"That, however, does not prevent its being essential to matter. No one has ever been able to discover the cause of sensation in animals; yet this sensation is so essential to them, that, if you exclude the idea of it, you no longer have the idea of an animal."
"Well, I will concede to you, for a moment, that motion is essential to matter—just for a moment, let it be remembered, for I am not much inclined to embroil myself with the theologians—and now, after this admission, tell me how one ball produces motion in another?"
"You are very curious and inquisitive; you wish me to inform you of what no philosopher ever knew."
"It appears rather curious, and even ludicrous, that we should know the laws of motion, and yet be profoundly ignorant of the principle of the communication of motion!"
"It is the same with everything else; we know the laws of reasoning, but we know not what it is in us that reasons. The ducts through which our blood and other animal fluids pass are very well known to us, but we know not what forms that blood and those fluids. We are in life, but we know not in what the vital principle consists."
"Inform me, however, at least, whether, if motion be essential to matter, there has not always existed the same quantity of motion in the world?"
"That is an old chimera of Epicurus revived by Descartes. I do not, for my own part, see that this equality of motion in the world is more necessary than an equality of triangles. It is essential that a triangle should have three angles and three sides, but it is not essential that the number of triangles on this globe should be always equal."
"But is there not always an equality of forces, as other philosophers express it?"
"That is a similar chimera. We must, upon such a principle, suppose that there is always an equal number of men, and animals, and moving beings, which is absurd."
By the way, what, let me ask, is the force of a body in motion? It is the product of its quantity multiplied by its velocity in a given time. Calling the quantity of a body four, and its velocity four, the force of its impulse will be equal to sixteen. Another quantity we will assume to be two, and its velocity two; the force with which that impels is as four. This is the grand principle of mechanics. Leibnitz decidedly and pompously pronounced the principle defective. He maintained that it was necessary to measure that force, that product, by the quantity multiplied by the square of the velocity. But this was mere captious sophistry and chicanery, an ambiguity unworthy of a philosopher, founded on an abuse of the discovery of the great Galileo, that the spaces traversed with a motion uniformly accelerated were, to each other, as the squares of the times and velocities.