Habermas argument in the opening chapter of The Philosophical Discourse of Modernity works within a philosophical tradition, which holds that since the Enlightenment, 'modernity' has been taken as the central concept in theorising about the self and society. This category has been used as the intellectual and historical philosophical framework within which to locate social and political thought, not just in the West, but across the world.

The significance of modernity rests on the perception of a ruptural break from the past and the emergence of a new civilisational complex which informs all spheres of human existence. It encapsulates the societal shift from traditional to modern societies as well as the intellectual and philosophical justification on which that society is founded---that is, the move was in detaching itself from the past and standing on its own reflexivity:---creating its normativity out of itself, as Habermas puts it. Modernity grounds itself. See the Young Hegelian..

Modernity can be seen as as a category to describe the coalescence of particular developments that occurred in Europe in the long sixteenth century that gave birth to a new epoch. These in turn both generated, and contributed, to particular philosophical developments associated with the Enlightenment and the freedom of subjectivity. This ethos of subjective freedom of individuals provides the critieria for subjects to orient themselves in the new order that had come into being.

It was held that, when modernity is understood as a particular kind of society and civilization and as a philosophical ethos associated with that civilization, it forms the ground of a particular experience that was seen to be both historically inevitable and universally applicable. Habermas explores this experience through the aesthetic understanding of modernity as expressed by Baudelaire and Benjamin: modernity is experienced as the transitory, the fleeting and the contingent: as a now time.

Habermas gives due weight to Hegel as the theoriest of philosophical modernity and to Hegel's argument that the subjective freedom of individuals is the philosophical principle of modernity that works away to transform the different institutions into embodying the principle of subjectivity.

Hegel argues that if modernity is self-grounding, then the critique of modernity can make use of no instrument other than reflection: out of the principle of the Enlightenment itself.

Can Hegel do it? Habermas has big doubts. He reckons that Hegel becomes ensnarred in dilemmas and contradictions. Have a look at the doubts about Hegel at this post by Allan Wittman over at Long Sunday