From Schiller's letters on aesthetics

"But we know that the condition of the human will always remains contingent, and that only in the Absolute Being physical coexists with moral necessity. Accordingly, if it is wished to depend on the moral conduct of man as on natural results, this conduct must become nature, and he must be led by natural impulse to such a course of action as can only and invariably have moral results. But the will of man is perfectly free between inclination and duty, and no physical necessity ought to enter as a sharer in this magisterial personality. If, therefore, he is to retain this power of solution, and yet become a reliable link in the causal concatenation of forces, this can only be effected when the operations of both these impulses are presented quite equally in the world of appearances. It is only possible when, with every difference of form, the matter of man's volition remains the same, when all his impulses agreeing with his reason are sufficient to have the value of a universal legislation."