The subject is a vast one; one could include in it aphasia and all the weaknesses resulting from the destruction of the brain-centres, whether those of motion or of sensation. M. Sollier has taken the pains to reduce it, however, to definite limits. He studies especially the subject of acquired amnesia (diminutive changes and disaggregations of the memory). He does not consider the subject of congenital amnesia, which is an absence and not a loss of memory. Strictly considered, the only cases of true amnesia, or _organic_ amnesia, are those which result from the destruction of the nerve-centres, since in this case there is an absolute loss of the power of forming mental images, and not simply an enfeebling or forgetting of them, which is characteristic of _functional_ amnesia. From a clinical standpoint amnesia exists only in the last case. Though the effect may be the same in both cases, the causes are not identical. Clinical investigation cannot, however, overlook the diminutive changes which take place in the memory and which, as early as 1817, were called by Louyer-Villermay dysmnesia, and which are always closely allied to organic modifications of the brain. As regards amnesia itself, it is important to distinguish simple amnesia from retrogressive and progressive amnesia. M. Sollier explains the motive causes of these different conditions with great lucidity, and renders them easy of comprehension by means of ingenious illustrations. I call attention to the information he gives us as to the conditions under which a revival of mental images takes place, p. 30; to his criticism of Ribot’s opinion, according to which the power of correctly locating events in the scale of time is the true characteristic of psychical memory: it is quite enough if it reproduces events as in the past, that is to say if there exists a conscious knowledge which shows that the mental conception belongs to the past, or is, simply, a remembrance, p. 35 and 40; to his remarks on the strengthening of mental images due to the repetition of remembrances, the necessary sequence of which is that a weakening of old memories follows the destruction of accumulated mental images, p. 48; to his explanation of the processes of retrogressive amnesia (coming suddenly after an attack of vertigo, a blow, etc.) which he bases upon a supposition of a group of mental conceptions in touch with one another, in such a way that the loss of one leading conception in a group deprives this group of sufficient consistency to form a conscious synthesis, p. 70.