the use of words, and of objects and incidents, as signs and symbols, with the proper method of reasoning upon them, to learn; and during their noviciate in these things their memories must labour under great imperfections. It appears also, that the imperfections peculiar to children correspond in kind as well as degree to the reasons here assigned for them. Their not being able to digest past facts in order of time is, in great measure, owing to their not having the proper use of the symbols, whereby time is denoted.
Seventhly, The peculiar imperfection of the memory in aged persons tallies also with the foregoing account. The vibrations, and dispositions to vibrate, in the small medullary particles, and their associations, are all so fixed by the callosity of the medullary substance, and by repeated impressions and recurrences, that new impressions can scarce enter, that they recur seldom, and that the parts which do recur bring in old trains from established associations, instead of continuing those which were lately impressed. Hence one may almost predict what very old persons will say or do upon common occurrences. Which is also the case frequently with persons of strong passions, for reasons that are not very unlike. When old persons relate the incidents of their youth with great precision, it is rather owing to the memory of many preceding memories, recollections, and relations, than to the memory of the thing itself.
Eighthly, In recovering from concussions, and other disorders of the brain, it is usual for the patient to recover the power of remembering the then present common incidents for minutes, hours, and days, by degrees; also the power of recalling the events of his life preceding his illness. At length he recovers this last power perfectly, and at the same time forgets almost all that passed in his illness, even those things which he remembered, at first, for a day or two. Now the reason of this I take to be, that upon a perfect recovery the brain recovers its natural state, i.e. all its former dispositions to vibrate; but that such as took place during the preternatural state of the brain, i.e. during his illness, are all obliterated by the return of the natural state. In like manner dreams, which happen in a peculiar state of the brain, i.e. in sleep, vanish, as soon as vigilance, a different state, takes place. But if they be recollected immediately upon waking, and thus connected with the state of vigilance, they may be remembered. But I shall have occasion to be more explicit on this head in the next Section.
Ninthly, It is very difficult to make any plausible conjectures why some persons of very weak judgments, not much below idiots, are endued with a peculiar extraordinary memory. This memory is generally the power of recollecting a large group of words, suppose, as those of a sermon, in a short time after they are heard, with wonderful exactness and readiness; but then the whole is obliterated, after a longer time, much more completely
