Solipsism (from the Latin ipse = "self" and solus = "alone") is the metaphysical belief that only oneself 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. This view is first recorded with the presocratic sophist Gorgias (c. 483-375 BC) who is quoted by Sextus Empiricus as having stated:
- Nothing exists
- Even if something exists, nothing can be known about it, and
- 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 solipsism, although in principle distinct, is the Brain in a Vat; i.e., the view that "I" may be trapped within some utterly unknowable reality, so that everything one thinks, one knows is illusion.
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.
The classic objection to solipsism is that people die. However, you have not died, and therefore you have not disproved it. This objection is also vulnerable to the criticism that one cannot say whether the mind lives on after death or not; hence, the theory is not disproven, because someone could still believe themselves to exist, even after death. Death, or someone killing the person, can also be seen as figments of the imagination - they may not have died at all!
A further objection is that life causes pain. Why would we create pain for ourselves? One response to this is that there may be some reason which we have decided to forget on purpose, such as the law of Karma, or a desire not to be bored. Another response is that the category of 'pain' is a conditioned perception originating from socio-cultural human value systems which are not necessarily universally valid. A solipsistic value system may not recognise pain, or the alleged fact of personal death, as real.
Another objection is that the practical solipsist needs a language to formulate his thoughts about solipsism. And language is an essential tool to communicate with other minds. Why does a solipsist universe need a language? Possible responses are similar to the last objection; that is, to keep from becoming bored, perhaps the solipsist imagines "other" minds, which would actually be only elements of his own mind, and which he has chosen to forget control of for the time being, inventing a language so as to interact with these more isolated segments of his mind.
Nearly all objections can be dispensed with by an appeal to the solipsist's free will.
The solipsist's universe may be divided into two parts: that part controlled by their conscious mind, and the part controlled by their unconscious. They will find that the unconscious part of their universe behaves with the same complexity as it would if it was external; i.e., not part of their self at all (realism). The distinction between the realist universe and the unconscious universe collapses when one notes that external and unconscious are simply two different words used to describe the same events occurring outside of conscious control. This leads to the conclusion that the unconscious is, for all practical purposes, someone else.
Thus, considering the external universe to actually be one's unconscious mind may be treated linguistically as a semantic distinction. It makes little difference whether one claims their body and the external universe comprises all of reality, or claims their conscious mind and their unconscious mind comprise their self as a whole. The claim that "only" oneself exists is a truism; "oneself" is the entire universe.
The only meaningful conclusion which may be drawn from this is that of Arthur Schopenhauer: Will is the otherwise unreachable external reality.