Huyghens and Leibniz both objected to Newton's inverse square law
because it postulated "action at a distance,"--for example, the
attractive force of the sun and the earth. This desire for "continuity"
in physical laws led to the supposition of an "ether." We may here
anticipate and state that the reason which prompted Huyghens to object
to Newton's law led Einstein in our own day to raise objections to
the "ether" theory. "In the formulation of physical laws, only those
things were to be regarded as being in causal connection which were
capable of being actually observed." And the "ether" has not been
"actually observed."
The idea of "continuity" implies distances between adjacent points
that are infinitesimal in extent; hence the idea of "continuity"
comes in direct opposition with the finite distances of Newton.
The statement relating to causal connection--the refusal to accept an
"ether" as an absolute base of reference--leads to the principle of
the relativity of motion.