“Secondly, I could not admit of impulsion; for to do this I must have known that a celestial matter was the agent.  But so far from knowing that there is any such matter, I have proved it to be merely imaginary.

“Thirdly, I use the word attraction for no other reason but to express an effect which I discovered in Nature—­a certain and indisputable effect of an unknown principle—­a quality inherent in matter, the cause of which persons of greater abilities than I can pretend to may, if they can, find out.”

“What have you, then, taught us?” will these people say further; “and to what purpose are so many calculations to tell us what you yourself do not comprehend?”

“I have taught you,” may Sir Isaac rejoin, “that all bodies gravitate towards one another in proportion to their quantity of matter; that these central forces alone keep the planets and comets in their orbits, and cause them to move in the proportion before set down.  I demonstrate to you that it is impossible there should be any other cause which keeps the planets in their orbits than that general phenomenon of gravity.  For heavy bodies fall on the earth according to the proportion demonstrated of central forces; and the planets finishing their course according to these same proportions, in case there were another power that acted upon all those bodies, it would either increase their velocity or change their direction.  Now, not one of those bodies ever has a single degree of motion or velocity, or has any direction but what is demonstrated to be the effect of the central forces.  Consequently it is impossible there should be any other principle.”

Give me leave once more to introduce Sir Isaac speaking.  Shall he not be allowed to say?  “My case and that of the ancients is very different.  These saw, for instance, water ascend in pumps, and said, ’The water rises because it abhors a vacuum.’  But with regard to myself; I am in the case of a man who should have first observed that water ascends in pumps, but should leave others to explain the cause of this effect.  The anatomist, who first declared that the motion of the arm is owing to the contraction of the muscles, taught mankind an indisputable truth.  But are they less obliged to him because he did not know the reason why the muscles contract?  The cause of the elasticity of the air is unknown, but he who first discovered this spring performed a very signal service to natural philosophy.  The spring that I discovered was more hidden and more universal, and for that very reason mankind ought to thank me the more.  I have discovered a new property of matter—­one of the secrets of the Creator—­and have calculated and discovered the effects of it.  After this, shall people quarrel with me about the name I give it?”

Vortices may be called an occult quality, because their existence was never proved.  Attraction, on the contrary, is a real thing, because its effects are demonstrated, and the proportions of it are calculated.  The cause of this cause is among the Arcana of the Almighty.