On the other hand, belief is caused by many influences that are not
evidence at all: such are (1) desire, which makes us believe in whatever
serves our purpose; fear and suspicion, which (paradoxically) make us
believe in whatever seems dangerous; (2) habit, which resists whatever
disturbs our prejudices; (3) vanity, which delights to think oneself
always right and consistent and disowns fallibility; (4) imitativeness,
suggestibility, fashion, which carry us along with the crowd. All these,
and nobler things, such as love and fidelity, fix our attention upon
whatever seems to support our prejudices, and prevent our attending to
any facts or arguments that threaten to overthrow them.

§ 3. Two departments of Logic are usually recognised, Deduction 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