§ 19. The question has often been discussed whether logic is a science or an art. The answer to it must depend upon the meaning we assign to these terms. § 20. Broadly speaking, there is the same difference between Science and Art as there is between knowing and doing. Science is systematized knowledge; Art is systematized action. Science is acquired by study; Art is acquired by practice. § 21. Now logic is manifestly a branch of knowledge, and does not necessarily confer any practical skill. It is only the right use of its rules in thinking which can make men think better. It is therefore, in the broad sense of the terms, wholly a science and not at all an art. § 22. But this word 'art,' like most others, is ambiguous, and is often used, not for skill displayed in practice, but for the knowledge necessary thereto. This meaning is better conveyed by the term 'practical science.' § 23. Science is either speculative or practical. In the first case we study merely that we may know; in the latter that we may do. Anatomy is a speculative science; Surgery is a practical science. In the first case we study the human frame in order that we may understand its structure; in the second that we may assist its needs. Whether logic is a speculative or a practical science depends entirely upon the way in which it is treated. If we study the laws of thought merely that we may know what they are, we are making it a speculative science; if we study the same laws with a view to deducing rules for the guidance of thought, we are making it a practical science. § 24. Hence logic may be declared to be both the science and the art of thinking. It is the art of thinking in the same sense in which grammar is the art of speaking. Grammar is not in itself the right use of words, but a knowledge of it enables men to use words correctly. In the same way a knowledge of logic enables men to think correctly, or at least to avoid incorrect thoughts. As an art logic may be called the navigation of the sea of thought. § 25. The laws of thought are all reducible to the three following axioms, which are known as The Three Fundamental Laws of Thought. (1) The Law of Identity-- Whatever is, is; or, in a more precise form, Every A is A. (2) The Law of Contradiction-- Nothing can both be and not be; Nothing can be A and not A. (3) The Law of Excluded Middle-- Everything must either be or not be; Everything is either A or not A. § 26. Each of these principles is independent and self-evident.