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II.: The Idea and Necessity of a Metaphysic of Morals. - Immanuel Kant, The Philosophy of Law: An Exposition of the Fundamental Principles of Jurisprudence as the Science of Right 
The Philosophy of Law: An Exposition of the Fundamental Principles of Jurisprudence as the Science of Right, by Immanuel Kant, trans. W. Hastie (Edinburgh: Clark, 1887).
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The Idea and Necessity of a Metaphysic of Morals.
The Laws of Nature Rational and also Empirical.—It has been shown in The Metaphysical Principles of the Science of Nature, that there must be Principles à priori for the Natural Science that has to deal with the objects of the external senses. And it was further shown that it is possible, and even necessary, to formulate a System of these Principles under the name of a ‘Metaphysical Science of Nature,’ as a preliminary to Experimental Physics regarded as Natural Science applied to particular objects of experience. But this latter Science, if care be taken to keep its generalizations free from error, may accept many propositions as universal on the evidence of experience, although if the term ‘Universal’ be taken in its strict sense, these would necessarily have to be deduced by the Metaphysical Science from Principles à priori. Thus Newton accepted the principle of the Equality of Action and Reaction as established by experience, and yet he extended it as a universal Law over the whole of material Nature. The Chemists go even farther, grounding their most general Laws regarding the combination and decomposition of the materials of bodies wholly upon experience; and yet they trust so completely to the Universality and Necessity of those laws, that they have no anxiety as to any error being found in propositions founded upon experiments conducted in accordance with them.
Moral Laws à priori and Necessary.—But it is otherwise with Moral Laws. These, in contradistinction to Natural Laws, are only valid as Laws, in so far as they can be rationally established à priori and comprehended as necessary. In fact, conceptions and judgments regarding ourselves and our conduct have no moral significance, if they contain only what may be learned from experience; and when any one is, so to speak, misled into making a Moral Principle out of anything derived from this latter source, he is already in danger of falling into the coarsest and most fatal errors.
If the Philosophy of Morals were nothing more than a Theory of Happiness (Eudæmonism), it would be absurd to search after Principles à priori as a foundation for it. For however plausible it may sound to say that Reason, even prior to experience, can comprehend by what means we may attain to a lasting enjoyment of the real pleasures of life, yet all that is taught on this subject à priori is either tautological, or is assumed wholly without foundation. It is only Experience that can show what will bring us enjoyment. The natural impulses directed towards nourishment, the sexual instinct, or the tendency to rest and motion, as well as the higher desires of honour, the acquisition of knowledge, and such like, as developed with our natural capacities, are alone capable of showing in what those enjoyments are to be found. And, further, the knowledge thus acquired, is available for each individual merely in his own way; and it is only thus he can learn the means by which he has to seek those enjoyments. All specious rationalizing à priori, in this connection, is nothing at bottom but carrying facts of Experience up to generalizations by induction (secundum principia generalia non universalia); and the generality thus attained is still so limited that numberless exceptions must be allowed to every individual in order that he may adapt the choice of his mode of life to his own particular inclinations and his capacity for pleasure. And, after all, the individual has really to acquire his Prudence at the cost of his own suffering or that of his neighbours.
But it is quite otherwise with the Principles of Morality. They lay down Commands for every one without regard to his particular inclinations, and merely because and so far as he is free, and has a practical Reason. Instruction in the Laws of Morality is not drawn from observation of oneself or of our animal nature, nor from perception of the course of the world in regard to what happens, or how men act.1 But Reason commands how we ought to act, even although no example of such action were to be found; nor does Reason give any regard to the Advantage which may accrue to us by so acting, and which Experience could alone actually show. For, although Reason allows us to seek what is for our advantage in every possible way, and although, founding upon the evidence of Experience, it may further promise that greater advantages will probably follow on the average from the observance of her commands than from their transgression, especially if Prudence guides the conduct, yet the authority of her precepts as Commands does not rest on such considerations. They are used by Reason only as Counsels, and by way of a counterpoise against seductions to an opposite course, when adjusting beforehand the equilibrium of a partial balance in the sphere of Practical Judgment, in order thereby to secure the decision of this Judgment, according to the due weight of the à priori Principles of a pure Practical Reason.
The Necessity of a Metaphysic of Morals. —‘Metaphysics’ designates any System of Knowledge à priori that consists of pure Conceptions. Accordingly a Practical Philosophy not having Nature, but the Freedom of the Will for its object, will presuppose and require a Metaphysic of Morals. It is even a Duty to have such a Metaphysic; and every man does, indeed, possess it in himself, although commonly but in an obscure way. For how could any one believe that he has a source of universal Law in himself, without Principles à priori? And just as in a Metaphysic of Nature there must be principles regulating the application of the universal supreme Principles of Nature to objects of Experience, so there cannot but be such principles in the Metaphysic of Morals; and we will often have to deal objectively with the particular nature of man as known only by Experience, in order to show in it the consequences of these universal Moral Principles. But this mode of dealing with these Principles in their particular applications will in no way detract from their rational purity, or throw doubt on their à priori origin. In other words, this amounts to saying that a Metaphysic of Morals cannot be founded on Anthropology as the Empirical Science of Man, but may be applied to it.
Moral Anthropology.—The counterpart of a Metaphysic of Morals, and the other member of the Division of Practical Philosophy, would be a Moral Anthropology, as the Empirical Science of the Moral Nature of Man. This Science would contain only the subjective conditions that hinder or favour the realization in practice of the universal moral Laws in human Nature, with the means of propagating, spreading, and strengthening the Moral Principles,—as by the Education of the young and the instruction of the people,—and all other such doctrines and precepts founded upon experience and indispensable in themselves, although they must neither precede the metaphysical investigation of the Principles of Reason, nor be mixed up with it. For, by doing so, there would be a great danger of laying down false, or at least very flexible Moral Laws, which would hold forth as unattainable what is not attained only because the Law has not been comprehended and presented in its purity, in which also its strength consists. Or, otherwise, spurious and mixed motives might be adopted instead of what is dutiful and good in itself; and these would furnish no certain Moral Principles either for the guidance of the Judgment or for the discipline of the heart in the practice of Duty. It is only by Pure Reason, therefore, that Duty can and must be prescribed.
Practical Philosophy in relation to Art.—The higher Division of Philosophy, under which the Division just mentioned stands, is into Theoretical Philosophy and Practical Philosophy. Practical Philosophy is just Moral Philosophy in its widest sense, as has been explained elsewhere.1 All that is practicable and possible, according to Natural Laws, is the special subject of the activity of Art, and its precepts and rules entirely depend on the Theory of Nature. It is only what is practicable according to Laws of Freedom that can have Principles independent of Theory, for there is no Theory in relation to what passes beyond the determinations of Nature. Philosophy therefore cannot embrace under its practical Division a technical Theory, but only a morally practical Doctrine. But if the dexterity of the Will in acting according to Laws of Freedom, in contradistinction to Nature, were to be also called an Art, it would necessarily indicate an Art which would make a System of Freedom possible like the System of Nature. This would truly be a Divine Art, if we were in a position by means of it to realize completely what Reason prescribes to us, and to put the Idea into practice.
[1 ]This holds notwithstanding the fact that the term ‘Morals,’ in Latin Mores, and in German Sitten, signifies originally only Manners or Mode of Life.
[1 ]In the Critique of the Judgment (1790).