e a _chance_ of being
mendacious.

V. The second series of questions will be of use in determining whether
there is any reason to distrust the accuracy of a statement. Was the
author in one of those situations which cause a man to make mistakes? As
in dealing with good faith, we must look for these conditions both as
affecting the document as a whole, and as affecting each of the
particular statements in it.

The practice of the established sciences teaches us the conditions of an
exact knowledge of facts. There is only one scientific procedure for
gaining knowledge of a fact, namely, _observation_; every statement,
therefore, must rest, directly or indirectly, upon an observation, and
this observation must have been made correctly.

The set of questions by the aid of which we investigate the
probabilities of error may be drawn up in the light of experience, which
brings before us the most common cases of error.

(1) The author was in a situation to observe the fact, and supposed he
really had observed it; he was, however, prevented from doing so by some
interior force of which he was unconscious, an hallucination, an
illusion, or a mere prejudice. It would be useless, as well as
impossible, to determine which of these agencies was at work; it is
enough to ascertain whether the author had a tendency to observe badly.
It is scarcely possible in the case of a particular statement to
recognise that it was the result of an hallucination or an illusion. At
the most we may learn, either from information derived from other
sources or by comparison, that an author had a _general_ propensity to
this kind of error.

There is a better chance of recognising whether a statement was due to
prejudice. In the life or the works of an author we may find the traces
of his dominant prejudices. With reference to each of his particular
statements, we ought to ask whether it is not the result of a
preconceived idea of the author on a class of men or a kind of facts.
This inquiry partly coincides with the search for motives of falsehood:
interest, vanity, sympathy, and antipathy give rise to prejudices which
alter the truth in the same manner as wilful falsehood. We therefore
employ the questions already formulated for the purpose of testing good
faith. But there is one to be added. In putting forward a statement has
the author been led to distort it unconsciously by the circumstance that
he was answering a question? This is the case o