§ 5. Two points should be carefully noted if we wish to avoid serious misapprehension. It might be objected, by a disciple of Kant or of Mill, that a thing may be real without ever being given as actual psychical fact in immediate apprehension, so long as its nature is such that it _would_ be psychical fact under known and specified conditions. Many, if not most, of the objects of scientific knowledge, it may be said, are of this kind; they have never entered, possibly never will enter, into the contents of any man’s direct apprehension, yet we rightly call them real, in the sense that they would be apprehended under certain known conditions. Thus I have never seen, and do not expect that any one ever will see, the centre of the earth, or, to take a still stronger case, no one has ever seen his own brain. Yet I call the centre of the earth or my own brain real, in the sense that _if_ I could, without ceasing to live, penetrate to a certain depth below the soil, I _should_ find the centre of the earth; _if_ an opening were made in my skull, and a suitable arrangement of mirrors devised, I _should_ see the reflection of my own brain. A comet may be rushing through unpeopled space entirely unbeheld; yet it does not cease for all that to be real, for _if_ I were there I _should_ see it, and so forth. Hence the Kantian will tell us that reality is constituted by relation to _possible_ experience; the follower of Mill, that it means “a permanent _possibility_ of sensation.” Now, there is, of course, an element of truth in these arguments. It is true that what immediately enters into the course of my own direct perception is but a fragment of the full reality of the universe. It is true, again, that there is much which in its own nature is capable of being perceived by human beings, but will, as far as we can judge, never be perceived, owing to the physical impossibility of placing ourselves under the conditions requisite for perception; there are other things which could only be perceived if some modification could be effected in the structure of our perceptive organs. And it may therefore be quite sufficient for the purposes of some sciences to define these unperceived realities as “possibilities of sensation,” processes which we do not perceive but might perceive under known or knowable conditions. But the definition, it will be seen, is a purely negative one; it takes note of the fact that we do not actually perceive certain things, without telling us anything positive as to their nature. In Metaphysics, where we are concerned to discover the very meaning of reality, we cannot avoid asking whether such a purely negative account of the reality of the greater part of the universe is finally satisfactory. And we can easily see that it is not. For what do we mean when we talk of the “possible”? Not simply “that which is not actual,” for this includes the merely imaginary and the demonstrably impossible. The events of next