Solipsism

Solipsism is the position in Metaphysics and Epistemology that the mind is the only thing that can be known to exist and that knowledge of anything outside the mind is unjustified. It is a skeptical hypothesis, and leads to the belief that the whole of reality and the external world and other people are merely representations of the individual self, having no independent existence of their own, and might in fact not even exist. It is not, however, the same as Skepticism (the epistemological position that one should refrain from even making truth claims).

Solipsism is therefore a pure variety of Idealism (more specifically Subjective Idealism or Subjectivism), and is opposed to concepts such as Materialism, Physicalism and Objectivism which hold that the only thing that can be truly proven to exist is matter.

The central assertion of Solipsism rests on the lack of a solid proof of the existence of the external world, and Strong Solipsism (as opposed to Weak Solipsism) asserts that no such proof can be made.

It is often considered a bankrupt philosophy, or at best bizarre and unlikely. Critics have argued that the very idea of communicating philosophical ideas would be entirely pointless to a true solipsist as, according to them, there is no other mind with whom they would communicate their beliefs. It also goes against the commonly observed tendency for sane adult humans in the western world to interpret the world as external and existing independently of themselves.

Positions somewhat similar to Solipsism are present in much of Eastern Philosophy, particularly in Taoism, several interpretations of Buddhism (especially Zen), and some Hindu models of reality.

The origins of Solipsism in Western Philosophy lie with the Greek Pre-Socratic Sophist Gorgias who claimed that: 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 cannot be communicated to others. While to some extent merely an ironic refutation and parody of the position of Parmenides and the Eleatic philosophers (that all being is one), Gorgias nevertheless captured at least the spirit of Solipsism.

Solipsism also lies at the heart of Descartes' view that the individual understands all psychological concepts (thinking, willing, perceiving, etc) by analogy with his or her own mental states (i.e. by abstraction from inner experience). Descartes' method of Cartesian Skepticism led him to doubt the existence of the world he perceived, and in his famous formulation "Cogito Ergo Sum" ("I think therefore I am") he retreated to the only thing he could not doubt, his own conscious self.

The Idealist philosopher George Berkeley argued that physical objects do not exist independently of the mind that perceives them, and that an item truly exists only so long as it is observed (otherwise it is not only meaningless but simply non-existent). Berkeley, however, further argued that there must also be an all-encompassing Mind (or God), so his position is not one of pure Solipsism.

• Metaphysical Solipsism is a type of Idealism which maintains that the individual self of an individual is the whole of reality, and that the external world and other persons are representations of that self and have no independent existence.
• Epistemological Solipsism is a type of Idealism according to which only the directly accessible mental contents of an individual can be known. The existence of an external world is regarded as an unresolvable question or an unnecessary hypothesis, rather than actually false.
• Methodological Solipsism is the epistemological thesis that the individual self and its mental states are the sole possible or proper starting point for philosophical construction. Therefore, all other truths must be founded on indisputable facts about an individual's own consciousness, and someone's beliefs about, say, water have absolutely nothing to do with the substance water in the outside world, but are determined internally.