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 differently colored billiard-balls in contact, so that when an object, the billiard-stick, strikes the first or white ball, the same motion propagates itself through the red, green, blue, black, &c. and sets the whole in motion. No! we must suppose the very same force, which constitutes the white ball, to constitute the red or black; or the idea of a circle to constitute the idea of a triangle; which is impossible.

But it may be said, that, by the sensations from the objects A and M, the nerves have acquired a disposition to the vibrations a and m, and therefore a need only be repeated in order to re-produce m. Now we will grant, for a moment, the possibility of such a disposition in a material nerve, which yet seems scarcely less absurd than to say, that a weather-cock had acquired a habit of turning to the east, from the wind having been so long in that quarter: for if it be replied, that we must take in the circumstance of life, what then becomes of the mechanical philosophy? And what is the nerve, but the flint which the wag placed in the pot as the first ingredient of his stone broth, requiring only salt, turnips, and mutton, for the remainder! But if we waive this, and pre-suppose the actual existence of such a disposition; two cases are possible. Either, every idea has its own nerve and correspondent oscillation, or this is not the case. If the latter be the truth,