The other principle, which we can lay down to start with is (_β_) that my knowledge of all the five propositions (_a_) to (_e_), is based, in the last resort, on experiences of mine consisting in the direct apprehension of sensibles and in the perception of relations between directly apprehended sensibles. It is _based_ on these, in at least this sense, that I should never have known any of these propositions if I had never directly apprehended any sensibles nor perceived any relations between them.
What, in view of these two principles, can be the sense in which my five propositions are true?
(1) It seems to me possible that the only _true_ interpretation which can be given to any of them is an interpretation of a kind which I can only indicate rather vaguely as follows: Namely, that all of them express only a kind of fact which we should naturally express by saying that, _if_ certain conditions were fulfilled, I or some other person, _should_ directly apprehend certain other sensibles. For instance the only _true_ thing that can be meant by saying that I really see _coins_ may be some such thing as that, _if_ I were to move my body in certain ways, I should directly apprehend _other_ sensibles, _e.g._ tactual ones, which I should not directly apprehend as a consequence of these movements, if these present visual experiences of mine were mere hallucinations or experiences of "images." Again, the only true thing that can be meant by saying that the upper sides of the coins are _really_ approximately circular may be some such thing as that, _if_ I were looking straight at them, I should directly apprehend circular sensibles. And similarly, the only true interpretation of (_c_) may be some such fact as that, _if_ I were to turn the coins over, or break them up, I _should_ have certain sensations, of a sort I can imagine very well; of (_d_) that _if_ I were at an equal distance from the half-crown and the florin, the sensible, I should then see corresponding to the half-crown would be bigger than that corresponding to the florin, whereas it is now smaller; of (_e_) that, _if,_ when my eyes were closed, they had been open instead, I should have seen certain sensibles.