From Immanuel Kant Metaphysics of Morals

Obligation is the necessity of a free action when viewed in relation to a categorical imperative of reason. An imperative is a practical rule by which an action, otherwise contingent in itself, is made necessary. It is distinguished from a practical law in that such a law, while likewise representing the action as necessary, does not consider whether it is internally necessary as involved in the nature of the agent -- say as a holy being -- or is contingent to him, as in the case of man as we find him; for where the first condition holds good, there is in fact no imperative. Hence an imperative is a rule which not only represents but makes a subjectively contingent action necessary; and it, accordingly, represents the subject as being (morally) necessitated to act in accordance with this rule. A categorical or unconditional imperative is one which does not represent the action in any way immediately through the conception of an end that is to be attained by it; but it presents the action to the mind as objectively necessary by the mere representation of its form as an action, and thus makes it necessary. Such imperatives cannot be put forward by any other practical science than that which prescribes obligations, and it is only the science of morals that does this. All other imperatives are technical, and they are altogether conditional. The ground of the possibility of categorical imperatives lies in the fact that they refer to no determination of the activity of the will by which a purpose might be assigned to it, but solely to its freedom.

Every action is allowed (licitum) which is not contrary to obligation; and this freedom not being limited by an opposing imperative, constitutes a moral right as a warrant or title of action (facultas moralis). From this it is at once evident what actions are disallowed or illicit (illicita).

Duty is the designation of any action to which anyone is bound by an obligation. It is therefore the subject-matter of all obligation. Duty as regards the action concerned may be one and the same, and yet we may be bound to it in various ways.

The categorical imperative, as expressing an obligation in respect to certain actions, is a morally practical law. But because obligation involves not merely practical necessity expressed in a law as such, but also actual necessitation, the categorical imperative is a law either of command or prohibition, according as the doing or not doing of an action is represented as a duty. An action which is neither commanded nor forbidden is merely allowed, because there is no law restricting freedom, nor any duty in respect of it. Such an action is said to be morally indifferent (indifferens, adiaphoron, res merae facultatis). It may be asked whether there are such morally indifferent actions; and if there are, whether in addition to the preceptive and prohibitive law (lex praeceptiva et prohibitiva, lex mandati et vetiti), there is also required a permissive law (lex permissiva), in order that one may be free in such relations to act, or to forbear from acting, at his pleasure? If it were so, the moral right in question would not, in all cases, refer to actions that are indifferent in themselves (adiaphora); for no special law would be required to establish such a right, considered according to moral laws.

An action is called an act -- or moral deed -- in so far as it is subject to laws of obligation, and consequently in so far as the subject of it is regarded with reference to the freedom of his choice in the exercise of his will. The agent -- as the actor or doer of the deed -- is regarded as, through the act, the author of its effect; and this effect, along with the action itself, may be imputed to him, if be previously knew the law in virtue of which an obligation rested upon him.

A person is a subject who is capable of having his actions imputed to him. Moral personality is, therefore, nothing but the freedom of a rational being under moral laws; and it is to be distinguished from psychological freedom as the mere faculty by which we become conscious of ourselves in different states of the identity of our existence. Hence it follows that a person is properly subject to no other laws than those he lays down for himself, either alone or in conjunction with others.

A thing is what is incapable of being the subject of imputation. Every object of the free activity of the will, which is itself void of freedom, is therefore called a thing (res corporealis).

Right or wrong applies, as a general quality, to an act (rectum aut minus rectum), in so far as it is in accordance with duty or contrary to duty (factum licitum aut illicitum), no matter what may be the subject or origin of the duty itself. An act that is contrary to duty is called a transgression (reatus).

An unintentional transgression of a duty, which is, nevertheless, imputable to a person, is called a mere fault (culpa). An intentional transgression -- that is, an act accompanied with the consciousness that it is a transgression -- constitutes a crime (dolus).

Whatever is juridically in accordance with external laws is said to be just (jus, iustum); and whatever is not juridically in accordance with external laws is unjust (unjustum).

