then undoubtedly the ideas thus called up will follow one another in the same order in which they were originally excited. But if we takeaway this topographical allotment of separate parcels of the brain to different ideas, and suppose the same substance or principle to be constantly impressed with a succession of different ideas, then there seems to be no assignable reason why a vibratory motion accompanied with thought in passing from one part of the thinking substance to the next should not excite any other idea which had been impressed there, as well as the one with which that particular vibration had been originally associated, or why it should not by one general impulse equally excite them all. It is like supposing that you might tread upon a nest of adders twined together, and provoke only one of them to sting you. On the other hand to say that this species of elective affinity is determined in its operation by the greater readiness with which the idea of a particular impression recalls the memory of another impression which co-existed with it in a state of sensible excitement is to describe the fact but not (that I can perceive) in any manner to account for it. Let any one compare this account with the one given by Hartley of his own principle, and he will be able to judge.
But farther, even if could be shewn that the doctrine of vibrations accounts satisfactorily for the association of the ideas of any one sense, (as those of the sight for example,) yet surely the very nature of that principle must cut off every sort of communication between the ideas of different senses, (as those of sight and hearing,) which may have been associated in the order of time, but which with respect to actual situation must be farther removed from one another than any ideas of the same sense, at whatever
