Deduction, in argument, is the form of reasoning by which a conclusion is formally reached from one or more premises. The premise(s) are said to "guarantee" the conclusion. In other words, deduction is reasoning to a conclusion that--given the known conditions--is a certainty. In valid deductive reasoning, the conclusion must be true if all the premises are true. Deduction is usually expressed in syllogisms, arguments in which two premises yield a single conclusion.
Fallacy, in argument, is an error in reasoning, or more precisely, an illicit maneuver made in the process of moving from the premises of an argument to its conclusion. As a result of a fallacy, the premises might or might not justify the conclusion. Whether an argument establishes a conclusion, fallacious or not, is audience-dependent. A distinction is often made between logical and so-called psychological fallacies.Induction, in argument, is sometimes said to be the process of reasoning from the particular to the general. This, however, is only a type of inductive reasoning and is not to be confused with a more inclusive and correct notion of induction, based on the assumption that if something is true in a number of observed instances, then it is also held to be true in similar, but unobserved, instances.
Syllogism is a deductive argument, consisting of a sequence of three propositions such that the first two imply the conclusion. The categorical syllogism comprises three categorical propositions, statements of the form all A are B, no A are B, some A are B, or some A are not B. A categorical syllogism contains precisely three terms: the major term, which is the predicate of the conclusion; the minor term, which is the subject of the conclusion; and the middle term, which appears in both premises but not in the conclusion.
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