complex cases, or the associations of associations, of which the memory, in its full extent, consists, the powers of the mind, deducible from this source, will be found much greater than any person, upon his first entrance on these inquiries, could well imagine.
This proposition may be deduced from the foregoing, in the same manner as the ninth has been from the eighth.
But it seems also deducible from the nature of vibrations, and of an animal body. Let A and B be two vibrations, associated synchronically. Now, it is evident, that the vibration A (for I will, in this proposition, speak of A and B in the singular number, for the sake of greater clearness) will, by endeavouring to diffuse itself into those parts of the medullary substance which are affected primarily by the vibration B, in some measure modify and change B, so as to make B a little different from what it would be, if impressed alone. For the same reasons the vibration A will be a little affected, even in its primary seat, by the endeavour of B to diffuse itself all over the medullary substance. Suppose now the vibrations A and B to be impressed at the same instant, for a thousand times; it follows, from the ninth proposition, that they will first overcome the disposition to the natural vibrations N, and then leave a tendency to themselves, which will now occupy the place of the original natural tendency to vibrations. When therefore the vibration A is impressed alone, it cannot be entirely such as the object would excite of itself, but must lean, even in its primary seat, to the modifications and changes induced by B, during their thousand joint impressions; and therefore much more, in receding from this primary seat, will it lean that way; and when it comes to the seat of B, it will excite B’s miniature a little modified and changed by itself.
Or thus: When A is impressed alone, some vibration must take place in the primary seat of B, both on account of the heat and pulsation of the arteries, and because A will endeavour to diffuse itself over the whole medullary substance. This cannot be that part of the natural vibrations N, which belongs to this region, because it is supposed to be overruled already. It cannot be that which A impressed alone would have propagated into this region, because that has always hitherto been overruled, and converted into B; and therefore cannot have begotten a tendency to itself. It cannot be any full vivid vibration, such as B, C, D, &c. belonging to this region, because all full vibrations require the actual impression of an object upon the corresponding external
