Metaphysics of Ethics
When it is said of the perfection that belongs to man generally (properly speaking, to humanity), that it is in itself a duty to make this our end, it must be placed in that which may be the effect of one's deed, not in that which is merely an endowment for which we have to thank nature; for otherwise it would not be duty. Consequently, it can be nothing else than the cultivation of one's power (or natural capacity) and also of one's will (moral disposition) to satisfy the requirement of duty in general.
When it is said of the perfection that belongs to man generally (properly speaking, to humanity), that it is in itself a duty to make this our end, it must be placed in that which may be the effect of one's deed, not in that which is merely an endowment for which we have to thank nature; for otherwise it would not be duty. Consequently, it can be nothing else than the cultivation of one's power (or natural capacity) and also of one's will (moral disposition) to satisfy the requirement of duty in general.
