the statement and
illustration of the typical forms of wrong procedure should accompany
_pari passu_ the exposition of the right procedure.

In accordance with this principle, I shall deal with special
fallacies, special snares or pitfalls--misapprehension of words,
misinterpretation of propositions, misunderstanding of arguments,
misconstruction of facts, evidences, or signs--each in connexion
with its appropriate safeguard. This seems to me the most profitable
method. But at this stage, it may be worth while, by way of
emphasising the need for Logic as a science of rational belief, to
take a survey of the most general tendencies to irrational belief,
the chief kinds of illusion or bias that are rooted in the human
constitution. We shall then better appreciate the magnitude of the
task that Logic attempts in seeking to protect reason against its own
fallibility and the pressure of the various forces that would usurp
its place.

It is a common notion that we need Logic to protect us against the
arts of the Sophist, the dishonest juggler with words and specious
facts. But in truth the Inner Sophist, whose instruments are our own
inborn propensities to error, is a much more dangerous enemy. For once
that we are the victims of designing Sophists, we are nine times the
victims of our own irrational impulses and prejudices. Men generally
deceive themselves before they deceive others.

Francis Bacon drew attention to these inner perverting influences,
these universal sources of erroneous belief, in his _De Augmentis_
and again in his _Novum Organum_, under the designation of _Idola_
([Greek: eidola]), deceptive appearances of truth, illusions. His
classification of _Idola--Idola Tribus_, illusions common to all men,
illusions of the race; _Idola Specus_, personal illusions, illusions
peculiar to the "den" in which each man lives; _Idola Fori_, illusions
of conversation, vulgar prejudices embodied in words; _Idola Theatri_,
illusions of illustrious doctrine, illusions imposed by the dazzling
authority of great names--is defective as a classification inasmuch as
the first class includes all the others, but like all his writings it
is full of sagacious remarks and happy examples. Not for the sake of
novelty, but because it is well that matters so important should be
presented from more than one point of view, I shall follow a division
adapted from the more scientific, if less picturesque, arrangement
of Professor Bain, in