We see, then, that the failure of our ordinary assurance of personal identity is due to the recognition of difference without similarity.  It arises from an act of memory—­for the mind must still be able to recall the past, dimly at least—­but from a memory which misses its habitual support in a recognized element of constancy.  If there is no memory, that is to say, if the past is a complete blank, the mind simply feels a rupture of identity without any transformation of self.  This is our condition on awaking from a perfectly forgotten period of sleep, or from a perfectly unconscious state (if such is possible) when induced by anaesthetics.  Such gaps are, as we have seen, easily filled up, and the sense of identity restored by a kind of retrospective “skipping.”  On the other hand, the confusion which arises from too great and violent a transformation of our remembered experiences is much less easily corrected.  As long as the recollection of the old feelings remains, and with this the sense of violent contrast between the old and the new ones, so long will the illusion of two sundered selves tend to recur.

The full development of this process of imaginative fission or cleavage of self is to be met with in mental disease.  The beginnings of such disease, accompanied as they commonly are with disturbances of bodily sensations and the recurring emotions, illustrate in a very interesting way the dependence of the recognition of self on a certain degree of uniformity in the contents of consciousness.  The patient, when first aware of these changes, is perplexed, and often regards the new feelings as making up another self, a foreign Tu, as distinguished from the familiar Ego.  And sometimes he expresses the relation between the old and the new self in fantastic ways, as when he imagines the former to be under the power of some foreign personality.

When the change is complete, the patient is apt to think of his former self as detached from his present, and of his previous life as a kind of unreal dream; and this fading away of the past into shadowy unreal forms has, as its result, a curious aberration in the sense of time.  Thus, it is said that a patient, after being in an asylum only one day, will declare that he has been there a year, five years, and even ten years.[135] This confusion as to self naturally becomes the starting-point of illusions of perception; the transformation of self seeming to require as its logical correlative (for there is a crude logic even in mental disease) a transformation of the environment.  When the disease is fully developed under the particular form of monomania, the recollection of the former normal self commonly disappears altogether, or fades away into a dim image of some perfectly separate personality.  A new ego is now fully substituted for the old.  In other and more violent forms of disease (dementia) the power of connecting the past and present may disappear altogether, and nothing but the disjecta membra of an ego remain.