body problem'. By avoiding the error of introjection we avoid at the same time the idealistic fallacy which leads to solipsism. It is easy to show that introjection is an error. When I see a green meadow the 'green' is declared to be a content of my consciousness, but it certainly is not inside my head. Inside my skull there is nothing but my brain; and if there should happen to be a green spot in my brain, it would obviously not be the green of the meadow, but the green of the brain.
But for our purpose it is not necessary to follow this train of thought; it is sufficient to restate the facts clearly.
It is a fact of experience that all data depend in some way or other upon the state of a certain body which has the peculiarity that its eyes and its back are never seen (except by means of a mirror). It is usually called 'my' body; but here, in order to avoid mistakes, I shall take the liberty of calling it the body 'M'. A particular case of the dependence just mentioned is expressed by the sentence, 'I do not perceive anything unless the sense-organs of the body M are affected'. Or, taking a still more special case, I may make the following statement: 'I feel pain only when the body M is hurt,' (P) I shall refer to this statement as 'proposition P Now let us consider another proposition (Q): 'I can feel only my pain.' (Q)
The sentence Q may be interpreted in various ways. Firstly, it may be regarded as equivalent to P, so that P and Q would just be two different ways of expressing one and the same empirical fact. The word 'can' occurring in Q would denote what we have called 'empirical possibility', and the words 'I' and 'my' would refer to the body M. It is of the utmost importance to realize that in this first interpretation Q is the description of a fact of experience, i.e., a fact which we could very well imagine to be different.
We could easily imagine (here I am closely following ideas expressed by Mr. Wittgenstein) that I experience a pain every time the body of my friend is hurt, that I am gay when his face bears a joyful expression, that I feel tired after he has taken a long walk, or even that I do not see anything when his eyes are closed, and so forth. Proposition Q (if interpreted as being equivalent to P) denies that these things ever happen; but if they
