This essay aims to clarify Hermann Cohen's view of the ideal in the context of Judaism. Kant secured the independence of the ethical sphere by a discrimination of 'ought' from 'is', but did not pay full attention to the realization of morality. Therefore Kant's ethics have been criticized for being unrealistic. Cohen insists that Kant should pursue the being of the 'ought-to-be' in accordance with 'a method of purity'. It is 'pure will' that is to be produced from the originative principle. Pure will can also be a realization of morality produced in relation only to time. However this reality is in principle very different from natural objects of thought, for it is characterized as a concrete individual idea, that is, the ideal. The ideal, which is said to have originated from Plato, is indispensable to pure ethics. Cohen discovers the moral reality of 'the ideal of eternity'. Needless to say, the word 'eternity' was also used in Kant's philosophy, for example, 'eternal peace'. According to Cohen, however, it originated from Judaism. In this way, he tries to reinterpret various concepts of Kant's ethics in the context of Jewish culture. In Western philosophy, Cohen is often considered as an epigone of modern rationalists such as Kant, Fichte, and Hegel. This valuation is not entirely inappropriate, but seems to neglect another dimension of Cohen's thought. We should now pain a fuller picture of Cohen as both a Kantian and a Jewish philosopher.