fallacy

fallacy in logic, a term used to characterize an invalid argument. Strictly speaking, it refers only to the transition from a set of premises to a conclusion, and is distinguished from falsity, a value attributed to a single statement. The laws of syllogisms were systematically elaborated by Aristotle, and for an argument to be valid, it must adhere to all the laws; to be fallacious, it need only break one (see syllogism ). The term fallacy has come to be used in a somewhat wider sense than the purely formal one. Informal fallacies are said to occur when statements are ambiguous or vague as to the logical form they represent, or when a multiplicity of meaning is present and the validity of the argument depends on switching meanings of a word or a phrase in midstream.

FALLACY. In general usage, a false and often deceitful idea; in logic, a line of reasoning (also known as a paralogism) that may seem valid but is not. Fallacies of discourse were first described in Greek and Latin, and many therefore retain their classical names, either uniquely or alongside a vernacular label. They include: (1) Argumentum ad baculum [Latin: argument backed by a stick]. Resorting to threat in order to have a point accepted. (2) Argumentum ad hominem [Latin: argument directed at the person]. Often called an ad hominem argument or an ad hominem attack. Seeking to disprove a point by attacking the people making that point, either in terms of their character or by referring to their personal circumstances as an explanation of why a position has been adopted. (3) Argumentum ad populum [Latin: argument directed at the people]. An appeal to popular opinion, bias, and inclination. (4) Non sequitur [Latin: it does not follow]. A statement in which the premisses of an argument do not lead to the conclusion provided. (5) Post hoc ergo propter hoc [Latin: after this therefore because of this]. Asserting that because A came before B, A caused B.

fallacy deception XV; logical flaw, delusive notion XVI; delusive nature XVIII. — L. fallācia, f. fallāx, fallāc-, f. fallere deceive.
So fallacious XVI. — (O)F. fallacieux. See -ACY, -ACIOUS.