manner, that is, by means of vibrations, it is difficult to conceive of its being produced by any other means than the accidental justling of these one against the other, which is what is meant by association.
There are two or three general observations which will be of use in conducting us through this inquiry. In the first place it appears to me certain that every impression or idea is produced in such a manner as to affect or be perceived by the whole brain at once, or in immediate succession, that is, before the impression ceases. For if we suppose a certain degree of resemblance to subsist between two ideas, the perception of the one will always be sure to excite a recollection of the other, if it is at all worth remembering. I mean for instance if any one should in some strange place suddenly see an excellent picture of an old and beloved friend, there can hardly be a doubt that the picture would call up the memory of the person whom it resembled with an instantaneous and irresistible force. Now this could not always happen but on the supposition that the visible impression of the picture was conveyed to every part of the brain, as otherwise it must be a mere accident whether it would ever come in contact with that part of it, where that correspondent image was lodged which it was calculated to excite, It is evident that the force with which the impression of the picture acts upon the mind is subsequent to the recollection of the likeness and not the cause of it, since the picture of any other person would act physically upon the mind in the same manner. It may be worth remarking here that the strength, or habitual or recent recurrence of any idea makes it more easily recollected. I might see a picture of a person whom I had not often seen and whose face did not at all interest me at the time, without recollecting whose it was, though the likeness
