American recreational mathematician and science writer [1914- ]
Martin Gardner (1999: 11) characterizes solipsism as:
the insane belief that only one’s self exists. All other parts of the universe, including other people, are unsubstantial figments in the mind of the single person who alone is truly real. It is almost the same as thinking one is god, and so far as I know, there has never been an authentic solipsist outside a mental institution or who in the past was not considered mad.
This echoes Arthur Schopenhauer’s famous comment (1896: 135) that as “a serious conviction” solipsism “could be found only in a madhouse: as such it would then need not so much a refutation as a cure.”
Gardner, Martin (1999) The Whys of a Philosophical Scrivener. St. Martin's Griffin, New York.
Schopenhauer, Arthur (1896) The World as Will and Idea. Book II, Chapter 19. Translated by R.B. Haldane and J. Kemp. London: Kegan Paul, Trench, Trübner & Co.
―How do I know I exist? ―Who’s asking?
Francisco José de Goya y Lucientes (1824-5) Man Looking for Fleas in His Shirt. Carbon black and watercolor on ivory. Museum of Fine Arts, Boston.