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.
- Really exciting novels are rare.
- But rare books are expensive.
- Therefore, Really exciting novels are expensive.
Here, the word “rare” is used in different ways in the two premises of the argument, so the link they seem to establish between the terms of the conclusion is spurious. In its more subtle occurrences, this fallacy can undermine the reliability of otherwise valid deductive arguments.
- A reckless motorist Thursday struck and injured a student who was jogging through the campus in his pickup truck.
- Therefore, it is unsafe to jog in your pickup truck.
In this example, the premise (actually heard on a radio broadcast) could be interpreted in different ways, creating the possibility of a fallacious inference to the conclusion.
The fallacy of accent arises from an ambiguity produced by a shift of spoken or written emphasis. Thus, for example:
- Jorge turned in his assignment on time today.
- Therefore, Jorge usually turns in his assignments late.
Here the premise may be true if read without inflection, but if it is read with heavy stress on the last word seems to imply the truth of the conclusion.
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).
- Every course I took in college was well-organized.
- Therefore, my college education was well-organized.
Even if the premise is true of each and every component of my curriculum, the whole could have been a chaotic mess, so this reasoning is defective.
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).
- Ocelots are now dying out.
- Sparky is an ocelot.
- Therefore, Sparky is now dying out.
Although the premise is true of the species as a whole, this unfortunate fact does not reflect poorly upon the health of any of its individual members.
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.