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Logic is the science and art which so directs the mind in the process of reasoning and subsidiary processes as to enable it to attain clearness, consistency, and validity in those processes. The aim of logic is to secure clearness in the definition and arrangement of our ideas and other mental images, consistency in our judgments, and validity in our processes of inference.
The Greek word 'logos' (reason) is the origin of the term logic--logike (techen, pragmateia, or episteme, understood), as the name of a science or art, first occurs in the writings of the Stoics. Aristotle, the founder of the science, designates it as "analytic", and the Epicureans use the term canonic. From the time of Cicero, however, the word logic is used almost without exception to designate this science. The names dialectic and analytic are also used.
The question whether logic is a science or an art is now generally decided by asserting that it is both. It is a science, in so far as it not merely formulates rules for right thinking, but deduces those rules from general principles which are based on the nature of mind and of truth. It is an art, in so far as it is directly and immediately related to performance, namely, to the acts of the mind. As the fine arts direct the painter or the sculptor in the actions by which he aims at producing a beautiful picture or a beautiful statue, so logic directs the thinker in the actions by which he aims at attaining truth, or expounding truth which he has attained.
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It is a curious fact that, although logic is the science which treats of definition, logicians are not agreed as to how logic itself should be defined. It would, of course, be impossible to enumerate even the principal definitions here. It will be sufficient to mention and discuss a few typical ones.
The Port Royal logic ("L'Art de penser", published 1662) defines logic as "the art of using reason well in the acquisition of the knowledge of things, both for one's own instruction and that of others." More briefly "Logic is the art of reasoning." The latter is Arnauld's definition. Definitions of this type are considered too narrow, both because they define logic in terms of art, not leaving room for its claim to be considered a science, and because, by the use of the term reasoning, they restrict the scope of logic to one class of mental processes.
Hegel goes to the other extreme when he defines logic as "the science of the pure idea." By idea he understands all reality, so that for him logic includes the science of subjective reality (logic of mental concepts) and the science of objective reality (logic of being, metaphysics). In like manner the definitions which fail to distinguish between logic and psychology, defining logic as "the science of mental processes", or "the science of the operations of the mind", are too wide. Definitions which characterize logic as "the science of sciences", "the art of arts", are also too wide: they set up too large a claim for logic.
St. Thomas Aquinas in his commentary on Aristotle's logical treatises (" In Post. Anal.", lect. i, Leonine ed., I, 138), he says: "Logic is the science and art which directs the act of the reason, by which a man in the exercise of his reason is enabled to proceed without error, confusion, or unnecessary difficulty." Taking reason in its broadest sense, so as to include all the operations of the mind which are strictly cognitive, namely, the formation of mental images, judgment, and ratiocination, we may expand St. Thomas' definition and define logic as "the science and art which so directs the mind in the process of reasoning and subsidiary processes as to enable it to attain clearness (or order), consistency, and validity in those processes". Logic is essentially directive. Therein it differs from psychology, which is essentially speculative or theoretical, and which concerns itself only in an Incidental and secondary manner with the direction of mental processes. Logic deals with processes of the mind. Therein it differs from metaphysics, which has for its field of inquiry and speculation the whole universe of being. Logic deals with mental processes in relation to truth or, more particularly, in relation to the attainment and exposition of truth by processes which aim at being valid, clear, orderly, and consistent. Therein it differs from ethics, which treats of human actions, external deeds as well as thoughts, in relation to man's final destiny. Validity, clearness, consistency, and order are logical qualities of thought, goodness and evil are ethical qualities. Finally, logic is not to be confounded with rhetoric. Rhetoric, in the old meaning of the word, was the art of persuasion; it used all the devices, such as emotional appeal, verbal arrangement, etc., in order to bring about a state of mind which had reference to action primarily, and to conviction only in a secondary sense. Logic is the science and art of conviction it uses only arguments, discarding emotional appeal and employing merely words as the symbols of thoughts.
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 (Gottfried Leibniz, 1670) "It is a good thing to proceed in order and to establish propositions (principles). This is the way to gain ground and to progress with certainty. ... I hold that the mark of a genuine idea is that its possibility can be proved, either a priori by conceiving its cause or reason, or a posteriori when experience teaches us that it is a fact in nature."
 (Albert Einstein, Physics and Reality, 1936) "Physics constitutes a logical system of thought which is in a state of evolution, whose basis (principles) cannot be distilled, as it were, from experience by an inductive method, but can only be arrived at by free invention. The justification (truth content) of the system rests in the verification of the derived propositions (a priori/logical truths) by sense experiences (a posteriori/empirical truths). ... Evolution is proceeding in the direction of increasing simplicity of the logical basis (principles). .. We must always be ready to change these notions - that is to say, the axiomatic basis of physics - in order to do justice to perceived facts in the most perfect way logically."
Editor: Haselhurst
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1. Catholic Encyclopedia: Logic, 1910
2. http://www.SpaceandMotion.com/Metaphysics-Principles-Reality.htm - Summary on Logical Truths / Principles of Physics, Empirical Truths and the Mind's Representation of our Senses. Quotes on Logic and Metaphysics from Aristotle, Gottfried Leibniz, David Hume, Immanuel Kant and Albert Einstein.
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