=Nature of Memory.=--Mention has been made of the retentive power of the nervous system, and of a consequent tendency for mental images to revive, or _re-present_, themselves in consciousness. It must now be noted that such a re-presentation of former experiences is frequently accompanied with a distinct recognition that the present image or images have a definite reference to past time. In other words, the present mental fact is able to be placed in the midst of other events believed to make up some portion of our past experience. Such an ideal revival of a past experience, together with a recognition of the fact that it formerly occurred within our experience, is known as an act of memory. =Neural Conditions of Memory.=--When any experience is thus reproduced, and recognized as a reproduction of a previous experience, there is physiologically a transmission of nervous energy through the same brain centres as were involved in the original experience. The mental reproduction of any image is conditioned, therefore, by the physical reproduction of a nervous impulse through a formerly established path. That this is possible is owing to the susceptibility of nervous tissue to take on habit, or to retain as permanent modifications, all impressions received. From this it is evident that when we say we retain certain facts in our mind, the statement is not in a sense true; for there is no knowledge stored up in consciousness as so many ideas. The statement is true, therefore, only in the sense that the mind is able to bring into consciousness a former experience by reinstating the necessary nervous impulses through the proper nervous arcs. What is actually retained, however, is the tendency to reinstate nervous movements through the same paths as were involved in the original experience. Although, therefore, retention is usually treated as a factor in memory, its basis is, in reality, physiological.