Forms of justification. On the structure and scope of self-refutation arguments in Plato, Cicero and Apel. - In this essay, the structure and scope of transcendental types of argumentation are analyzed, compared and criticized on the basis of the reception of two antiskeptical types of reasoning in ancient philosophy (Plato, Parmenides 135b-c; Cicero, Lucullus § 28) by a contemporary philosophical author (Karl-Otto Apel). Plato puts forward a transcendental argument for the inevitability of a final knowledge. Cicero argues that a principle of fallibilism occurring with universal claim contradicts itself. In Karl-Otto Apel's transcendental pragmatic variant of philosophy, the two types of self-refutation arguments developed in Plato's and Cicero's dialogues are brought together into a single argumentation.