His research and teaching focused on the fundamental questions of philosophy and on the most important figures of its history, from Plato to Heidegger.
Rosen's first two books, a study of Plato's Symposium and Nihilism: A Philosophical Essay, represent his abiding concerns.
On the one hand he continuously returned to the roots of the philosophical tradition, in particular to Plato, and, on the other, he thought through modern and postmodern philosophy by confronting their most powerful representatives.
The most notable feature of this engagement was the justice done to the two main strands of contemporary philosophy, the continental and analytic movements, represented by their most influential members, Heidegger and Wittgenstein, as preparation for Rosen's criticism and positive proposals.
One of the central themes of Rosen's work is the claim that the extraordinary discourses of philosophy have no other basis than the intelligent understanding of the features of ordinary life or human existence.