Joachim Renn’s following comments (in his extremely insightful review of TJ which I have discovered only a month or so ago) on how to ground Habermas’ methodological distinction between understanding and observation in his ontology of weak naturalism seems to me quite fascinating:
“Habermas stresses – again in accord with Putnam – primarily the normative status of rationality to avoid naturalistic interpretations of the relation between the biological history of humanity and the history of a reconstructive (internal) perspective with an (external) observational point of view, i.e. the continuity between nature and culture, on a ‘meta-theoretical’ level (TJ: pp. 38-9). The strategic value of this argument lies both in avoiding the reduction of inner-worldly learning to the evolutionary adaptation of the human species to its environment, and in maintaining the connection between rational justification (as an emergent level in comparison to a natural history of adaptation) and the problems in the real world.” (One world is Enough, p. 490).
Thus, the unique Habermasian synthesis depends on the realisation of continuities (and discontinuities) between language and action on the one and language and the world on the other hand.