Equivocation is when an ambiguous expression is used in more than one of its meanings in a single context. Ambiguity by itself is not fallacious, but is a lack of clarity in language that occurs when a claim has two (or more) meanings and it is not clear which is intended. The fallacy of Equivocation occurs when that context is an argument, and the conclusion depends on shifting the meaning of the expression while treating it as if it remains the same.
Premises: On or more premises are presented that contain an equivocation.
Conclusion: Claim C is drawn from these premises.
The sort of “reasoning” presented above is fallacious because the evidence only appears to support the conclusion because the same word is being used. Because the meaning of the word is shifted, the evidence does not support the conclusion.
In some cases, the error is obvious. For example, if someone said, “Sally is standing on my right, I’m a moderate and people to the right of me are conservative, so Sally is a conservative”, most people would see it as a lame joke. Other cases of equivocation, especially ones that occur with a more subtle equivocation, can be more tempting.
A variation of this fallacy, called the motte-and-bailey fallacy or doctrine, was presented by fellow philosopher Nicholas Shackel. Briefly put, this fallacy involves conflating two similar positions. One of the positions is controversial while the other is more modest and easier to defend. The technique is to advance the controversial position and then, when it is challenged, shift to the more modest position as if nothing had changed. This can also be seen as like Moving the Goal Posts, although it involves only one move.
Equivocation, like amphiboly, is often used in humor. Such uses are not intended as serious arguments and would not (generally) count as fallacies. Perhaps the most famous example is from Alice in Wonderland:
Who did you pass on the road?’ the King went on, holding out his hand to the Messenger for some more hay.
`Nobody,’ said the Messenger.
`Quite right,’ said the King: `this young lady saw him too. So of course, Nobody walks slower than you.
`I do my best,’ the Messenger said in a sulky tone. `I’m sure nobody walks much faster than I do!’
`He can’t do that,’ said the King, `or else he’d have been here first. However, now you’ve got your breath, you may tell us what’s happened in the town.’
Defense: The defense is to watch out for attempts to exploit equivocation to deceitfully (or accidentally) switch meaning in the context of an argument. This involves checking to see if the expression has the same meaning throughout the argument. In the case of the motte and bailey variant and other swapping variations, the defense is to watch out for the shift.
“A blue whale is an animal; therefore, a small blue whale is a small animal.”
“A feather is light. What is light is not dark. So, feathers cannot be dark.”
Rex: “I can’t believe that Sally still doesn’t believe me.”
Ted: “Why not?”
Rex: “Well, I gave her the reason why I did it and I learned in logic that reasons support claims. So, she should believe me.”
“Every day we see miracles such as antibiotics, the internet, and space travel. So, when those atheists say there are no miracles, they are wrong. So, that pretty much wraps it up for the atheists’ claim.”