“... The fallacious use of the principle that you cannot look into a man's mind. It is said you cannot do that: therefore what follows? It is said that you are to have fixed rules to tell you that he must have meant something, one way or the other, when certain exterior phenomena arise. The answer is that there is no such thing as an absolute criterion which gives you certain index to a man's mind. There is nothing outside his mind which is an absolute indication of what is going on inside. So far from saying that you cannot look into a man's mind, you must look into it, if you are going to find fraud against him: and unless you think you see what must have been in his mind, you cannot find him guilty of fraud.”

Source: Guide for the Perplexed (c. 1190), Part I, p.106 (1881) Tr. Friedlander

Source: Blood Bound

21 September 1830
Table Talk (1821–1834)

Source: Short fiction, Home is the Hangman (1975), p. 138

Variant translations: Are you really sure that a floor can't also be a ceiling?
I can't keep from fooling around with our irrefutable certainties. It is, for example, a pleasure knowingly to mix up two and three dimensionalities, flat and spatial, and to make fun of gravity.
1950's, On Being a Graphic Artist', 1953
Context: In my prints I try to show that we live in a beautiful and orderly world and not in a chaos without norms, as we sometimes seem to. My subjects are also often playful. I cannot help mocking all our unwavering certainties. It is, for example, great fun deliberately to confuse two and three dimensions, the plane and space, or to poke fun at gravity. Are you sure that a floor cannot also be a ceiling? Are you absolutely certain that you go up when you walk up a staircase? Can you be definite that it is impossible to eat your cake and have it?

Source: After Many a Summer Dies the Swan

Mind
Source: I am That, P.121.

McKenna interview (1992)