Fallacy of Equivocation
Fallacy of Ambiguity
Explanation of the Fallacy of Equivocation
This fallacy is perhaps the most simple and obvious of the fallacies of ambiguity. This fallacy occurs when a single term is used with two or more meanings in the same argument.
The basic form of this fallacy is:
a. Premise: [statement(s) using term X in sense 1]
b. Premise: [statement using term X in sense 2] AND/OR Conclusion: [statement using term X in sense 2]
The term equivocation comes from the Latin equi (equal) and vox (voice) and means "with equal voice". When a term is used univocally in an argument ("one voice"), it always has the same meaning, but when it is used equivocally, more than one meaning is given equal voice.
Examples and Discussion of the Fallacy of Equivocation
Here is an amusing example of an argument using this fallacy:
1. It is well known that the average family has 2.5 children (premise #1). Well, Jane's family is very average (premise #2), so they must have 2.5 children (conclusion).
The key term average is used in more than one sense in this argument. In the first premise, average is used in the sense of statistical averages. In the second premise a different sense of average is used, this time meaning not unusual. By equating the two, the absurd conclusion of a family having fractional children is reached.
We can also find an example of equivocation in a common argument against abortion:
2. It is wrong to kill innocent human beings. (premise #1) Fetuses are innocent human beings. (premise #2) Therefore, it is wrong to kill fetuses. (conclusion)
It isn't hard to show that the term innocent human being is being used in more than one sense here. Normally, when the first premise is used, what is meant is a human being who is capable of moral choices, but who has not in fact chosen any immoral acts. But in the second premise, what is meant has to be more along the lines of a human being who is not capable of any moral choices in the first place.
However, if it is argued that the exact same sense is meant in both instances of innocent human being, then the argument is guilty of the fallacy of Begging the Question, and is still invalid. You can't have a valid argument showing that it is wrong to kill a member of class X by simply assuming that it's wrong to kill any member of class X.
Some religious arguments rely on equivocation fallacies, for example:
3. It is not possible for the universe to exist without a cause, therefore there must have been a First Cause, which we can reasonably call "God." I already believe in the God of the Bible, and now you have no excuse for not doing so as well.
4. There exist laws of nature. Laws imply the existence of a lawgiver. Therefore, there must be a cosmic lawgiver just as there are societal lawgivers.
Example #3 is the First Cause argument and the term God is being used in two entirely different ways. In the first sense, "God" is used to describe a First Cause of the universe, with no particular attributes beyond that which is necessary to cause a universe. In the second sense, "God" is used for something much more specific and with many more attributes: a traditional Christian conception of God.
Example #4 is an argument that used to be more common, but perhaps people have finally realized its fallaciousness because we don't see it too often any more. In the first premise, the idea of laws being applied to nature refer to the concept of observed regularities. In the conclusion, the idea of laws as they are in society refer to the concept of authoritative command or rules. Because two different meanings to the term law are used, the argument is invalid.
This is why it's important to ask people to define their terms clearly. If someone is going to try to prove that God exists, and they define their god in terms of traditional Christian theology, then we will immediately be able to tell that arguments like the above fail because, even if valid, they "prove" an entirely different sort of entity.
A common argument used by creationists also commits this fallacy:
5. Evolution is just a theory. Therefore, it shouldn't be taught in the public schools as fact.
The term theory has two meanings, a general meaning used in common vernacular and a technical meaning used in science. With the former, it means "an assumption based on limited information or knowledge; a conjecture." Under this meaning, the above argument would be correct.
But biological evolution is a scientific theory, and thus science's usage of the term is more appropriate: "A set of statements or principles devised to explain a group of facts or phenomena, especially one that has been repeatedly tested or is widely accepted and can be used to make predictions about natural phenomena."
Because the term theory is used in the first sense in the premise, but the second sense in the conclusion, the fallacy of equivocation is committed. This is similar to example #4 above because we have a conflict between common usages of terms (law, theory) and technical or scientific uses of those same terms. This conflict is a common source of equivocation and should be watched for.
Sometimes, this fallacy is known as "Bait and Switch." That is a concept usually applied to deceptive commercial advertising: a store advertises the availability of some wonderful product at a great price, but once you get there you find that none are left and all that remain are much more expensive items. They "baited" you with something attractive, but then "switched" it for something less desirable (and more profitable for them).
The Fallacy of Equivocation is similar to such deceptive advertising because it starts out sounding reasonable and gets you to agree to certain ideas. But after that, the meanings of key terms are changed, thus causing you to agree to things you never accepted at the beginning.
That is why one of the keys to dealing with this sort of fallacy is keep it from occurring in the first place: always make an effort to define, or ask for definitions of, the most important terms — especially when you know that they can be used in multiple ways. When clear definitions are provided, it is harder to subtly or accidentally alter them later on.
There are times, however, when use of such a fallacy is valid (in a sense) because it makes a good rhetorical point, such as this comment from Benjamin Franklin:
6. If we don't hang together, we will hang separately.
Obviously he is using the term 'hang' in more than one way, but in this case it was valid because if the American revolutionaries did not stick together, there was a good chance that they would fail and be executed by hanging. By using the same word in more than one way, the ideas were emphasized and a memorable quote was created.