SIDELIGHTS ON RELATIVITY

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to assume that empty space has no physical qualities whatever. The
fundamental facts of mechanics do not harmonize with this view.
For the mechanical behaviour of a corporeal system hovering freely
in empty space depends not only on relative positions (distances)
and relative velocities, but also on its state of rotation, which
physically may be taken as a characteristic not appertaining to the
system in itself. In order to be able to look upon the rotation of
the system, at least formally, as something real, Newton objectivises
space.

Since he classes his absolute space together with real things, for
him rotation relative to an absolute space is also something real.
Newton might no less well have called his absolute space "Ether";
what is essential is merely that besides observable objects, another
thing, which is not perceptible, must be looked upon as real,
to enable acceleration or rotation to be looked upon as something
real.