Ancient Greek philosophy was divided into three sciences: physics, ethics, and logic. This division is perfectly suitable to the nature of the thing; and the only improvement that can be made in it is to add the principle on which it is based, so that we may both satisfy ourselves of its completeness, and also be able to determine correctly the necessary subdivisions.
All rational knowledge is either material or formal: the former considers some object, the latter is concerned only with the form of the understanding and of the reason itself, and with the universal laws of thought in general without distinction of its objects. Formal philosophy is called logic. Material philosophy, however, which has to do with determinate objects and the laws to which they are subject, is again twofold; for these laws are either laws of nature or of freedom. The science of the former is physics, that of the latter, ethics; they are also called natural philosophy and moral philosophy respectively.
Logic cannot have any empirical part; that is, a part in which the universal and necessary laws of thought should rest on grounds taken from experience; otherwise it would not be logic, i.e., a canon for the understanding or the reason, valid for all thought, and capable of demonstration. Natural and moral philosophy, on the contrary, can each have their empirical part, since the former has to determine the laws of nature as an object of experience; the latter the laws of the human will, so far as it is affected by nature: the former, however, being laws according to which everything does happen; the latter, laws according to which everything ought to happen. Ethics, however, must also consider the conditions under which what ought to happen frequently does not.
We may call all philosophy empirical, so far as it is based on grounds of experience: on the other hand, that which delivers its doctrines from a priori principles alone we may call pure philosophy. When the latter is merely formal it is logic; if it is restricted to definite objects of the understanding it is metaphysic.
In this way there arises the idea of a twofold metaphysic- a metaphysic of nature and a metaphysic of morals. Physics will thus have an empirical and also a rational part. It is the same with Ethics; but here the empirical part might have the special name of practical anthropology, the name morality being appropriated to the rational part.
All trades, arts, and handiworks have gained by division of labour, namely, when, instead of one man doing everything, each confines himself to a certain kind of work distinct from others in the treatment it requires, so as to be able to perform it with greater facility and in the greatest perfection. Where the different kinds of work are not distinguished and divided, where everyone is a jack-of-all-trades, there manufactures remain still in the greatest barbarism. It might deserve to be considered whether pure philosophy in all its parts does not require a man specially devoted to it, and whether it would not be better for the whole business of science if those who, to please the tastes of the public, are wont to blend the rational and empirical elements together, mixed in all sorts of proportions unknown to themselves, and who call themselves independent thinkers, giving the name of minute philosophers to those who apply themselves to the rational part only- if these, I say, were warned not to carry on two employments together which differ widely in the treatment they demand, for each of which perhaps a special talent is required, and the combination of which in one person only produces bunglers. But I only ask here whether the nature of science does not require that we should always carefully separate the empirical from the rational part, and prefix to Physics proper (or empirical physics) a metaphysic of nature, and to practical anthropology a metaphysic of morals, which must be carefully cleared of everything empirical, so that we may know how much can be accomplished by pure reason in both cases, and from what sources it draws this its a priori teaching, and that whether the latter inquiry is conducted by all moralists (whose name is legion), or only by some who feel a calling thereto.
As my concern here is with moral philosophy, I limit the question suggested to this: Whether it is not of the utmost necessity to construct a pure thing which is only empirical and which belongs to anthropology? for that such a philosophy must be possible is evident from the common idea of duty and of the moral laws. Everyone must admit that if a law is to have moral force, i.e., to be the basis of an obligation, it must carry with it absolute necessity; that, for example, the precept, "Thou shalt not lie," is not valid for men alone, as if other rational beings had no need to observe it; and so with all the other moral laws properly so called; that, therefore, the basis of obligation must not be sought in the nature of man, or in the circumstances in the world in which he is placed, but a priori simply in the conception of pure reason; and although any other precept which is founded on principles of mere experience may be in certain respects universal, yet in as far as it rests even in the least degree on an empirical basis, perhaps only as to a motive, such a precept, while it may be a practical rule, can never be called a moral law.
Thus not only are moral laws with their principles essentially distinguished from every other kind of practical knowledge in which there is anything empirical, but all moral philosophy rests wholly on its pure part. When applied to man, it does not borrow the least thing from the knowledge of man himself (anthropology), but gives laws a priori to him as a rational being. No doubt these laws require a judgement sharpened by experience, in order on the one hand to distinguish in what cases they are applicable, and on the other to procure for them access to the will of the man and effectual influence on conduct; since man is acted on by so many inclinations that, though capable of the idea of a practical pure reason, he is not so easily able to make it effective in concreto in his life.
