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