A collision of duties or obligations (collisio officiorum s. obligationum) would be the result of such a relation between them that the one would annul the other, in whole or in part. Duty and obligation, however, are conceptions which express the objective practical necessity of certain actions, and two opposite rules cannot be objective and necessary at the same time; for if it is a duty to act according to one of them, it is not only no duty to act according to an opposite rule, but to do so would even be contrary to duty. Hence a collision of duties and obligations is entirely inconceivable (obligationes non colliduntur). There may, however, be two grounds of obligation (rationes obligandi), connected with an individual under a rule prescribed for himself, and yet neither the one nor the other may be sufficient to constitute an actual obligation (rationes obligandi non obligantes); and in that case the one of them is not a duty. If two such grounds of obligation are actually in collision with each other, practical philosophy does not say that the stronger obligation is to keep the upper hand (fortior obligatio vincit), but that the stronger ground of obligation is to maintain its place (fortior obligandi ratio vincit).

Obligatory Laws for which an external legislation is possible are called generally external laws. Those external laws, the obligatoriness of which can be recognised by reason a priori even without an external legislation, are called natural laws. Those laws, again, which are not obligatory without actual external legislation, are called positive laws. An external legislation, containing pure natural laws, is therefore conceivable; but in that case a previous natural law must be presupposed to establish the authority of the lawgiver by the right to subject others to obligation through his own act of will.

The principle which makes a certain action a duty is a practical law. The rule of the agent or actor, which he forms as a principle for himself on subjective grounds, is called his maxim. Hence, even when the law is one and invariable, the maxims of the agent may yet be very different.

The categorical imperative only expresses generally what constitutes obligation. It may be rendered by the following formula: "Act according to a maxim which can be adopted at the same time as a universal law." Actions must therefore be considered, in the first place, according to their subjective principle; but whether this principle is also valid objectively can only be known by the criterion of the categorical imperative. For reason brings the principle or maxim of any action to the test, by calling upon the agent to think of himself in connection with it as at the same time laying down a universal law, and to consider whether his action is so qualified as to be fit for entering into such a universal legislation.

The simplicity of this law, in comparison with the great and manifold consequences which may be drawn from it, as well as its commanding authority and supremacy without the accompaniment of any visible motive or sanction, must certainly at first appear very surprising. And we may well wonder at the power of our reason to determine the activity of the will by the mere idea of the qualification of a maxim for the universality of a practical law, especially when we are taught thereby that this practical moral law first reveals a property of the will which the speculative reason would never have come upon either by principles a priori, or from any experience whatever; and even if it had ascertained the fact, it could never have theoretically established its possibility. This practical law, however, not only discovers the fact of that property of the will, which is freedom, but irrefutably establishes it. Hence it will be less surprising to find that the moral laws are undemonstrable, and yet apodeictic, like the mathematical postulates; and that they, at the same time, open up before us a whole field of practical knowledge, from which reason, on its theoretical side, must find itself entirely excluded with its speculative idea of freedom and all such ideas of the supersensible generally.

The conformity of an action to the law of duty constitutes its legality; the conformity of the maxim of the action with the law constitutes its morality. A maxim is thus a subjective principle of action, which the individual makes a rule for himself as to how in fact he will act.

On the other hand, the principle of duty is what reason absolutely, and therefore objectively and universally, lays down in the form of a command to the individual, as to how he ought to act.

The supreme principle of the science of morals accordingly is this: "Act according to a maxim which can likewise be valid as a universal law." Every maxim which is not qualified according to this condition is contrary to Morality.

Laws arise from the will, viewed generally as practical reason; maxims spring from the activity of the will in the process of choice. The latter in man is what constitutes free-will. The will which refers to nothing else than mere law can neither be called free nor not free, because it does not relate to actions immediately, but to the giving of a law for the maxim of actions; it is therefore the practical reason itself. Hence as a faculty, it is absolutely necessary in itself, and is not subject to any external necessitation. It is, therefore, only the act of choice in the voluntary process that can be called free.

The degree of the imputability of actions is to be reckoned according to the magnitude of the hindrances or obstacles which it has been necessary for them to overcome. The greater the natural hindrances in the sphere of sense, and the less the moral hindrance of duty, so much the more is a good deed imputed as meritorious. This may be seen by considering such examples as rescuing a man who is an entire stranger from great distress, and at very considerable sacrifice. Conversely, the less the natural hindrance, and the greater the hindrance on the ground of duty, so much the more is a transgression imputable as culpable. Hence the state of mind of the agent or doer of a deed makes a difference in imputing its consequences, according as he did it in passion or performed it with coolness and deliberation.