An apagogic argument is a form of argument by the absurd, which argues that unreasonable interpretations of the law must be rejected:

The apagogic argument assumes that the legislator is reasonable and could not have admitted an interpretation of the law that would lead to illogical or unfair consequences. (Perelman 1979, p. 58)

It parallels the psychological argument, presupposing that the legislator is rational and benevolent, V. Absurd; Juridical arguments.

According to Alexy, the apagogic argument is one of the four types of arguments prevailing in law, the others being the arguments by analogy, a contrario (opposites) and a fortiori, (1989, quoted in Kloosterhuis 1995, p. 140).