Our sense of personal identity may be said to be rooted in that special side of the mnemonic process which consists in the linking of all sequent events together by means of a thread of common consciousness. It is closely connected with that smooth, gliding movement of imagination which appears to involve some more or less distinct consciousness of the uniting thread of similarity. And so long as this movement is possible, so long, that is to say, as retrospective imagination detects the common element, which we may specifically call the recurring consciousness of self, so long is there the undisturbed assurance of personal identity. Nay, more, even when such a recognition might seem to be difficult, if not impossible, as in linking together the very unlike selves, viewed both on their objective and subjective sides, of childhood, youth, and mature life, the mind manages, as we have seen, to feign to itself a sufficient amount of such similarity. But this process of linking stage to stage, of discerning the common or the recurring amid the changing and the evanescent, has its limits. Every great and sudden change in our experience tends, momentarily at least, to hinder the smooth reflux of imagination. It makes too sharp a break in our conscious life, so that imagination is incapable of spanning the gap and realizing the then and the now as parts of a connected continuous tissue.[133] 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 coenaesthesis, 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]