The notion of duty is in itself already the notion of a constraint of the free elective will by the law; whether this constraint be an external one or be self-constraint. The moral imperative, by its categorical (the unconditional ought) announces this constraint, which therefore does not apply to all rational beings (for there may also be holy beings), but applies to men as rational physical beings who are unholy enough to be seduced by pleasure to the transgression of the moral law, although they themselves recognize its authority; and when they do obey it, to obey it unwillingly (with resistance of their inclination); and it is in this that the constraint properly consists. * Now, as man is a free (moral) being, the notion of duty can contain only self-constraint (by the idea of the law itself), when we look to the internal determination of the will (the spring), for thus only is it possible to combine that constraint (even if it were external) with the freedom of the elective will. The notion of duty then must be an ethical one. {INTRODUCTION ^paragraph 5} * Man, however, as at the same time a moral being, when he considers himself objectively, which he is qualified to do by his pure practical reason, (i.e. according to humanity in his own person), finds himself holy enough to transgress the law only unwillingly; for there is no man so depraved who in this transgression would not feel a resistance and an abhorrence of himself, so that he must put a force on himself. It is impossible to explain the phenomenon that at this parting of the ways (where the beautiful fable places Hercules between virtue and sensuality) man shows more propensity to obey inclination than the law. For, we can only explain what happens by tracing it to a cause according to physical laws; but then we should not be able to conceive the elective will as free. Now this mutually opposed self-constraint and the inevitability of it makes us recognize the incomprehensible property of freedom. The impulses of nature, then, contain hindrances to the fulfilment of duty in the mind of man, and resisting forces, some of them powerful; and he must judge himself able to combat these and to conquer them by means of reason, not in the future, but in the present, simultaneously with the thought; he must judge that he can do what the law unconditionally commands that he ought. Now the power and resolved purpose to resist a strong but unjust opponent is called fortitude (fortitudo), and when concerned with the opponent of the moral character within us, it is virtue (virtus, fortitudo moralis). Accordingly, general deontology, in that part which brings not external, but internal, freedom under laws is the doctrine of virtue. {INTRODUCTION ^paragraph 10}