Solipsism is the idea that a person's mind is the only thing that actually exists. It is a philosophical argument that maintains reality is based on the perceptions of one's mind, and therefore nothing really exists except for that perceptual reality of one's mind. Solipsism was first introduced by the Greek philosopher Gorgias and perpetuated to a great degree by Descartes, who emphasized the importance of finding the origins of knowledge and understanding through epistemology. This led to epistemological solipsism, in which only the knowledge and understanding of the individual philosopher can be known.
The general thought behind solipsism is that everything one sees, does, or feels is simply an extension of one's subconscious. It is a belief that everything in reality takes place within one's own mind, and therefore there are no actual outside forces acting upon the self. This philosophy calls into question the very essence of existence, and it also contends that existence and experience reside only in the mind and therefore existence may not be real at all. This relates most closely to metaphysical solipsism, a philosophical argument in which a person is the only thing existence and everything else is just a projection of that person's mental state.
Methodological solipsism is the idea that all philosophical arguments must be built from the individual's knowledge and understanding of reality. It contends that the self is the only proper starting point for building such arguments. However, there are several counter-arguments to such a philosophy, which include the certainty of death, the fact that life can be unpleasant and it would seem antithetical for one to create a reality that is painful and imperfect, and a person's inability to imitate another person's skills.
Related only tenuously to the philosophy of solipsism is solipsism syndrome, which is a dissociative psychiatric condition that causes the subject to believe all reality is internal and everything outside of them does not exist or exists simply as an ethereal or dreamlike state. People experiencing this syndrome often feel a detachment from reality, apathy, indifference, and intense loneliness, which can be dangerous and lead to other serious or life-threatening mental conditions. Astronauts living in space for long periods of time have experienced this syndrome, and it is believed that infants experience this type of world-view until they are old enough to grow out of it.