Solipsism belief

Solipsism (from the Latin ipse = "self" and solus = "alone") is the epistemological belief that one's self is the only thing that can be known with certainty and verified (sometimes called egoism). Solipsism is also commonly understood to encompass the metaphysical belief that only one's self exists, and that "existence" just means being a part of one's own mental states — all objects, people, etc, that one experiences are merely parts of one's own mind. Solipsism is first recorded with the presocratic sophist Gorgias (c. 483-375 BC) who is quoted by Sextus Empiricus as having stated:
1. Nothing exists
2. Even if something exists, nothing can be known about it, and
3. Even if something could be known about it, knowledge about it can't be communicated to others.
Solipsism is generally identified with statement 2 and 3 from Gorgias.

A thought-experiment related to solipsis, although in principle distinct, is the Brain in a Vat. The person performing the thought-experiment considers the possibility that they are trapped within some utterly unknowable reality, much like that illustrated in the movie "The Matrix". A mad scientist could be sending the same impulses to a brain in a vat that a brain understood to be in the "real world" could receive, thereby exactly replicating the world as one knows it. Yet, for the brain in the vat, that world would obviously not be real. This raises the possibility that everything one thinks or knows is illusion. Or at the least that one cannot know, with any certainty, whether one's brain is in the "real world" or in a vat receiving impulses that would create an equivalent consciousness.

Thought similar to solipsism is present in much of eastern philosophy. Taoism and several interpretations of Buddhism, especially Zen, teach that drawing a distinction between self and universe is nonsensical and arbitrary, and merely an artifact of language rather than an inherent reality. Giovanni Gentile postulated a form of solipsism with his own brand of Idealism, which maintained that one's dependent view of reality only existed in so far as it related to the world it created itself into.

Another variation is a sort of materialistic agnosticism, stating simply that nothing outside of one's own thoughts can be absolutely proven to exist; it may all simply be the illusion/imagination/whatever of the thinker.
Wikipedia