Fallacy, unsound argument, or mode of arguing, leading to a specious but erroneous conclusion. The ingenious puzzles of the later Greek sophists, and the more serious arguments, seemingly unassailable yet leading to a palpably false conclusion, of the Eleatic and Megarian schools in Greece, were of great use in stimulating logical theory. Many of these were dealt with by Aristotle in the earliest treatise on fallacies, the Sophistici Eleuchi. Later logicians classified fallacies as formal and material. In the former the fault is in the reasoning; in the latter it is in the premises, and they are therefore outside the scope of logic proper. A number of formal fallacies are enumerated in the ordinary text-books of logic. Bacon's enumeration of idola, or phantoms, which prevent the reception of scientific truth (individual and racial prejudices: prejudices due to language, and false philosophical theories), is in some degree a classification of fallacies. J. S. Mill classed fallacies under five heads: (1) Simple Inspection; (2) Observation; (3) Generalisation; (4) Ratiocination; (5) Confusion.