With the imperative Form of Ethics, which in Chapter II. we proved to be a petitio principii, is directly connected a favourite idea of Kant's, that may be excused, but cannot be adopted.

Sometimes we see a physician, after having employed a certain remedy with conspicuous success, henceforth prescribing it for almost all diseases; to such a one Kant may be likened. By separating the a priori from the a posteriori in human knowledge he made the most brilliant and pregnant discovery that Metaphysics can boast of. What wonder then that thereafter he should try to apply this method, this sundering of the two forms, everywhere, and should consequently make Ethics also consist of two parts, a pure, i.e. an a priori knowable part, and an empirical? The latter of these he rejects as unreliable for the purpose of founding Ethics.