The three main classes of logical inference are Deduction, Induction,
and Hypothesis. These correspond to three chief modes of action of the
human soul. In deduction the mind is under the dominion of a habit or
association by virtue of which a general idea suggests in each case
a corresponding reaction. But a certain sensation is seen to involve
that idea. Consequently, that sensation is followed by that reaction.
That is the way the hind legs of a frog, separated from the rest of the
body, reason, when you pinch them. It is the lowest form of psychical
manifestation.
By induction, a habit becomes established. Certain sensations, all
involving one general idea, are followed each by the same reaction; and
an association becomes established, whereby that general idea gets to be
followed uniformly by that reaction.
Habit is that specialisation of the law of mind whereby a general idea
gains the power of exciting reactions. But in order that the general
idea should attain all its functionality, it is necessary, also, that
it should become suggestible by sensations. That is accomplished by a
psychical process having the form of hypothetic inference. By hypothetic
inference, I mean, as I have explained in other writings, an induction
from qualities. For example, I know that the kind of man known and
classed as a “mugwump” has certain characteristics. He has a high
self-respect and places great value upon social distinction. He laments
the great part that rowdyism and unrefined good-fellowship play in the
dealings of American politicians with their constituency. He thinks that
the reform which would follow from the abandonment of the system by which
the distribution of offices is made to strengthen party organisations
and a return to the original and essential conception of office-filling
would be found an unmixed good. He holds that monetary considerations
should usually be the decisive ones in questions of public policy. He
respects the principle of individualism and of _laisser-faire_ as the
greatest agency of civilisation. These views, among others, I know to
be obtrusive marks of a “mugwump.” Now, suppose I casually meet a man
in a railway-train, and falling into conversation find that he holds
opinions of this sort; I am naturally led to suppose that he is a
“mugwump.” That is hypothetic inference. That is to say, a number of
readily verifiable marks of a mugwump being selected, I find this man
has these, and infer that he has all the other characters which go to
make a thinker of that stripe. Or let us suppose that I meet a man of a
semi-clerical appearance and a sub-pharisaical sniff, who appears to look
at things from the point of view of a rather wooden dualism. He cites
several texts of scripture and always with particular attention to their
logical implications; and he exhibits a sternness, almost amounting to
vindictiveness, toward evildoers, in general. I readily conclude that
he is a minister of a certain denomination. Now the mind acts in a way
similar to this, every time we acquire a power of coördinating reactions
in a peculiar way, as in performing any act requiring skill. Thus, most
persons have a difficulty in moving the two hands simultaneously and in
opposite directions through two parallel circles nearly in the medial
plane of the body. To learn to do this, it is necessary to attend, first,
to the different actions in different parts of the motion, when suddenly
a general conception of the action springs up and it becomes perfectly
easy. We think the motion we are trying to do involves this action, and
this, and this. Then, the general idea comes which unites all those
actions, and thereupon the desire to perform the motion calls up the
general idea. The same mental process is many times employed whenever we
are learning to speak a language or are acquiring any sort of skill.
Thus, by induction, a number of sensations followed by one reaction
become united under one general idea followed by the same reaction;
while by the hypothetic process, a number of reactions called for by one
occasion get united in a general idea which is called out by the same
occasion. By deduction, the habit fulfils its function of calling out
certain reactions on certain occasions.