The motte-and-bailey fallacy (named after the motte-and-bailey castle) is a form of argument and an informal fallacy where an arguer conflates two positions that share similarities, one modest and easy to defend (the "motte") and one much more controversial and harder to defend (the "bailey").[1] The arguer advances the controversial position, but when challenged, insists that only the more modest position is being advanced.[2][3] Upon retreating to the motte, the arguer can claim that the bailey has not been refuted (because the critic refused to attack the motte)[1] or that the critic is unreasonable (by equating an attack on the bailey with an attack on the motte).[4]
A skilled pseudoscientist switches back and forth between different versions of his theory, and may even exploit his own equivocations to accuse his critics of misrepresenting his position. Philosopher Nicholas Shackel has termed this strategy the 'Motte and Bailey Doctrines' (Shackel 2005; see also Fusfield 1993), after the medieval defense system in which a stone tower (the Motte) is surrounded by an area of open land (the Bailey) ...Boudry and Braekman said that a retreat to the motte in a motte-and-bailey doctrine is a "deflationary revision" that is used by pseudoscientists to "immunize" a theory or belief system against refutation.
For my purposes the desirable but only lightly defensible territory of the Motte and Bailey castle, that is to say, the Bailey, represents a philosophical doctrine or position with similar properties: desirable to its proponent but only lightly defensible. The Motte is the defensible but undesired position to which one retreats when hard pressed ...
Some people have spoken of a Motte and Bailey Doctrine as being a fallacy and others of it being a matter of strategic equivocation. Strictly speaking, neither is correct. ... So it is, perhaps, noting the common deployment of such rhetorical trickeries that has led many people using the concept to speak of it in terms of a Motte and Bailey fallacy. Nevertheless, I think it is clearly worth distinguishing the Motte and Bailey Doctrine from a particular fallacious exploitation of it.