The inductive and hypothetic forms of inference are essentially probable
inferences, not necessary; while deduction may be either necessary or
probable.
But no mental action seems to be necessary or invariable in its
character. In whatever manner the mind has reacted under a given
sensation, in that manner it is the more likely to react again; were
this, however, an absolute necessity, habits would become wooden and
ineradicable, and no room being left for the formation of new habits,
intellectual life would come to a speedy close. Thus, the uncertainty of
the mental law is no mere defect of it, but is on the contrary of its
essence. The truth is, the mind is not subject to “law,” in the same
rigid sense that matter is. It only experiences gentle forces which
merely render it more likely to act in a given way than it otherwise
would be. There always remains a certain amount of arbitrary spontaneity
in its action, without which it would be dead.
Some psychologists think to reconcile the uncertainty of reactions with
the principle of necessary causation by means of the law of fatigue.
Truly for a _law_, this law of fatigue is a little lawless. I think it is
merely a case of the general principle that an idea in spreading loses
its insistency. Put me tarragon into my salad, when I have not tasted
it for years, and I exclaim “What nectar is this!” But add it to every
dish I taste for week after week, and a habit of expectation has been
created; and in thus spreading into habit, the sensation makes hardly
any more impression upon me; or, if it be noticed, it is on a new side
from which it appears as rather a bore. The doctrine that fatigue is
one of the primordial phenomena of mind I am much disposed to doubt.
It seems a somewhat little thing to be allowed as an exception to the
great principle of mental uniformisation. For this reason, I prefer to
explain it in the manner here indicated, as a special case of that great
principle. To consider it as something distinct in its nature, certainly
somewhat strengthens the necessitarian position; but even if it be
distinct, the hypothesis that all the variety and apparent arbitrariness
of mental action ought to be explained away in favor of absolute
determinism does not seem to me to recommend itself to a sober and sound
judgment, which seeks the guidance of observed facts and not that of
prepossessions.