From G.E. Moore's 'Proof of an External World':
Consider any kind of thing, such that anything of that kind, if there is anything of it, must be 'to be met with in space': e.g. consider the kind 'soap-bubble.' If I say of anything which I am perceiving, 'That is a soap-bubble', I am, it seems to me, certainly implying that there would be no contradiction in asserting that it existed before I perceived it and that it will continue to exist, even if I cease to perceive it. This seems to me to be part of what is meant by saying that it is a real soap-bubble, as distinguished, for instance, from an hallucination of a soap-bubble. Of course, it by no means follows, that if it really is a soap-bubble, it did in fact exist before I perceived it or will continue to exist after I perceive it: soap-bubbles are an example of a kind of 'physical object' and 'thing to be met with in space,' in the case of which it is notorious that particular specimens of the kind often do exist only so long as they are perceived by a particular person. But a thing which I perceive would not be a soap-bubble unless its existence at any given time were logically independent of my perception of it at that time; unless, that is to say, from the proposition, with regard to a particular time, that it existed at that time, it never follows that I perceived it at that time. . . . That is to say, from the proposition with regard to anything which I am perceiving that it is a soap-bubble, there follows the proposition that it is external to my mind. But if, when I say that anything which I perceive is a soap-bubble, I am implying that it is external to my mind, I am, I think, certainly also implying that it is also external to all other minds. . . . I think, therefore, that from any proposition of the form "There's a soap-bubble!' there really does follow the proposition 'There's an external object!' 'There's an object external to all our minds!' (144-45)
