Note 146 — Chapter IV · SPACE AND TIME

Of course, a physical vacuum is not the same thing as empty space. For the purposes of any special science a vacuum means a space not occupied by contents of the special kind in which that special science is interested. Thus, in the ordinary parlance of Physics, a vacuum means simply a space in which there is no mass. Whether it is desirable, for the purposes of physical science, to assume the existence of vacuum, is altogether a question for Physics itself, and to decide it in the affirmative is not to maintain the existence of that unmeaning abstraction, absolutely empty space. In any case, it may be observed that the widespread notion that motion is only possible in a physical vacuum is a mistake, motion being perfectly possible in a fluid plenum.