In addition to the fallacies of relevance and presumption we examined in our previous lessons, there are several patterns of incorrect reasoning that arise from the imprecise use of language. An
ambiguous word, phrase, or sentence is one that has two or more distinct meanings. The inferential relationship between the propositions included in a single argument will be sure to hold only if we are careful to employ exactly the same meaning in each of them. The fallacies of ambiguity all involve a confusion of two or more different senses.
An equivocation trades upon the use of an ambiguous word or phrase in one of its meanings in one of the propositions of an argument but also in another of its meanings in a second proposition.
The fallacy of accent arises from an ambiguity produced by a shift of spoken or written emphasis. Thus, for example:
The fallacy of composition involves an inference from the attribution of some feature to every individual member of a class (or part of a greater whole) to the possession of the same feature by the entire class (or whole).
Notice that this is distinct from the fallacy of converse accident, which improperly generalizes from an unusual specific case (as in "My philosophy course was well-organized; therefore, college courses are well-organized."). For the fallacy of composition, the crucial fact is that even when something can be truly said of each and every individual part, it does not follow that the same can be truly said of the whole class.
Similarly, the fallacy of division involves an inference from the attribution of some feature to an entire class (or whole) to the possession of the same feature by each of its individual members (or parts).
Again, be sure to distinguish this from the fallacy of accident, which mistakenly applies a general rule to an atypical specific case (as in "Ocelots have many health problems, and Sparky is an ocelot; therefore, Sparky is in poor health"). The essential point in the fallacy of division is that even when something can be truly said of a whole class, it does not follow that the same can be truly said of each of its individual parts.
Informal fallacies of all seventeen varieties can seriously interfere with our ability to arrive at the truth. Whether they are committed inadvertently in the course of an individual's own thinking or deliberately employed in an effort to manipulate others, each may persuade without providing legitimate grounds for the truth of its conclusion. But knowing what the fallacies are affords us some protection in either case. If we can identify several of the most common patterns of incorrect reasoning, we are less likely to slip into them ourselves or to be fooled by anyone else.