THE LIMITS TO THE EXPLANATION OF LAWS OF NATURE. HYPOTHESES.
The constant tendency of science, operating by the Deductive Method, is
to resolve all laws, even those which once seemed ultimate and not
derivative, into others still more general. But no process of
_resolving_ will ever reduce the number of ultimate laws below the
number of those varieties of our feelings which are distinguishable in
quality, and not merely in quantity or degree. The _ideal_ limit of the
explanation of natural phenomena is to show that each of these ultimate
facts has (since the differences in the different cases of it affect our
sensations as differences in degree only, and not in quality) only one
sort of cause or mode of production; and that all the seemingly
different modes of production or causes of it are resolvable into one.
But _practically_ this limit is never attained. Thus, though various
laws of Causes of Motion have been resolved into others (e.g. the fall
of bodies to the earth, and the motions of the planets, into the one law
of mutual attraction), many causes of it remain still unresolved and
distinct.
Hypotheses are made for the sake of this resolving and explaining of
laws. When we do not _know_ of any more general laws into which to
resolve an uniformity, we then (either on no or on insufficient
evidence) _suppose_ some, imagining either causes (as, e.g. Descartes
did the Vortices), or the laws of their operation (as did Newton
respecting the planetary central force); but we never feign both cause
and law. The use of a hypothesis is to enable us to apply the Deductive
Method before the laws of the causes have been ascertained by Induction.
In those cases where a false law could not have led to a true result (as
was the case with Newton's hypothesis as to the law of the Attractive
force) the third part of the process in the Deductive Method, viz.
Verification, which shows that the results deduced are true, amounts to
a complete induction, and one conforming to the canon of the Method of
Difference. But this is the case only when either the cause is known to
be one given agent (and only its law is unknown), or to be one of
several given agents.
An assumed cause, on the other hand, cannot be accepted as true simply
_because_ it explains the phenomena (since two conflicting hypotheses
often do this even originally, or, as Dr. Whewell himself allows, may at
any rate by modifications be made to do it); nor _because_ it moreover
leads to the prediction of other results which turn out true (since this
shows only what was indeed apparent already from its agreement with the
old facts, viz. that the phenomena are governed by laws partially
identical with the laws of other causes); nor _because_ we cannot
imagine any other hypothesis which will account for the facts (since
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.