It may, however, be asked: Is there any reason to suppose that they ever do? And the reason, which weighs with me most, is one which applies, I think, to a certain class of sensibles _only_; a class which I will try to define by saying that it consists of those which _would_ (under certain conditions which actually exist) be experienced in a _sensation proper, if only_ a living body, having a certain constitution, existed under those conditions in a position in which no such body does actually exist. I think it is very probable that this definition does not define at all accurately the kind of sensibles I mean; but I think that what the definition aims at will become clearer when I proceed to give my reasons for supposing that sensibles, of a kind to be defined in _some_ such way, do exist unexperienced. The reason is simply that, in Hume's phrase, I have "a strong propensity to believe" that, _e.g.,_ the visual sensibles which I directly apprehend in looking at this paper, still exist unchanged when I merely alter the position of my body by turning away my head or closing my eyes, _provided_ that the physical conditions outside my body remain unchanged. In such a case it is certainly true in some sense that I _should_ see sensibles like what I saw the moment before, _if only_ my head were still in the position it was at that moment or my eyes unclosed. But if, in such a case, there is reason to think that sensibles which I should see, if the position of my body were altered, exist in spite of the fact that I do not experience them, there is, I think, an equal reason to suppose it in other cases. We must, for instance, suppose that the sensibles which I should see now, if I were at the other end of the room, or if I were looking under the table, exist at this moment, though they are not being experienced. And similarly we must suppose that the sensibles which _you_ would see, if you were in the position in which I am now, exist at this moment, in spite of the fact that they may be more or less different from those which I see, owing to the different constitution of our bodies. All this implies of course, that a vast number of sensibles exist at any moment, which are not being experienced at all. But still it implies this only with regard to sensibles of a strictly limited class, namely sensibles which would be experienced _in a sensation proper,_ if a body, having a certain constitution, were in a position in which it is not, under the given physical conditions. It does not, for instance, imply that any _images,_ of which it may be true that I _should_ have them, under present physical conditions, if the position of my body were altered, exist now; nor does it imply that sensibles which _would_ be experienced by me now in a sensation proper, if the physical conditions external to my body were different from what they are, exist now.