I am aware, that, when you consider, for the first time, this assertion of an infinite variety of _states_, corresponding with all the innumerable varieties of direction and velocity, in the tendencies of a simple billiard-ball, which, in the various circumstances supposed, appears to us precisely the same, in all its sensible qualities, you may be apt to conceive, that the assertion must be founded on a mistake, and, from the influence of former prejudice may be inclined to think, that, when it exhibits a tendency to begin to move east, at one time, and, at another time, a beginning tendency to move north, this does not arise from any difference of state in itself, but from its being merely carried along by the first ball, which was itself previously moving in one or other of these particular lines of direction. When the elastic billiard-ball, however, bounds away from the ball which strikes it, this supposition is manifestly inapplicable;--and, in all cases, it is the influence only of former prejudice which can lead you to this opinion,--the influence of that prejudice, by which you may have been accustomed to consider impulse, not as inducing a tendency to motion at some little distance, but as involving the necessity of actual contact. To destroy this prejudice, a very little reflection on the phenomena of elastic bodies, in their shocks and mutual retrocessions, is surely all that can be requisite; and if the motion of B, and consequently its tendency to motion, have begun, without contact of A, as it afterwards continues while A, the elastic body which struck it, is moving back in an opposite direction, it could not be by mechanical trusion, as carried along by A, which is still at some points of distance from it when its motion begins, and at still greater distance the longer the motion continues, that B has assumed any one of its variety of states,--that, for example, in which, in one case, it tends to move east, in another case to move north, in one case to move rapidly, in another slowly. To say that the body acquires this new tendency because it is impelled, is only to say that it is impelled because it is impelled. It is an equally idle use of language, to affirm, as if a word could obviate the difficulty instead of merely stating it,--that A, in communicating a different tendency to B, which was before at rest, does this by a principle, or power of repulsion; for this, as I have said, is merely to state in a single word, the regularity in certain circumstances of the very fact asserted. The different tendencies of B, and consequently the different states in which B exists,--are not the less different, in whatever manner the difference may have been produced, or by whatever word, or combination of words, the difference may be expressed. There is no magic, in the phrase, _principle_ of repulsion, or _power_ of repulsion, which can render the _same_, states or tendencies that are