of those processes obtaining clear recognition in consciousness at any one time: the remainder affect the clearest portion more or less, but exist themselves only dimly in the marginal region to which I am not attending. Again, we must not suppose that in ascribing to the mental state active properties we mean to imply that the mental states could exist or manifest those properties apart from the physical processes which form their basis, or that they can act upon them in any way from outside, as the older psychology supposed. I do not think, as most psychologists do, that this notion is inconsistent with the doctrine of parallelism, but it certainly can not be derived from it, or from the facts upon which it rests. Guarding against these two errors, then, we may justly regard the mental state as an active, dynamic thing, subject to laws and possessed of properties into which it is the business of the psychologist to inquire.
We all know the difference between red as seen and red as we think it, although the difference is hard to describe. In most persons the sensation red is peculiarly intense and vivid, while the idea is lacking in some indescribable way in both these traits. Now, we have reason to believe that both mental states are of the same general kind, and that the idea is capable of passing into a state indistinguishable from the sensation. Such a transition is known as development. In some persons certain ideas are normally already developed, so to speak, to their maximum degree. A friend of mine tells me that, so far as vividness and intensity go, it makes little difference to him whether his eyes are open or shut—what he sees is about the same in either case. But more commonly the idea must be much heightened before it reaches sensational intensity. Another friend of mine by thinking intently of a friend's appearance can see that friend slowly taking shape, at first as a shadowy outline, then gaining in clearness and solidity until the shadowy outline has become the perfect form of a real person. Now, if we regard the dimmest idea as zero and the clear sensation as the maximum, we may say that any mental state may conceivably run through all the intervening grades, and we have reason to believe that every mental state tends to run through some grades. This I would express by saying that every mental state tends to develop within limits which we can not at present assign. The first property of the mental state, then, is that of development. It is of importance in explaining the phenomena of dreams and hallucinations.
The second group of properties which I shall mention depend upon the transmissibility of the physical impulse. I have already shown in my first paper how mental elements become agglutinated into systems in which any one tends to awaken the others. It is also true of the relation of any one system to another; they tend
