Weak naturalism starts with an assumption:
Our “learning process” are the continuation of the learning process that have been going on in the natural and biological stages of evolution.
Habermas differentiates between “continuation” and “mere continuation”: Here is the key passage (from Habermas’ discussion of Peirce):
“If the learning processes of human species merely continue, in reflexive form, those of nature, then argumentation, or what one human being has to say to another, and the power of the better argument to convince, both lose the weight and value that are proper to them. The unforced agreement of individuals who hold one another accountable, and who are faced with opinions that differ form person to person, ought to issue form argumentation by virtue of the latter’s specific character. But this specific achievement falls victim to the leveling force of a universalism propelling itself inferentially from within reality itself. The multivocal character of intersubjectivity becomes an epiphenomenon.” (TJ: 109, italics added).
The assumption of weak naturalism understood in the above sense (or terms) can be regarded as the assumption about the emergence of what Allison has called absolute spontaneity (as against relative spontaneity) from within “nature itself.”
The above is supposed to be an answer to the questions like this:
If every thing is to be described ultimately in naturalistic terms (broadly construed) then it needs to be explained how freedom comes here in the first place.
The above explains in part how Kant can be reconciled with Darwin. Habermas does not give an overall argument for how freedom emerges from nature (apart from his assumption of weak naturalism and his conception of language as half transcendence) but he gives us an overall indication of how the Kantian notion of freedom can be reconciled with Darwinain naturalism.
A main part of Habermas’ strategy in this context is to distinguish between objective nature and primordial nature, a strategy that is reminiscent of Schelling (cf. PT: 20). [Relavent here is the difference between “objective nature,” “subjective nature,” and “nature in itself.” The significance of Habermas distinction between “instrumental” conception of reason and communicative reason and his methodological dualism is obvious in this context].
But with regard to the above different authors have pointed out the dilemma that emerges when we try to maintain the concept of “nature in itself.” within an overall Kantianism. Habermas' replies to this worry by spelling out his conception of the pragmatic conception of knowledge (TJ: introduction]. Habermas maintains that the aporias pointed out by different authors only apply to if we presuppose a representational model of knowledge (TJ: 22).
It is only then that we can start thinking of reconciling Kant and Hegel or alternatively start thinking of how transcending character of reason can be reconciled with a thoroughgoing detranscendentalisation.