From a psychological point of view, a fallacy is often
defined as a mistake in reasoning or argumentation. Often fallacies are
used for deceptive purposes and many of the informal fallacies described
here are often used for the manipulation of opinion.
Many of these mistakes in reasoning occur so often they deserve special study.
This section investigates informal fallacies—those dependent upon
language. An informal fallacy is one that arises from the content of
an argument (the meaning what is declared), not a fallacy arising from the
grammatical structure (the syntax of how the argument is expressed).
Our classification of fallacies, like that of I.M. Copi's, is arranged in
terms of mistakes in reasoning arising from appeals to irrelevant factors as
well as mistakes arising from unsupported assumptions.
Nevertheless, as Joseph says in his Introduction to Logic: “Truth may have its norm, but error is infinite in its aberrations, and they cannot be digested in any classification.”
Not all irrelevant appeals or unsupported assumptions are fallacies. Fallacies
only occur in argumentative discourse. Thus, if no argument is present in a
passage, no fallacy can be present.
Links to Lecture Notes
FALLACIES OF RELEVANCE
FALLACIES OF PRESUMPTION
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