"Karl-Otto Apel has succeeded in revealing the buried dimension of the nondeductive justification of basic ethical norms. He revives the transcendental mode of justification using the tools of a pragmatics of language. One of the key elements of Apel's transcendental-pragmatic line of argumentation is the notion of a performative contradiction. A PC occurs when a constative speech act rests on non-contingent presuppositions whose propositional content contradicts the asserted proposition. <...>
This debate about a "minimal logic,"...is of interest to Apel insofar as it refutes the skeptic's claim that it is impossible to ground moral principles. But it does not thereby relieve the ethical cognitivist of the burden of proof. ... In "argumentation as such" Apel has gained a reference point that is as fundamental for the analysis of unavoidable rules as the "i think" or "consciousness as such" is for the philosophy of reflection. Just as someone interested in a theory of knowledge cannot adopt a standpoint outside his own cognitive acts (and thus remains caught in the self-referentialisty of the subject of cognition), so too a person engaged in developing a theory of moral argumentation cannot adopt a standpoint outside the situation defined by the fact that he is taking part in a process of argumentation. The theorists of argumentation becomes aware of the self-referentiality of his arguments as the epistemologist becomes aware of the self-referentiality of his knowledge. Such awareness means giving up futile attempts at a deductive grounding of "ultimate" principles and returning to the explication of "unavoidable," presuppositions.
<....> "the present essay is rightly characterized as a sketch or a proposal. <..> Participants in argumentation cannot avoid the presupposition that, owing to certain characteristics that require formal description, the structure of their communication rules out all external or internal coercion other than the force of the better argument and thereby also neutralizes all motives other than that of the cooperative search for truth. <i trust you know what these are or will look the ISS up on the 'net>
If these considerations (the ISS) are to amount to more than a definition favoring an ideal form of communication and thus prejudging everything else, we must show that these rules of discourse are not mere conventions; rather, they are inescapable presuppositions."
Moral Consciousness and Communicative Action, 80-90
The presuppositions themselves are identified by convincing a person who contest the hypothetical reconstructions offered (in the ISS) that he is caught up in performative contradictions.