reasoning. Many philosophers have failed to accept the logical consequences of their own most fundamental commitments and preconceptions.
Internet Encyclopedia of Philosophy: Solipsism and the Problem of Other Minds
In philosophy, Idealism is the group of metaphysical philosophies which assert that reality, or reality as humans can know it, is fundamentally mental, mentally constructed, or otherwise immaterial.
Idealism and materialism are both impossible to prove or disprove, of course — they’re unfalsifiable statements, which means there’s no neutral test that could weigh them against each other. The test, ultimately, has to be one of intuition, or “gut reaction.” Many people find that materialism makes more sense because, after all, everyone has the experience of interacting with an outside world and believing that’s really “out there.” On the other hand, it’s impossible for us to step “outside” our own minds, so how can we be so sure that there really is an “out there” at all?
As with the past and future, no amount of evidence can help decide the case. It comes down to preference, not reason.