In logic, the fallacy of equivocation occurs when a word or phrase that has more than one possible meaning is used in different senses in an argument.
Consider, for example, if a person were to say: “The ancient Greeks were mistaken in asserting that ‘Man is a rational animal,’ since we need but open our eyes to see how frequently people’s beliefs and behavior are utterly irrational.”
The problem with the inference this person is making is that it depends on an equivocation on the term “rational.”
In one sense, “rational” can be understood to mean something along the lines of “being of a species whose members have the capacity to shape their beliefs according to evidence and logic, and their behavior according to what means are suitable to achieving their ends.” A rational being in this sense can be contrasted with entities that have no such capacity to rationally assess potential beliefs and acts and make choices accordingly-that is, entities that either don’t have beliefs and don’t perform actions at all, or that only do so in some automatic or instinctual way.
This is the sense that the ancients meant in saying “Man is a rational animal.” They meant that humanity as a species could be contrasted with non-rational things like rocks, flowers and earthworms.
But one can also use “rational” in the sense not of capacity or potential, but in the sense of beliefs actually best fitting the available evidence, and behavior actually being the most efficient choices in achieving one’s ends. Here rationality is to be contrasted not with non-rationality, but with irrationality.
This is the sense the speaker has in mind when pointing out that people frequently behave and do things that are irrational, i.e., that are not good reason-based choices at all.
Which is certainly true. No one is a perfect “Mr. Spock” type. We have all kinds of rational imperfections affecting what we believe and how we live, from prejudices to wishful thinking to lazy thinking to making hasty decisions and on and on.
But look how the argument changes if we spell out the two senses of “rational” it uses: “The ancient Greeks were mistaken in asserting that people are capable of believing and acting in accordance with evidence and reasons, since people frequently fail to believe and act in the ways most in accordance with the available evidence and reasons.” Now the fallaciousness of the inference is apparent. Just because we don’t always exercise our rational capacities perfectly, it doesn’t follow that we are non-rational like rocks, flowers and earthworms.
But let’s look at another example, to make sure this concept is clear. Consider the following argument:
* Premise: Burt and Marjorie claim to believe in the commandment “Thou Shalt Not Kill.”
* Premise: Burt eats meat, which involves killing animals, and even Marjorie, who is a vegan, routinely kills lettuce and carrots and other living things for her food.
* Conclusion: Therefore, Burt and Marjorie are hypocrites violating a principle they claim to believe.
The flaw in this argument is an equivocation on the word “kill.” In the context of the Old Testament, “Thou Shalt Not Kill” clearly is a reference to killing people. (In fact, it’s even a subset of that. A closer translation would probably be “murder,” in that the commandment doesn’t forbid all killing of people; it forbids all “wrongful” or “illegal” killing of people, i.e., murder.) So one does not violate this commandment by killing non-human animals or plants.
Spotting the fallacy of equivocation requires being sensitive to the multiple meanings, and even subtle nuances and shadings, that words and phrases can have.
I’ll close with yet one more example, of a kind that some might find humorous.
Many years ago, there was a cult run by a shameless con artist Eastern guru type who called himself the “Bhagwan.” The Bhagwan was quite the connoisseur of luxury cars, and, rumor had it, had at least as much interest in helping himself to the favors of the more impressionable, nubile young maidens who found themselves at his commune.
In the course of a televised interview, a spokesperson for the cult was asked if there were any truth to the rumors that the cult members practiced “free love.” “Of course,” she replied, “Prostitution is absolutely against Bhagwan’s teachings! Bhagwan would never countenance that!”
Here the spokesperson’s “defense” of cult practices makes use of an equivocation on the word “free.” In the question, the word was used as part of the phrase “free love,” and clearly referred to the kind of promiscuous, orgy-style group sex and partner swapping sometimes associated with certain hippie communes and the like. But in the spokesperson’s comically deceptive response, “free” is used in its ordinary sense of something that isn’t bought and sold for money, to make it seem like anyone who disagrees with cult practices is an advocate of prostitution.