Modern psychology recognizes no such pigeonhole apparatus in unconscious mind. On the purely psychical side, memory is nothing but an occasional reappearance of a past mental experience. And the sole mental conditions of this reappearance are to be found in the circumstances of the moment of the original experience and in those of the moment of the reappearance.
Among these are to be specially noted, first of all, the degree of impressiveness of the original experience, that is to say, the amount of interest it awakened and of attention it excited. The more impressive any experience, the greater the chances of its subsequent revival. Moreover, the absence of impressiveness in the original experience may be made good either by a repetition of the actual experience or, in the case of non-recurring experiences, by the fact of previous mnemonic revivals.
In the second place, the pre-existing mental states at the time of revival are essential conditions. It is now known that every recollection is determined by some link of association, that every mnemonic image presents itself in consciousness only when it has been preceded by some other mental state, presentative or representative, which is related to the image. This relation may be one of contiguity, that is to say, the original experiences may have occurred at the same time or in close succession; or one of similarity (partial and not amounting to identity), as where the sight of one place or person recalls that of another place or person. Finally, it is to be observed that recollection is often an act, in the full sense of that term, involving an effort of voluntary attention at the moment of revival.
Modern physiology has done much towards helping us to understand the nervous conditions of memory. The biologist regards memory as a special phase of a universal property of organic structure, namely, modifiability by the exercise of function, or the survival after any particular kind of activity of a disposition to act again in that particular way. The revival of a mental impression in the weaker form of an image is thus, on its physical side, due in part to this remaining functional disposition in the central nervous tracts concerned. And so, while on the psychical or subjective side we are unable to find anything permanent in memory, on the physical or objective side we do find such a permanent substratum.
With respect to the special conditions of mnemonic revival at any time, physiology is less explicit. In a general way, it informs us that such a reinstatement of the past is determined by the existence of certain connections between the nervous structures concerned in the reviving and revived mental elements. Thus, it is said that when the sound of a name calls up in the mind a visual image of a person seen some time since, it is because connections have been formed between particular regions and modes of activity of the auditory and the visual centres. And it is supposed that the existence of such connections is somehow due to the fact that the two regions acted simultaneously in the first instance, when the sight of the person was accompanied by the hearing of his name. In other words, the centres, as a whole, will tend to act at any future moment in the same complex way in which they have acted in past moments.
All this is valuable hypothesis so far as it goes, though it plainly leaves much unaccounted for. As to why this reinstatement of a total cerebral pulsation in consequence of the re-excitation of a portion of the same should be accompanied by the specific mode of consciousness which we call recollection of something past, it is perhaps unreasonable to ask of physiology any sort of explanation.[114]
Thus far as to the general or essential characteristics of memory on its mental and its bodily side. But what we commonly mean by memory is, on its psychical side at least, much more than this. We do not say that we properly recollect a thing unless we are able to refer it to some more or less clearly defined region of the past, and to localize it in the succession of experiences making up our mental image of the past. In other words, though we may speak of an imperfect kind of recollection where this definite reference is wanting, we mean by a perfect form of memory something which includes this reference.
Without entering just now upon a full analysis of what this reference to a particular region of the past means, I may observe that it takes place by help of an habitual retracing of the past, or certain portions of it, that is to say, a regressive movement of the imagination along the lines of our actual experience. Setting out from the present moment, I can move regressively to the preceding state of consciousness, to the penultimate, and so on. The fact that each distinct mental state is continuous with the preceding and the succeeding, and in a certain sense overlaps these, makes any portion of our experience essentially a succession of states of consciousness, involving some rudimentary idea of time. And thus, whether I anticipate a future event or recall a past one, my imagination, setting out from the present moment, constructs a sequence of experiences of which the one particularly dwelt on is the other term or boundary. And our idea of the position of this last in time, like that of an object in space, is one of a relation to our present position, and is determined by the length of the sequence of experiences thus run over by the imagination.[115] It may be added that since the imagination can much more easily follow the actual order of experience than conceive it as reversed, the retrospective act of memory naturally tends to complete itself by a return movement forwards from the remembered event to the present moment.
In practice this detailed retracing of successive moments of mental life is confined to very recent experiences. If I try to localize in time a remote event, I am content with placing it in relation to a series of prominent events or landmarks which serves me as a rough scheme of the past. The formation of such a mnemonic framework is largely due to the needs of social converse, which proceeds by help of a common standard of reference. This standard is supplied by those objective, that is to say, commonly experienced regularities of succession which constitute the natural and artificial divisions of the years, seasons, months, weeks, etc. The habit of recurring to these fixed divisional points of the past renders a return of imagination to any one of them more and more easy. A man has a definite idea of "a year ago" which the child wants, just because he has had so frequently to execute that vague regressive movement by which the idea arises. And though, as our actual point in time moves forward, the relative position of any given landmark is continually changing, the change easily adapts itself to that scheme of time-divisions which holds good for any present point.
Few of our recollections of remote events involve a definite reference to this system of landmarks. The recollections of early life are, in the case of most people, so far as they depend on individual memory, very vaguely and imperfectly localized. And many recent experiences which are said to be half forgotten, are not referred to any clearly assignable position in time. One may say that in average cases definite localization characterizes only such supremely interesting personal experiences as spontaneously recur again and again to the mind. For the rest it is confined to those facts and events of general interest to which our social habits lead us repeatedly to go back.[116]