dilemma an argument or argument form in which one of the premises is a disjunction. Constructive dilemmas take the form ‘If A and B, if C then D, A or C; therefore B or D’ and are instances of modus ponens in the special case where A is C and B is D; destructive dilemmas are of the form ‘If A then B, if C then D, not-B or not-D; therefore not-A or not-C’ and are likewise instances of modus tollens in that special case. A dilemma in which the disjunctive premise is false is commonly known as a false dilemma. See also MORAL DILEMM. G.F.S.