The history of thought is hardly intelligible without it.
As the science of proof, Logic gives an account of the _general_ nature of evidence deductive and inductive, as applied in the physical and social sciences and in the affairs of life.

The _general_ nature of such evidence: it would be absurd of the logician to pretend to instruct the chemist, economist and merchant, as to the _special_ character of the evidence requisite in their several spheres of judgment.

Still, by investigating the general conditions of proof, he sets every man upon his guard against the insufficiency of evidence.
One application of the science of proof deserves special mention: namely, to that department of Rhetoric which has been the most developed, relating to persuasion by means of oratory, leader-writing, or pamphleteering.

It is usually said that Logic is useful to convince the judgment, not to persuade the will: but one way of persuading the will is to convince the judgment that a certain course is advantageous; and although this is not always the readiest way, it is the most honourable, and leads to the most enduring results.

Logic is the backbone of Rhetoric.
It has been disputed whether Logic is a science or an art; and, in fact, it may be considered in both ways.