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 22 It is instructive to note some of the attempts that have been made to classify the main sources of error in reaching beliefs. Francis Bacon, for example, at the beginnings of modern scientific inquiry, enumerated four such classes, under the somewhat fantastic title of "idols" (Gr. ειδωλα, images), spectral forms that allure the mind into false paths. These he called the idols, or phantoms, of the (a) tribe, (b) the marketplace, (c) the cave or den, and (d) the theater; or, less metaphorically, (a) standing erroneous methods (or at least temptations to error) that have their roots in human nature generally; (b) those that come from intercourse and language; (c) those that are due to causes peculiar to a specific individual; and finally, (d) those that have their sources in the fashion or general current of a period. Classifying these causes of fallacious belief somewhat differently, we may say that two are intrinsic and two are extrinsic. Of the intrinsic, one is common to all men alike (such as the universal tendency to notice instances that corroborate a favorite belief more readily than those that contradict it), while the other resides in the specific temperament and habits of the given individual. Of the extrinsic, one proceeds from generic social conditions—like the tendency to suppose that there is a fact wherever there is a word, and no fact where there is no linguistic term—while the other proceeds from local and temporary social currents.

Locke's method of dealing with typical forms of wrong belief is less formal and may be more enlightening. We can hardly do better than quote his forcible and quaint language, when, enumerating different classes of men, he shows different ways in which thought goes wrong: