The Equivocation fallacy is where the arguer switches the meaning of a term at least twice within an argument, using the term in multiple ways without clarification.(IEP, Dowden) Ambiguity in of itself is not fallacious but can be if the meaning of the term itself is central or core to the argument, therefore affecting the conclusion in a substantial way.(LaBossiere, 2023, p. 224)
Example:(IEP, Dowden)
Brad is a nobody, but since nobody is perfect, Brad must be perfect, too.
The term “nobody” changes its meaning without warning in the passage. Equivocation can sometimes be very difficult to detect, as in this argument from Walter Burleigh:(IEP, Dowden)
If I call you a swine, then I call you an animal.
If I call you an animal, then I’m speaking the truth.
Therefore, if I call you a swine, then I’m speaking the truth.
Motte-and-bailey doctrine
The Motte-and-bailey fallacy is an informal fallacy and variant of Equivocation,(LaBossiere, 2023, pp. 225, 350) named after a motte and bailey castle and initially coined as the Motte-and-bailey Doctrine.(Shackel, p. 2) It is where an arguer conflates two separate arguments, one that is easier to defend but undesired by the arguer, the “Motte,” and the harder to defend but is more desired by the arguer, the “Bailey.”(Boudry, Braeckman, p. 150) (Shackel, p. 2)
Example from Alice in Wonderland:(LaBossiere, 2023, pp. 225-226)
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.’
Another example:(LaBossiere, 2023, p. 227)
“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.”