In the modern philosophical culture the concept of duty plays a dominant role in the ethics of Kant. Hence duty becomes specifically not only a lawful action of reason, but also an act undertaken in view and in accordance with that law. Thus there is a clear distinction between action compliant with the law and moral action or duty, i.e. action taken by the law, that is, apart from the natural inclinations and often in opposition with them. The ability to act for duty becomes the witness and expression of human freedom as "autonomy" or obedience to the inner law of reason.
Kant defines, in fact, a moral duty as independent (since it does not come from external sources) and categorical (valid in itself and not to the achievement of other purposes): any motivation or utilitarian purpose corrupts the moral act in its purity. He understands duty as freedom of a rational being who questions and obliges himself, thereby linking closely duty to the essence of morality.
At the end of the nineteenth century the concept of duty was subjected to harsh criticism. First, as part of a utilitarian morality, from Bentham, it has replaced the concept of duty with that of interest, and as a result the duties towards themselves or others have become acts in the name of an individual or social interest.
But the main attack came from Nietzsche, in whose pages the criticism of the idea of duty coincides with the exaltation of the superman, who imposes its morality of an hero and does not accept a preconceived ethical and universal order. To the morality of duty, Nietzsche replaces that of the will (Thus Spoke Zarathustra, 1892): the superman denies traditional and universal values and asserts his own freedom and the will of power, thus re-establishing a new state of innocence which initiates a New era.
In the Critique of Practical Reason Kant deals with duty and argues that morality must be based on something absolutely certain and firm: duty. Everyone perceives morality, safely and aware, as a duty. Man, endowed with reason, the one with reason, feels in front of certain situations of having to make a choice, to be followed by the moral behavior. Even the most wicked men, who still retain at least part of rationality, will feel having to pose the problem of moral choice, or how to behave. This is the moment that precedes any real moral action.
Morality is thus a matter of reason. Every rational being has morality, because he feels the duty and the need to choose. The duty has nothing to do with causality and determinism of the material world: it concerns only the sphere of morality.
But if morality is duty, then how will compulsion be reconciled with the absolute formal freedom of choice? The answer lies in the concept of autonomy. The morality of the rational being is such that he must obey a command (mandatory) that he has freely given (freedom), in accordance with his rational nature.
Man who performs a certain action according to the moral duty knows that, in so far as his decision can be explained naturally (also with psychological motivations), the real substance of his morality does not lie in this causal chain but in a free will that corresponds to the rational essence of his being.
Man, in short, is a 'being' belonging to two worlds: inasmuch is gifted with sensory capabilities, he belongs to the natural one, and therefore is subject to the phenomenal laws; as a rational creature, however, he belongs to what Kant calls the "intelligible" world or noumenon, that is, the world as it is in itself independently of our feelings or our cognitive ties, and therefore in it he is absolutely free (autonomous) , a freedom manifested in obedience to the moral law, to the '’categorical imperative".
The analytical philosopher Anscombe recognizes as empty and meaningless the various 'boxes' in which to enter her concepts of" obligation " " duty, " " right, " " wrong "(in a moral sense). According to her theory, in fact, the concepts of obligation and duty exist only as psychological survival, because they are based on a conception of ethics, grounded on the belief in a divine lawmaker, no longer existing. The concept of "moral duty" was intrinsically tied to a certain ethical conception that, by virtue of the beliefs and practices that characterized it, meant that it took on a special meaning, regulatory, and clearly intelligible.
The issue is that the "pseudo notion” of moral obligation, on the one hand, shares its characteristics with the correct concept which appears in legalistic conception of ethics, but at the same time does not qualify for the background, intended as a set of practices and thought, necessary for the intelligibility of a concept with these features. That of the concept of moral duty is a case where we transfer an expression from one context to another without affecting the use completely but at most some meaning, which is however insufficient to the intelligibility of the concept.