The imprint of an object is not absolutely permanent and is probably never quite true. It begins to lose sharpness at once, but if the object be frequently observed and much remembered, it will retain its general character for years. The exercise of memory, instead of wearing out the imprint as would be the case with a material negative or engraved plate, keeps the channels open[6]. Persons of little experience remember well, for their energy of attention is not distributed over many different ideas; it travels continuously round a small circuit. One hears ignorant persons recounting events that happened years ago, with as much detail and with almost as much sentiment as if they had taken place the day before. A 'good memory' is no proof that the quality of mind or thought is good. All experience is not worth remembering. One of the most difficult things in moral culture is to get rid of the imprints of ideas that are out of harmony with our improved sentiment. Although the imprints in our mind may close up and leave scarce a cicatrice, the part that has been once disturbed is never the same as the virgin plasm. It remains a little more tender. It may not reopen to ordinary stimuli, but an extra agitation of the plasm will rip up the closed furrows, and give us back scenes in our life that had long ceased to be recollected. A great agitation in all parts of the mind may revive what appears to be the whole of our past experience in a simultaneous recollection. So I explain the extraordinary lucidity that sometimes occurs in fevers and in moments of extreme terror. It is also conceivable that the egoistic energy may be so strong as to destroy outright the moulds of thought, as a flood sweeps away the banks of a river. 'We sometimes find a disease quite strip the mind of all its ideas, and the flames of a fever in a few days calcine all those images to dust and confusion which seemed to be as lasting as if graved in marble'[7]. What is called 'decay of the mind' in old age is merely the loss of the plasmic images. Since intellect would not have been formed in the first instance if it had not been wanted, it is to be expected that it will fade out of the mind when it is no longer wanted. So far as the realistic intellect is concerned, we return to 'second childhood' and the uniform sensibility we had at birth. No philosophy but the substantial explains memory. Idealists and metaphysicians, who recognise only consciousness, are utterly unable to account for the revival of a shadowy sort of objects in the absence of their original causes. Here is the melancholy confession of John Stuart Mill on the subject:--