there may be causes unknown to our present experience which will equally
account for them). The utility of such assumptions _of causes_ depends
on their being, in their own nature, _capable_ (as Descartes' Vortices
were not, though possibly the Luminiferous Ether may be) of being, at
some time or other, proved directly by independent evidence to be the
causes. And this was, perhaps, all that Newton meant by his _verae
causae_, which alone, he said, may be assigned as causes of phenomena.
Assumptions of causes, which fulfil this condition, are, in science,
even indispensable, with a view both to experimental inquiry, and still
more to the application of the Deductive Method. They may be accepted,
not indeed, as Dr. Whewell thinks they may be, as proof, but as
suggesting a line of experiment and observation which may result in
proof. And this is actually the method used by practical men for
eliciting the truth from involved statements. They first extemporise,
from a few of the particulars, a rude theory of the mode in which the
event happened; and then keep altering it to square with the rest of the
facts, which they review one by one.

The attempting, as in Geology, to conjecture, in conformity with known
laws, in what former collocations of known agents (though _not_ known to
have been formerly present) individual existing facts may have
originated, is not Hypothesis but Induction; for then we do not
_suppose_ causes, but legitimately infer from known effects to unknown
causes. Of this nature was Laplace's theory, whether weak or not, as to
the origin of the earth and planets.

CHAPTER XV.

PROGRESSIVE EFFECTS, AND CONTINUED ACTION OF CAUSES.

Sometimes a complex effect results, not (as has been supposed in the
last four chapters) from several, but from _one_ law. The following is
the way.

Some effects are instantaneous (e.g. some sensations), and are
prolonged only by the prolongation of the causes; others are in their
own nature permanent. In some cases of the latter class, the original is
also the proximate cause (e.g. Exposure to moist air is both the
original and the proximate cause of iron rust). But in others of the
same class, the permanency of the effect is only the permanency of a
series of changes. Thus, e.g. in cases of Motion, the original force is
only the _remote_ cause of any link (after the very first) in the
series; and the motion immediately preceding it, being itself a compound
of the