ILLUSTRATIONS OF FALLACIES OF OPPOSITION.
(1) Since some men are wise, then I may conclude that all men are wise. (2) I have contradicted his statement “all men are honest” by proving that _no men are honest_.
There is little difference between fallacies like (1) and fallacies of converse accident. Concerning illustration (2), _both_ statements are false; but to contradict we know that if one is false, the other must be true.
“Two negatives are equivalent to one affirmative,” is the principle underlying obversion. The most common fallacy in obversion springs from using one negative instead of two.
(a) Original: Some men are not wise. Obverse: (incorrect) Some men are wise.
(b) Original: All true teachers are just. Obverse: (incorrect) All true teachers are not just.
Conversion involves the interchanging of the subject and predicate of a proposition without affecting the distribution; in consequence the usual fallacy incident to this interchange is _distributing_ an undistributed term.
(a) Original: All fixed stars are heavenly bodies. Converted: (incorrectly) All heavenly bodies are fixed stars.
(b) Original: Some men are not wise. Converted: (incorrectly) Some wise beings are not men.
As this process involves the two steps of obversion and conversion, fallacies appertaining to contraversion would relate to these two steps.
ILLUSTRATIONS OF FALLACIES OF CONTRAVERSION.
(a) Original: No honest man fails to pay his debts. Contraverted: (incorrectly) Some who do not pay their debts are honest men.
(b) Original: Some animals are quadrupeds. Contraverted: (incorrectly) Some not-quadrupeds are not animals.
The formal fallacies of categorical, hypothetical, and disjunctive arguments have received detailed treatment in chapters 11, 14 and 15; we may, therefore, devote our attention to the material fallacies without further delay.
=6. FALLACIES OF LANGUAGE. (Equivocation.)=
These are the fallacies of _double meaning_. It is known that an equivocal term is one which permits two or more interpretations; similarly a _proposition_ which admits of two or more interpretations may be denominated equivocal. Thus the term equivocation has come to stand for all errors in language resulting from a possibility of more than one interpretation. This justifies the position of referring to all of the six fallacies in language as fallacies also of equivocation.