TOWARDS THE END of his life the German philosopher Immanuel Kant wrote a treatise entitled Religion Within the Limits of Reason Alone. In it he defended the thesis of the autonomy of philosophical ethics and the inherent rationality of the laws of morality. On the basis of this principle he concluded that if religion is to be admitted as a legitimate mode of thought and practice it would have to be measured by reason. In particular, revealed, or historical, faiths, such as Judaism, Christianity, and Islam, containing dogmas, mysteries, and ceremonial laws, would have to be judged by whether or not they are compatible with and conducive to the promotion of ‘pure religion’, the religion of morality based upon reason. Using this principle Kant further concluded that of the three historical faiths only Christianity in its Protestant form came close to satisfying the rational and moral conditions that reason and morality prescribe. Perhaps influenced by Spinoza, Kant maintained that Judaism was more a national polity primarily focused upon ceremonial law and having no intrinsic relevance to the moral law; hence it possessed no application or validity to all mankind. Islam fares no better: according to Kant it is simply a religion of conquest and subjugation, in which moral autonomy and reason are despised. Only a religion purified of irrational and compulsory dogma and mystery, containing minimal ritual law, could, according to Kant, attain the status of a ‘religion within the limits of reason alone’.

A little over a century later the Jewish neo-Kantian philosopher Hermann Cohen accepted for the most part Kant's general conception of a religion of reason but rejected his judgement concerning Judaism. In his last important work, The Religion of Reason out of the Sources of Judaism, Cohen attempted to show the essential rationality of Judaism and its foundation in the moral law. Having adopted the common Christian view of Judaism as a legalistic religion, Cohen argued, Kant intentionally glossed over the moral teachings of the prophets and was totally ignorant of the doctrines of the sages and the medieval Jewish philosophers and moralists. Cohen's work was an attempt to correct Kant as well as an argument for the thesis that Judaism is a religion of reason. Nevertheless, for Cohen, no less than for Kant, the guiding principle is that reason is the ultimate authority in religion.