This book is the first complete intellectual biography of Hermann Cohen (1842–1918), the only one to cover all his major philosophical and Jewish writings. It pays special attention to Cohen’s intellectual development, to its breaks and continuities. From its beginning to its end, Cohen’s intellectual career is seen as the development of a radical rationalism, one committed to unending enquiry and the unlimited rights of criticism. Cohen’s thought was resolutely opposed to any form of irrationalism or mysticism, which would act as arbitrary and artificial limits on criticism and enquiry. This interpretation is therefore opposed to those who see a proto-existentialism (Rosenzweig) or mysticism (Adelmann and Köhnke) in Cohen. Cohen’s Judaism was not a limit to his radical rationalism but a consistent development of it. Judaism was the religion of reason, which committed the believer to the unending search for truth and to striving to achieve the cosmopolitan or universal values of reason. Most interpretations of Cohen’s Judaism fail to appreciate its philosophical depth and sophistication.