Concerning the several degrees of lasting, wherewith ideas are imprinted on the memory, we may observe,—­that some of them have been produced in the understanding by an object affecting the senses once only, and no more than once; others, that have more than once offered themselves to the senses, have yet been little taken notice of:  the mind, either heedless, as in children, or otherwise employed, as in men intent only on one thing; not setting the stamp deep into itself.  And in some, where they are set on with care and repeated impressions, either through the temper of the body, or some other fault, the memory is very weak.  In all these cases, ideas in the mind quickly fade, and often vanish quite out of the understanding, leaving no more footsteps or remaining characters of themselves than shadows do flying over fields of corn, and the mind is as void of them as if they had never been there.

5.  Causes of oblivion.

Thus many of those ideas which were produced in the minds of children, in the beginning of their sensation, (some of which perhaps, as of some pleasures and pains, were before they were born, and others in their infancy,) if in the future course of their lives they are not repeated again, are quite lost, without the least glimpse remaining of them.  This may be observed in those who by some mischance have lost their sight when they were very young; in whom the ideas of colours having been but slightly taken notice of, and ceasing to be repeated, do quite wear out; so that some years after, there is no more notion nor memory of colours left in their minds, than in those of people born blind.  The memory of some men, it is true, is very tenacious, even to a miracle.  But yet there seems to be a constant decay of all our ideas, even of those which are struck deepest, and in minds the most retentive; so that if they be not sometimes renewed, by repeated exercise of the senses, or reflection on those kinds of objects which at first occasioned them, the print wears out, and at last there remains nothing to be seen.  Thus the ideas, as well as children, of our youth, often die before us:  and our minds represent to us those tombs to which we are approaching; where, though the brass and marble remain, yet the inscriptions are effaced by time, and the imagery moulders away.  The pictures drawn in our minds are laid in fading colours; and if not sometimes refreshed, vanish and disappear.  How much the constitution of our bodies are concerned in this; and whether the temper of the brain makes this difference, that in some it retains the characters drawn on it like marble, in others like freestone, and in others little better than sand, I shall here inquire; though it may seem probable that the constitution of the body does sometimes influence the memory, since we oftentimes 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.