Janus faced conception of truth (which should not be confused with the Janus faced nature of validity claims in general) is based on the Heideggerian distinction between ‘ready to hand’ and ‘present at hand.’ In Habermas’ opinion we operate in the lifeworld and discourse with two different notions of truth. In the lifeworld we operate with what might be termed a non objectified conception of truth, where truth operates as behavioural certainty. Here we are in constant touch with surprising reality and behavioural certainties are constantly tested through our need to cope with reality.
However once behavioural certainties are shattered the question of truth is objectified and taken into the realm of discourse which is relatively detached from action imperatives that cannot be avoided within lifeworld. Here we cannot rely on ‘resisting’ and ‘surprising’ reality. Here we can only rely on ‘reasons’ and ‘counter reasons’ of our fellow beings and whether those reasons are good or not is itself decided within discourse. To be sure, the resistance of reality works here as well but only indirectly, through being converted into discursive reasons:
“A perception that is contrary to our beliefs destabilizes our certainties about how to act. Only if agents distance themselves from their practical coping with the world and enter into rational discourse, objectifying the situation that was originally “ready to hand” in order to reach understanding with one another about something in the world, can such a perception become a discursively mobilized “reason.” It then enters as criticism into the conceptual economy and semantic inferential resources attached to existing views, setting in motion revisions, if necessary.” (TJ: 154-155).
Since participants in discourse must return to the lifeworld sooner or later they test their provisional conclusions reached within discourse against the surprising reality through engaging in action and coping with reality. Problematised certainties can further be discussed in the discourse and this process in principle remains continued and open ended.
A complete conception of truth must combine the above two notions of truth and must be based on a dialectical* relation between the discursive conception of truth and a life based conception of truth. Habermas dramatises the distinction between the two conceptions of truth in order to bring home the point that discourses are ultimately embedded in the lifeworld.
* In a non Hegelian sense.