I’m sure everyone, or at least if you read Slate Star Codex, has heard of the motte and bailey (MAB) fallacy, which is as follows:
Motte and bailey (MAB) is a combination of bait-and-switch and equivocation in which someone switches between a “motte” (an easy-to-defend and often common-sense statement, such as “culture shapes our experiences”) and a “bailey” (a hard-to-defend and more controversial statement, such as “cultural knowledge is just as valid as scientific knowledge”) in order to defend a viewpoint. Someone will argue the easy-to-defend position (motte) temporarily, to ward off critics, while the less-defensible position (bailey) remains the desired belief, yet is never actually defended.
In short: instead of defending a weak position (the “bailey”), the arguer retreats to a strong position (the “motte”), while acting as though the positions are equivalent. When the motte has been accepted (or found impenetrable) by an opponent, the arguer continues to believe (and perhaps promote) the bailey.
The MAB is a combination of bait and switch and false equivocation between an extreme position that is hard to defend and and a position that is almost tacitly true.
Here’s an example:
A: “Libertarianism is the best form of government”
B: “I disagree because of X, Y, and z”
A: “But shouldn’t people have individual autonomy/freedom?”
A: “Then I’m right” (B leaves discussion)
But no one actually argues this way. ‘B’ will almost never let ‘A’ get away with making such a baldfaced equivocation. More likely, ‘B’ will object, “Yes, but that does not prove libertarianism is the best government and or the only way to achieve freedom.” ‘A’, on the other hand, would likely not make such a poorly supported claim, but rather would say, “Libertarianism is a superior form of government for reasons X, Y, and Z.” Second, ‘A’ understands that freedom is not the same as libertarianism–and although they may be related, A has enough intellectual honesty to know they are not equivalent. In real life, MABs are probably more subtle, and this leads to the second reason why the MAB isn’t a fallacy.
Persuasive arguments require some degree of equivocation. In order to convince someone of a declarative statement, which we’ll call ‘A’, one must bring forth smaller premises that lend support to ‘A’. Although theses smaller premises are not equivalent to A, they are related or may be causal. For example, “Libertarianism is a good form of government” (declarative statement). Premise: “It maximizes individual freedom.”
Also there is a degree of subjectivity in that more extreme equivocations are likely to be interpreted as a MAB. For example, the statement “if you want to be healthy, stop drinking” is less likely to raise objection than “if you want to be healthy, convert to Mormonism.” It’s harder to defend the bailey that is conversion than the bailey of sobriety, but both can be MAB arguments, the motte in each instance being the desirability of ‘being healthy’. In either case, an implication that A (an action) leads to B (an outcome) is unavoidable, and the reader must suspend skepticism and accept that sobriety leads to better health. IF the reader rejects this conclusion, it does not mean the argument is fallacious, but either the evidence was insufficient or the reader is incredulous. The only way it becomes fallacious is if the author literally equates health with sobriety, rather than being positively correlated.