These changes may be either objective or subjective. Any sudden alteration of our bodily appearance sensibly impedes the movement of imagination. A patient after a fever, when he first looks in the glass, exclaims, "I don't know myself." More commonly the bodily changes which affect the consciousness of an enduring self are such as involve considerable alterations of cœnæsthesis, or the mass of stable organic sensation. Thus, the loss of a limb, by cutting off a portion of the old sensations through which the organism may be said to be immediately felt, and by introducing new and unfamiliar feelings, will distinctly give a shock to our consciousness of self.
Purely subjective changes, too, or, to speak correctly, such as are known subjectively only, will suffice to disturb the sense of personal unity. Any great moral shock, involving something like a revolution in our recurring emotional experience, seems at the moment to rupture the bond of identity. And even some time after, as I have already remarked, such cataclysms in our mental geology lead to the imaginative thrusting of the old personality away from the new one under the form of a "dead self."[134]
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 anæsthetics. 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.
Enough has, perhaps, been said to show how much of uncertainty and of self-deception enters into the processes of memory. This much-esteemed faculty, valuable and indispensable though it certainly is, can clearly lay no claim to that absolute infallibility which is sometimes said to belong to it. Our individual recollection, left to itself, is liable to a number of illusions even with regard to fairly recent events, and in the case of remote ones it may be said to err habitually and uniformly in a greater or less degree. To speak plainly, we can never be certain on the ground of our personal recollection alone that a distant event happened exactly in the way and at the time that we suppose. Nor does there seem to be any simple way by mere reflection on the contents of our memory of distinguishing what kinds of recollection are likely to be illusory.
How, then, it may be asked, can we ever be certain that we are faithfully recalling the actual events of the past? Given a fairly good, that is, a cultivated memory, it may be said that in the case of very recent events a man may feel certain that, when the conditions of careful attention at the time to what really happened were present, a distinct recollection is substantially correct. Also it is obvious that with respect to all repeated experiences our memories afford practically safe guides. When memory becomes the basis of some item of generalized knowledge, as, for example, of the truth that the pain of indigestion has followed a too copious indulgence in rich food, there is little room for an error of memory properly so called. On the other hand, when an event is not repeated in our experience, but forms a unique link in our personal history, the chances of error increase with the distance of the event; and here the best of us will do well to have resort to a process of verification or, if necessary, of correction.
In order thus to verify the utterances of memory, we must look beyond our own internal mental states to some external facts. Thus, the recollections of our early life may often be tested by letters written by ourselves or our friends at the time, by diaries, and so on. When there is no unerring objective record to be found, we may have recourse to the less satisfactory method of comparing our recollections with those of others. By so doing we may reach a rough average recollection which shall at least be free from any individual error corresponding to that of personal equation in perception. But even thus we cannot be sure of eliminating all error, since there may be a cause of illusion acting on all our minds alike, as, for example, the extraordinary nature of the occurrence, which would pretty certainly lead to a common exaggeration of its magnitude, etc., and since, moreover, this process of comparing recollections affords an opportunity for that reading back a present preconception into the past to which reference has already been made.
The result of our inquiry is less alarming than it looks at first sight. Knowledge is valuable for action, and error is chiefly hurtful in so far as it misdirects conduct. Now, in a general way, we do not need to act upon a recollection of single remote events; our conduct is sufficiently shaped by an accurate recollection of single recent events, together with those bundles of recollections of recurring events and sequences of events which constitute our knowledge of ourselves and our common knowledge of the world about us. Nature has done commendably well in endowing us with the means of cultivating our memories up to this point, and we ought not to blame her for not giving us powers which would only very rarely prove of any appreciable practical service to us.