We may fairly wonder how, after all previous explanations of the principles of duty, so far as it is derived from pure reason, it was still possible to reduce it again to a doctrine of happiness; in such a way, however, that a certain moral happiness not resting on empirical causes was ultimately arrived at, a self-contradictory nonentity.  In fact, when the thinking man has conquered the temptations to vice, and is conscious of having done his (often hard) duty, he finds himself in a state of peace and satisfaction which may well be called happiness, in which virtue is her own reward.  Now, says the eudaemonist, this delight, this happiness, is the real motive of his acting virtuously.  The notion of duty, says be, does not immediately determine his will; it is only by means of the happiness in prospect that he is moved to his duty.  Now, on the other hand, since he can promise himself this reward of virtue only from the consciousness of having done his duty, it is clear that the latter must have preceded:  that is, be must feel himself bound to do his duty before he thinks, and without thinking, that happiness will be the consequence of obedience to duty.  He is thus involved in a circle in his assignment of cause and effect.  He can only hope to be happy if he is conscious of his obedience to duty:  and he can only be moved to obedience to duty if be foresees that he will thereby become happy.  But in this reasoning there is also a contradiction.  For, on the one side, he must obey his duty, without asking what effect this will have on his happiness, consequently, from a moral principle; on the other side, he can only recognize something as his duty when he can reckon on happiness which will accrue to him thereby, and consequently on a pathological principle, which is the direct opposite of the former.

I have in another place (the Berlin Monatsschrift), reduced, as I believe, to the simplest expressions the distinction between pathological and moral pleasure.  The pleasure, namely, which must precede the obedience to the law in order that one may act according to the law is pathological, and the process follows the physical order of nature; that which must be preceded by the law in order that it may be felt is in the moral order.  If this distinction is not observed; if eudaemonism (the principle of happiness) is adopted as the principle instead of eleutheronomy (the principle of freedom of the inner legislation), the consequence is the euthanasia (quiet death) of all morality.

{Preface ^paragraph 10}