Note: Moving forward this post due to recent comments.
Habermas’ debate with Apel can be considered as a debate about how far detranscendentalisation should be taken and how much detranscendentalisation should be carried out. While Habermas is concerned with how this detranscendentalisation turn is carried out (it needs a balancing act according to some of his interpreters), this does not inhibit him to carry out the turn thoroughly.
For Habermas’ typical response to those who he deems to have carried out the turn before him (like Hegel and Heidegger) is that they have not carried out it thoroughly and wholeheartedly. There is no stage at which this thoroughgoing detranscendentalisation should stop.
One of the most fascinating thought Habermas has is that in order to preserve the transcending character of reason it is necessary (though obviously not sufficient) to carry out the detranscendentalisation more and not less robustly. Thus we can differentiate between robust and non robust conceptions of detranscendentalisation.
Apel has been a colleague and mentor of Habermas for a long time. In Habermas’ own words, “under the living philosophers non one has more lastingly determined the direction of my thinking than Karl-Otto Apel.” Habermas and Apel share the detranscendentalisation turn. However despite this a difference has emerged in recent days between them about what kind of justification or what notion of justification is compatible with the detranscendentalisation turn. Apel thinks that we can maintain a version of “ultimate grounding or justification,” despite the turn. Habermas on the other hand thinks that ultimate justification of grounding is incompatible with the turn. Habermas insists that only a modest conception of grounding and justification is compatible with the turn.
The debate between Apel and Habermas can be read as a debate about which of the two notions of detranscendentalisation (roust or non-robust) are to be espoused. Habermas claims that detranscendentalisation should be carried out thoroughly and nothing should be immune form it. Apel on the other hand thinks that there is a limit to detranscendentalisation (thus he adopts a less robust conception of it). According to Apel there are certain principles and certain hierarchy among principles that is immune from detranscendentalisation and hence they retain their old transcendental character.
Habermas considers this half hearted detranscendentalisation not only incoherent but also anti-modern in its spirit (as it is more clear in is debate with Henrich on the status of metaphysics).
Apel's critique of Habermas
two sense of linguistic turn
Apel and Habermas
Kant and Darwin
Morality,law and deliberative democracy