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.