A metaphysic of morals is therefore indispensably necessary, not merely for speculative reasons, in order to investigate the sources of the practical principles which are to be found a priori in our reason, but also because morals themselves are liable to all sorts of corruption, as long as we are without that clue and supreme canon by which to estimate them correctly. For in order that an action should be morally good, it is not enough that it conform to the moral law, but it must also be done for the sake of the law, otherwise that conformity is only very contingent and uncertain; since a principle which is not moral, although it may now and then produce actions conformable to the law, will also often produce actions which contradict it. Now it is only in a pure philosophy that we can look for the moral law in its purity and genuineness (and, in a practical matter, this is of the utmost consequence): we must, therefore, begin with pure philosophy (metaphysic), and without it there cannot be any moral philosophy at all. That which mingles these pure principles with the empirical does not deserve the name of philosophy (for what distinguishes philosophy from common rational knowledge is that it treats in separate sciences what the latter only comprehends confusedly); much less does it deserve that of moral philosophy, since by this confusion it even spoils the purity of morals themselves, and counteracts its own end.
Let it not be thought, however, that what is here demanded is already extant in the propaedeutic prefixed by the celebrated Wolf to his moral philosophy, namely, his so-called general practical philosophy, and that, therefore, we have not to strike into an entirely new field. Just because it was to be a general practical philosophy, it has not taken into consideration a will of any particular kind- say one which should be determined solely from a priori principles without any empirical motives, and which we might call a pure will, but volition in general, with all the actions and conditions which belong to it in this general signification. By this it is distinguished from a metaphysic of morals, just as general logic, which treats of the acts and canons of thought in general, is distinguished from transcendental philosophy, which treats of the particular acts and canons of pure thought, i.e., that whose cognitions are altogether a priori. For the metaphysic of morals has to examine the idea and the principles of a possible pure will, and not the acts and conditions of human volition generally, which for the most part are drawn from psychology. It is true that moral laws and duty are spoken of in the general moral philosophy (contrary indeed to all fitness). But this is no objection, for in this respect also the authors of that science remain true to their idea of it; they do not distinguish the motives which are prescribed as such by reason alone altogether a priori, and which are properly moral, from the empirical motives which the understanding raises to general conceptions merely by comparison of experiences; but, without noticing the difference of their sources, and looking on them all as homogeneous, they consider only their greater or less amount. It is in this way they frame their notion of obligation, which, though anything but moral, is all that can be attained in a philosophy which passes no judgement at all on the origin of all possible practical concepts, whether they are a priori, or only a posteriori.
Intending to publish hereafter a metaphysic of morals, I issue in the first instance these fundamental principles. Indeed there is properly no other foundation for it than the critical examination of a pure practical reason; just as that of metaphysics is the critical examination of the pure speculative reason, already published. But in the first place the former is not so absolutely necessary as the latter, because in moral concerns human reason can easily be brought to a high degree of correctness and completeness, even in the commonest understanding, while on the contrary in its theoretic but pure use it is wholly dialectical; and in the second place if the critique of a pure practical Reason is to be complete, it must be possible at the same time to show its identity with the speculative reason in a common principle, for it can ultimately be only one and the same reason which has to be distinguished merely in its application. I could not, however, bring it to such completeness here, without introducing considerations of a wholly different kind, which would be perplexing to the reader. On this account I have adopted the title of Fundamental Principles of the Metaphysic of Morals instead of that of a Critical Examination of the pure practical reason.
But in the third place, since a metaphysic of morals, in spite of the discouraging title, is yet capable of being presented in popular form, and one adapted to the common understanding, I find it useful to separate from it this preliminary treatise on its fundamental principles, in order that I may not hereafter have need to introduce these necessarily subtle discussions into a book of a more simple character.
The present treatise is, however, nothing more than the investigation and establishment of the supreme principle of morality, and this alone constitutes a study complete in itself and one which ought to be kept apart from every other moral investigation. No doubt my conclusions on this weighty question, which has hitherto been very unsatisfactorily examined, would receive much light from the application of the same principle to the whole system, and would be greatly confirmed by the adequacy which it exhibits throughout; but I must forego this advantage, which indeed would be after all more gratifying than useful, since the easy applicability of a principle and its apparent adequacy give no very certain proof of its soundness, but rather inspire a certain partiality, which prevents us from examining and estimating it strictly in itself and without regard to consequences.