Even in the normal state the co-ordination is often sufficiently loose to allow several series to coexist separately. We can walk or perform manual work with a vague and intermittent consciousness of the movements, at the same time singing, musing; but if the activity of thought increases, the singing will cease. With many people it is a kind of substitute for intellectual activity, an intermediate state between thinking and not-thinking.
The unity of the ego, in a psychological sense, is, therefore, the cohesion, during a given time, of a certain number of clear states of consciousness, accompanied by others less clear, and by a multitude of physiological states which, without being accompanied by consciousness like the others, yet operate as much as, and even more than, the former. Unity, in fact, means co-ordination. The conclusion to be drawn from the above remarks is namely this, that the consensus of consciousness being subordinate to the consensus of the organism, the problem of the unity of the ego is, in its ultimate form, a biological problem. To biology pertains the task of explaining, if it can, the genesis of organisms and the solidarity of their component parts. Psychological interpretation can only follow in its wake.
Ideas, after being experienced in consciousness, become dormant (conserved as physiological dispositions) and may or may not afterward be reawakened in consciousness as memories. Many such ideas, under conditions with some of which we are all familiar, tend to form part of our voluntary or involuntary memories and many do not. But when such is the case, the memories do not ordinarily include the whole of a given mental experience, but only excerpts or abstracts of it. Hence one reason for the fallibility of human memory and consequent testimony.
Now under special conditions, the ideas making up an experience at any given moment tend to become organized into a system or complex, so that when we later think of the experience or recall any of the ideas belonging to it, the complex as a whole is revived. This is one of the principles underlying the mechanism of memory. Thus it happens that memory may, to a large extent, be made up of complexes. These complexes may be very loosely organized in that the elementary ideas are weakly bound together, in which case, when we try to recall the original experience, only a part of it is recalled. Or a complex may be very strongly organized, owing to the conditions under which it is formed, and then a large part of the experience can be recalled. In this case, any idea associated with some element in the complex may, by the law of association, revive the whole original complex. If, for instance, we have gone through a railroad accident involving exciting incidents, loss of life, etc., the words "railroad," "accident," "death," or a sudden crashing sound, or the sight of blood, or even riding in a railroad train may recall the experience from beginning to end, or at least the prominent features in it, i.e., so much as was organized. The memory of the greater part of this experience is well organized, while the earlier events and those succeeding the accident may have passed out of all possibility of voluntary recall.