In this inaugural lecture at Frankfurt University which was later published as the appendix of his KHI (“Knowledge and Human Interest: A General Perspective” KHI pp. 301-317). Habermas starts with stating what he terms as the fundamental assumption of the ‘traditional ontology’. The assumption is that “the only knowledge that can truly orient action is knowledge that frees itself from mere human interests and is based on ideas” (KHI p. 301). The traditional ontology according to this conception is supposed to be based on the assumption that knowledge consists of pure ideas (that depict reality as it is) without intervention from (or of) subject. This assumption, Habermas elaborates, is based on fundamental distinction and demarcation between “Being and time” (ibid.). Thus the concept of knowledge devoid of interest, as pure theory and contemplation and the notion of static universe (in essence) lies at the heart of Habermas’ understanding of what he calls ‘traditional’ ontology .
The second aspect of the Greek conception of theory, Habermas claims, pertains to the idea that theory is to guide the ‘conduct of life’: “Through the soul’s likening itself to the ordered motion of the cosmos, theory enters the conduct of life. In ethos theory moulds life to its form and is reflected in the conduct of those who subject themselves to its discipline” (p. 302, emphasis retained) . Through the conception of ethos theory is connected to practice or conduct of life. Similar observations recur in Habermas’ following remarks regarding Husserl’s idea of theory and crisis: “Like almost all philosophers before him, Husserl, without second thought took as the norm of his critique an idea of knowledge that preserves the platonic connection of pure theory with the conduct of life” (ibid., emphasis added).
Thus a particular conception of knowledge as pure theory and belief in the efficacy of pure theory to guide the conduct of life forms the basis of Habermas’ understanding of ontology. This is confirmed by Habermas’ further comments on relation between positivism and ontology on the one hand and on neo Kantianism on the other hand, although Habermas concedes that in positivism, and neo Kantianism original intention of classical theory to connect theory with practice is ultimately lost (ibid: 303-304).
Habermas sees “a real connection between the positivistic self understanding of the sciences and traditional ontology” (ibid: 302). The connection is stated in the following manner: “Both are committed to a theoretical attitude that frees those who take it from dogmatic association with the natural interests of life and their irritating influence; and both share the cosmological intention of describing the universe theoretically in its lawlike order, just as it is” (ibid: 303). According to this conception ontology is based on a particular conception of knowledge that dissociates knowledge from ‘natural interests’ and consequently sees knowledge as pure theory. Similarly ontology is based on conception of universe as a static (immutable) order that can be stated and understood in terms of ‘law’ and the assumption that knowledge can describe universe ‘as it is’, or ‘in itself’ independent of knower (ibid: 304).
Habermas mentions three aspects of this ‘unconditional commitment’ of modern sciences which ultimately boil down to the same thing or at least different aspects of the same thing: “psychologically an unconditional commitment to theory and epistemologically the severance of knowledge from interest . . . in logic . . . the distinction between descriptive and prescriptive statements, which makes grammatically obligatory the filtering out of merely emotive from cognitive contents” (ibid.). All three aspects have a commonality: purging knowledge of any shades of interest. The ‘theory’ does this by purging practice, epistemology through purging interest and logic through positing emotive as outside the realm of knowledge.
To summarise: According to Habermas’ conception following assumptions are involved in a particular notion of ontology he is rejecting:
a) The notion of pure knowledge devoid of interests, emotions etc. Knowledge, which depicts reality as it is and does not involve anything else, thus a conception of knowledge as pure depiction, the notion of knowledge independent of the knowing subject. Hence the rejection of knowledge as constitutive of reality.
b) The notion of universe or world as ordered cosmos behaving according to immutable laws. Thus the rejection of dynamic universe. Hence the rejection of the concept of world as constituted.
c) The notion of universe having structures, which are independent of the knowing subject. Hence the rejection of the notion of being in itself or facts independent of knower.
The above three aspects are in fact highly interrelated and can be discussed separately only with some abstraction.