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Consider for awhile, with me, how much is, or may come to be, involved in this thought, viz. that external facts are of their very essence facts which exist for other conscious beings besides ourselves. One conscious being may have in his mind any content you please, however fleeting, incommunicable, ineffable, or insane. But if he is to regard given facts which are represented in his own experience as existent also for beings external to himself, his notion of what his social relations to these beings are, will inevitably determine in some measure, and perhaps very deeply, the sorts of facts which he can regard as thus verifiably objective and common in their character. For the facts that can be common to the experience of many beings can only be distinguished from the facts that are peculiar to the inner life of the isolated individual, through an application of the very important criterion expressible by saying that the common, the external facts, must be such as are apt material for social communication, for description, for definite imitative characterization. For instance, to take the case of visual perception in space, if I am to become sure that I see what you see, there must be some way in which you and I can agree as to the whereabouts of the object of our common perception. And this, the conditio sine qua non of our social communications regarding the objects of our common sense perceptions, may itself be expressed as a principle determining a priori the nature of whatever experiences you and I are to learn to regard as standing for external things.

Every external thing must occupy, at any moment, one exactly definable portion of space; this is a simple axiom, but it is by no means self-evident. It is true a priori, but it is true of external, not of internal things. There is no contradiction for the inner life, in our having an experience which involves elements of space perception, but which is essentially vague and undetermined as to these, its spatial characters. An internal bodily pain or pleasure of the massive type may be dimly localized, so that for consciousness it appears as having