Argumentation is reflexive, that is, it has a unique self-referential structure. As any theory of argument has to make use of arguments for its justification, argumentation theory is part of its own subject area. Moreover, ground-level argumentation and meta-argumentation proceed in a concurrent process, so that this two-leveled structure provides for the endeavor to remain open. "Rational" argument, as defined by economical or logical criteria, lacks this openness. With regard to epistemic criteria, three stages of argumentation can be specified: natural, scientific, and philosophical argumentation.