The Critique of Practical Reason · Immanuel Kant
REMARK II.
For it is every man's own special feeling of pleasure and pain that decides in what he is to place his happiness, and even in the same subject this will vary with the difference of his wants according as this feeling changes, and thus a law which is subjectively necessary (as a law of nature) is objectively a very contingent practical principle, which can and must be very different in different subjects and therefore can never furnish a law; since, in the desire for happiness it is not the form (of conformity to law) that is decisive, but simply the matter, namely, whether I am to expect pleasure in following the law, and how much. Principles of self-love may, indeed, contain universal precepts of skill (how to find means to accomplish one's purpose), but in that case they are merely theoretical principles; * as, for example, how he who would like to eat bread should contrive a mill; but practical precepts founded on them can never be universal, for the determining principle of the desire is based on the feeling pleasure and pain, which can never be supposed to be universally directed to the same objects. * Propositions which in mathematics or physics are called practical ought properly to be called technical. For they have nothing to do with the determination of the will; they only point out how a certain effect is to be produced and are, therefore, just as theoretical as any propositions which express the connection of a cause with an effect. Now whoever chooses the effect must also choose the cause.
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