o 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 hav