duction and
Induction; that is, to describe them briefly, proof from principles, and
proof from facts. Classification is sometimes made a third department;
sometimes its topics are distributed amongst those of the former two. In
the present work the order adopted is, Deduction in chaps. ii. to xiii.;
Induction in chaps. xiii. to xx.; and, lastly, Classification. But such
divisions do not represent fundamentally distinct and opposed aspects of
the science. For although, in discussing any question with an opponent
who makes admissions, it may be possible to combat his views with merely
deductive arguments based upon his admissions; yet in any question of
general truth, Induction and Deduction are mutually dependent and imply
one another.

This may be seen in one of the above examples. It was argued that a
certain metal must be copper, because every metal is copper that turns
green when dipped in vinegar. So far the proof appealed to a general
proposition, and was deductive. But when we ask how the general
proposition is known to be true, experiments or facts must be alleged;
and this is inductive evidence. Deduction then depends on Induction. But
if we ask, again, how any number of past experiments can prove a general
proposition, which must be good for the future as well as for the past,
the uniformity of causation is invoked; that is, appeal is made to a
principle, and that again is deductive proof. Induction then depends
upon Deduction.

We may put it in this way: Deduction depends on Induction, if general
propositions are only known to us through the facts: Induction depends
on Deduction, because one fact can never prove another, except so far as
what is true of the one is true of the other and of any other of the
same kind; and because, to exhibit this resemblance of the facts, it
must be stated in a general proposition.

Sec. 4. The use of Logic is often disputed: those who have not studied it,
often feel confident of their ability to do without it; those who have
studied it, are sometimes disgusted with what they consider to be its
superficial analysis of the grounds of evidence, or needless
technicality in the discussion of details. As to those who, not having
studied Logic, yet despise it, there will be time enough to discuss its
utility with them, when they know something about it; and as for those
who, having studied it, turn away in disgust, whether they are justified
every man must judge for himself, when