Habermas claims that the methodological distinction between external and internal perspectives is ontologically neutral. A question arises however: in what sense is the methodological distinction to be considered ontologically neutral? The distinction is not ontologically neutral in that it does not have any ontological implications. The distinction is only ontologically neutral in the sense that it is compatible with more than one ontological position. Thus both idealism and naturalism are in principle compatible with the methodological dualism presupposed by a transcendentalist approach. Deriving an ontological position from a methodological distinction involves committing two types of fallacies, viz. an idealistic fallacy and a naturalistic fallacy:
“Only the idealistic fallacy of inferring an ontological difference between mind and body (or Being and beings) from a methodological distinction misleads us into locating the transcendental conditions of objective experience in a transmundane realm of intelligible – or of the history of Being. Conversely the naturalistic fallacy is but the other side of the same coin; it simply assimilates transcendental conditions to empirical conditions, without considering aporias of self-referentiality, and projects them onto a scientifically objectified realm.”
In order to provide ontological support to the above mentioned methodical distinction one needs to take an additional step. Habermas’ weak naturalist hypothesis is meant to provide this additional step. Thus ‘weak’ naturalism is an ontological position that can maintain the methodological distinction between internal and external perspectives without committing either to idealism (which according to Habermas has been discredited) or strong naturalism which Habermas wants to avoid.
Also read Epistemic dualism vs. Ontological monism