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Source Critique of Practical Reason, Book I, Chapter I, Theorem I, 5:22

All practical principles that presuppose an object (matter) of the faculty of desire as the determining ground of the will are, without exception, empirical and can furnish no practical laws.

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    The answer to any question along the lines of “What did Kant mean by…..” is almost always unknown which is why millions of people are still trying to understand his writings. The vagueness of his writings are either seen as profound and revolutionary or as a trick in making meaninglessness seem profound. You decide which :) Commented Apr 7 at 4:43
  • the nerve to call this wordsalad bs a "theorem"...
    – ac15
    Commented Apr 7 at 4:45
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    "Supposing that pure reason contains in itself a practical motive, that is, one adequate to determine the will, then there are practical laws; otherwise all practical principles will be mere maxims." Kant has a very high standard for what he considers a "law". As he sees it, all "practical laws", if any, must be a priori and come from pure reason in its practical application. Desires explicitly refer to appearances (objects from experience), hence they are empirical and not a priori, hence they cannot furnish any "practical laws".
    – Conifold
    Commented Apr 7 at 6:13
  • The key is Kant's use of desire mentioned here are meant to be colloquial which may not align with other philosophers' usage, and thus this could be said to be a deep and central passage in Kantian categorical imperative of practical laws as opposed to the mundanely conceived empirical hypothetical imperative... Commented Apr 7 at 6:26
  • Well I agree that colloquialism is sometimes unavoidable, I think Kant is using an inherited technical philosophical language that's coming from Christian Wolff which he further modifies for his own use. In other words, I don't think he's using a colloquial language but rather technical philosophical language
    – Gerry
    Commented Apr 7 at 21:18

1 Answer 1

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Kant has argued in his Groundwork for the Metaphysics of Morals that a practical law must be devoid of all (empirical) content and contain "only the form of a law".

The whole argumentation can be read step by step in this answer of mine.

Thus, given the contents of the faculty of desire are empirical, they cannot give practical laws. The question remains why they are empirical. Here, one has to read carefully since it says

an object (matter) of the faculty of desire

The second critique is all about why the moral law itself is the only 'object' (and only of practical reason in a very peculiar, unique sense) that is given to man not from the outside, through the senses (thus empirically). Hence, every object given to the faculty of desire is, in his philosophy, empirical by definition.

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  • I think I see, it is not that practical reason is without objective, but that a particular kind of objective, what Kant refers to as empirical, is not moral as it is in some way incompatible with the form of practical reason.
    – Gerry
    Commented Apr 13 at 4:05
  • @Gerry Almost. If practical reason is moral, its objective is not empirical. On the other hand, we follow desires and empirical objectives all the time, and do so using practical reason. Thus, moral behaviour, in a sense, is special because we use practical reason alone, without input from anywhere else. That this is possible is what the second critique aims to show.
    – Philip Klöcking
    Commented Apr 13 at 11:06
  • My fault, I should have punctuated better. There needs to be a period between ". . . a particular kind of objective " and "what Kant refers to. . ." So if the objective is not empirical, what kind of objective is it? It could be the moral law itself AND/OR an empirical objective that is secondary and compatible with the form, not the basis of the moral law, but an objective the moral law can act upon because it is not in conflict with the form, e.g., humanity as a end.
    – Gerry
    Commented Apr 13 at 23:00

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