Central elements in Kant's ethic are:
 --denial of consequentialist considerations as relevant to right or wrong
 --notion of obligation or duty to the moral law as of sole moral worth
 --developing concept of the rational will as a central human characteristics that enables knowledge of the moral law (vs. the heteronomous will)
 --the categorical imperative as the command of the rational will that provides the form of that law

Kant parallels moral law to physical law in the sense that both lay claim to:
 --universality: it is true everywhere for everyone, i.e.  binding on all agents
 --necessity: it is always true i.e. is binding on every occasion
 --a priori: it is neither gained from nor falsified by experience (controversial in physics)

But moral law is unlike physical law in that its laws are prescriptive rather than descriptive, i.e. they tell us what should be done. Indeed, Kant claims that even if nobody ever performed a morally worthy act we would still know whether some sort of intended action was of moral worth

Foundation of the Metaphysic of Morals

Kant believes he is making explicit what we already in a sense know: ask yourself about an action "What if everyone did that"?
 --yet the deontologist does not focus on consequences
 --rather, the point is to try to will the universality and necessity of an act consistently with reason: an immoral act is one that is contradictory or incoherent when universalized

From this sort of consideration, the rational will issues the categorical imperative, a maxim ( or rule ) whose general form is: "I can universally will that in a moral community people perform action x"
 --the act is evaluated not by its consequences but by its motive, i.e. by the rational will (not just any motive such as inclination but only by the rational will)
 --a good will (i.e. the rational will) is all that matters morally

An a priori knowledge of ethics is assumed when making a moral judgement, which is evident to us in our everyday concept of duty:
 --duty commands necessarily and without exception, for all rational beings
 --duty is an a priori concept, i.e. cannot be known from experience yet is present always in experience
 --(the concept of the "a priori" is an element in Kant's critical philosophy: given what we know about the world, we discover the underlying structure of the mind and how it conditions all possible experience)
 --all we can know from experience is what people do, not what they should do, yet we know that there are things that people should do: thus such knowledge is a priori
 --duty involves universality and necessity, which are not empirically given but are essential to the notion of law and unlike physical law it involves values, not facts
 --so, the foundation of ethics is uncovered in duty which embodies these features of universality and necessity
 --rational ethics seeks the a priori element of morality which duty, not experience, provides
 --duty consists in letting reason be the motive and guide in all our actions, not acting according to inclination or natural disposition which are unreliable and partisan

So, an action may be done (1) from inclination and be inconsistent with duty; (2) from inclination and be consistent with duty; or (3) from duty and of course consistent with duty: only the last (#3) is a morally worthy act. One cannot logically do one's duty from inclination.

Categorical Imperative (CI): There are three formulations that Kant gives for the CI. The first is the

1. Natural Law formulation:
--"Act only on that maxim whereby thou canst at the same time will that it should become a universal law"
 --it is construed as a test of the rightness of an act
 --note the natural law phrasing, capturing the universality and necessity of law: choose as though your choice has the power to transform the world so that your action is an universal law
 --it ties morality to formal characteristics of rationality, universality, and necessity and denies the moral worth of expectations of good outcomes or anything contingent
 --basically it prohibits us from letting inclination guide our actions; willing is all and the rational will always appeals to the CI

Kant divides moral imperatives that result from the CI into two sorts, each of which may involve a duty to self or a duty to others:
 --perfect duties:
  --those binding on all rational beings under all circumstances without exception
  --to will the opposite results in a logical contradiction when CI is applied
 --imperfect duties:
  --those binding on all rational beings but not required on all occasions, i.e they admit exceptions as to when (but not whether) they are fulfilled
  --to do otherwise on a given occasion does not result in a logical contradiction, but a contradiction in rational agency: a rational agent could not will that they never be fulfilled
Examples:
 --perfect duty to self: refrain from suicide
 --perfect duty to others: repaying debts
 --imperfect duty to self: developing one's talents
 --imperfect duty to others: giving to charity

2. Persons as Ends formulation--the second formulation of the CI is:
--"So act as to treat humanity, whether in thine own person or in that of any other, in every case as an end withal, never as a means only"
 --when we try to make "exceptions" of ourselves by engaging in immoral practices, i.e. when we follow the will of inclination rather than the rational will, not only do we violate the universality of the CI, but we effectively are willing a world in which others are means to our ends.
 --this formulation of the CI (which Kant believes to be equivalent to the first) captures the sense of the unconditional worth of the human being
 --the CI is grounded in the human being's rational will which gives human beings this unconditioned worth; to treat a human being as a means is to recognize in them only a hypothetical worth as an instrumental means, not as a subject possessing inherent worth, i.e. an end.
 --it is generally felt that this formulation most directly establishes the absoluteness that is necessary to ground rights, although each formulation does at least indirectly --examples of perfect & imperfect duties to self and others, derived from this formulation, are furnished on pp. 61-2.

3. Kingdom of Ends formulation--the third formulation of the CI is:
--"Therefore every rational being must so act as if he were by his maxims in every case a legislating member in the universal kingdom of ends"
 --this means that one must take no action that is inconsistent with it being a law followed by all rational agents: collectively, a kingdom of ends
 --a kingdom of ends is composed of rational beings who are able to and simultaneously must will so as to legislate for all rational agents
 --since freedom is a necessary condition for any willful action, Kant, by this formulation, seeks to reconcile the freedom, necessary for willing, with the notion of following an imperative or command
 --it is only insofar as we are simultaneously givers and receivers of the law that the imperative is not imposed or coercive but freely issued and obeyed by the rational agent
 --so, acting heteronomously (conditionally, with regard to consequences, habits, inclinations, restraints etc.) is not autonomous action and not moral action: one must be governed by a law that one freely imposes on oneself (and all rational agents)
 --thus freedom and law are unified in this formulation: the moral world is law governed, as in the physical world, but not deterministic insofar as it presumes autonomy which, again, is necessary for truly moral action.

PROBLEMS
 --Kant thinks that each formulation entails the same duties as the other two but it isn't clear that this is true, and he supports them with different examples
 --no guidelines are offered as to when or how often we must perform imperfect duties
 --no decision making criterion is offered to determine a rational imperative when perfect duties conflict: they all seem inviolable yet they can and do come into conflict. We need a way to prioritize them to establish which are more important than others and it isn't clear that Kant's theory can generate such a criterion.