The account which Hartley has in one place given of successive association as distinct from synchronous seems to have no necessary connection with the last-mentioned principle. He says, p. 69, "If A and B be vibrations impressed successively, then will the latter part of A, viz. that part which remains after the impression of the object ceases, be modified and altered by B, at the same time that it will a little modify and alter it, till at last it be quite overpowered by it, and end in it. It follows therefore that the successive impression of A and B sufficiently repeated will so alter the medullary substance, as that when A is impressed alone, the latter part shall not be such as the sole impression of A requires, but lean towards B, and end in c at last. But B will not excite a in a retrogade order, since, by supposition, the latter part of B was not modified and altered by A, but by some other vibration, such as C or D." First of all, this account seems to imply that the associated impressons A and B are the only ones made on the mind, and that they extend over the whole medullary substance. In this case when the action of A ceases or grows very weak, we may suppose that the tendency to B will be gradually revived, and at last completely overpower that of A, because these are the only impressions existing in the mind, and it must consequently incline to one or other of them, which would be equally the case, whether they had been impressed together, or not. Otherwise we must suppose the impressions thus made successively to have a distinct local communication with each other, or there is no reason given why A should excite b more than any other vibration impressed on the brain in general, or on the seat of b in particular. We must besides this suppose the vibrations A and B to have a particular line of direction, as well
