(Greek: logos, speech, reason)
In ordinary language, refers to sound reasoning.
Logicians have never agreed as to how logic should be defined, and have doubted as to whether it is a mode of knowing or an instrument of knowledge, a science or an art.
Modern works are in three groups; those which attempt to apply mathematical methods to the processes of thought (symbolic logic); those which follow Hegel in making logic a criticism of the understanding practically identical with metaphysics; and those which accept the traditional view of Aristotle and the Schoolmen.
In accordance with Aristotle, founder of logic, Saint Thomas defines logic as the science or the art of exact reasoning:
"Logic is the science and art which directs the act of reason, by which man is able to proceed in the pursuit of truth without error, confusion, or difficulty."
Reason includes all mental operations; hence the aim of logic is to direct the acts of the mind so as to secure clearness in definition and arrangement of our mental images, consistency in our judgments, and validity in our inferences.
By reason of its directive role, logic differs from psychology, which deals with the mental processes in themselves, and is primarily speculative.
In opposition to the Hegelian viewpoint logic cannot be identified with metaphysics, since the latter deals not with the processes of thought in particular, but with the whole universe of being.
According to the traditional division formal logic studies mental processes simply to secure clearness and order among their contents; material logic concerns the truth of these contents.
In the modern usage the field of material logic is reserved to epistemology, and the term logic is applied only to that portion formerly designated as formal logic.
New Catholic Dictionary