cause. This may be technically arguable, given a sufficient
latitude to the word sufficiency: nevertheless, it is convenient
to distinguish between mere sufficiency to explain the phenomena
in question, and the proof otherwise that the cause assigned really
exists _in rerum natura_, or that it operated in the given case. The
frequency with which the expression _vera causa_ has been used since
Newton's time shows that a need is felt for it, though it may be hard
to define "verity" precisely as something apart from "sufficiency". If
we examine the common usage of the expression we shall probably find
that what is meant by insisting on a _vera causa_ is that we must
have some evidence for the cause assigned outside the phenomena in
question. In seeking for verification of a hypothesis we must extend
our range beyond the limited facts that have engaged our curiosity and
that demand explanation.

There can be little doubt that Newton himself aimed his rule at the
Cartesian hypothesis of Vortices. This was an attempt to explain the
solar system on the hypothesis that cosmic space is filled with a
fluid in which the planets are carried round as chips of wood in a
whirlpool, or leaves or dust in a whirlwind. Now this is so far a
_vera causa_ that the action of fluid vortices is a familiar one: we
have only to stir a cup of tea with a bit of stalk in it to get an
instance. The agency supposed is sufficient also to account for the
revolution of a planet round the sun, given sufficient strength in the
fluid to buoy up the planet. But if there were such a fluid in space
there would be other phenomena: and in the absence of these other
phenomena the hypothesis must be dismissed as imaginary. The fact that
comets pass into and out of spaces where the vortices must be assumed
to be in action without exhibiting any perturbation is an _instantia
crucis_ against the hypothesis.

If by the requirement of a _vera causa_ were meant that the cause
assigned must be one directly open to observation, this would
undoubtedly be too narrow a limit. It would exclude such causes as the
ether which is assumed to fill interstellar space as a medium for
the propagation of light. The only evidence for such a medium and
its various properties is sufficiency to explain the phenomena. Like
suppositions as to the ultimate constitution of bodies, it is of the
nature of what Professor Bain calls a "Representative Fiction": the
only condition is that it mus