Thus, we ask: When was Julius Caesar born? We are given a year and a day. How is the time which has elapsed since measured? By changes in the physical world, by revolutions of the earth about the sun. We ask: When did he conceive the plan of writing his Commentaries? If we get an answer at all, it must be an answer of the same kind--some point in the series of physical changes which occur in real time must be indicated. Where else should we look for an answer? In point of fact, we never do look elsewhere. Again. We have distinguished between apparent space and real space (section 34). We have seen that, when we deny that a mental image can occupy any portion of space, we need not think of it as losing its parts and shrivelling to a point. We may still attribute to it apparent space; may affirm that it seems extended. Let us mark the same distinction when we consider time. The psychologist speaks of the duration of a sensation. Has it real duration? It is not in time at all, and, of course, it cannot, strictly speaking, occupy a portion of time. But we can try to measure the duration of the physical concomitant, and call this the real duration of the sensation. We all distinguish between the real time of mental phenomena, in the sense indicated just above, and the apparent time. We know very well that the one may give us no true measure of the other. A sermon _seems_ long; was it _really_ long? There is only one way of measuring its real length. We must refer to the clock, to the sun, to some change in the physical world. We _seem_ to live years in a dream; was the dream _really_ a long one? The real length can only be determined, if at all, by a physical reference. Those apparent years of the dream have no place in the real time which is measured by the clock. We do not have to cut it and insert them somewhere. They belong to a different order, and cannot be inserted any more than the thought of a patch can be inserted in a rent in a real coat. We see, thus, when we reflect upon the matter, that mental phenomena cannot, strictly speaking, be said to have a time and place. He who attributes these to them materializes them. But their physical concomitants have a time and place, and mental phenomena can be ordered by a reference to these. They can be assigned a time and place of existing in a special sense of the words not to be confounded with the sense in which we use them when we speak of the time and place of material things. This makes it possible to relate every mental phenomenon to the world system in a definite way, and to distinguish it clearly from every other, however similar.