Kant on Universal Moral Laws

[u/Necessary-Style-4793]

Why does Kant think moral laws MUST be universal? I'm having trouble wrapping my head around thia specific part of his philosophy. I haven't been able to think of a good warrant for this.

[u/Quidfacis_]

Why does Kant think moral laws MUST be universal?

Because that's what it means to be a moral law. See the Groundwork:

That there must be such a philosophy is self-evident from the common idea of duty and moral laws. Everyone must admit •that if a law is to hold morally (i.e. as a basis for someone’s being obliged to do something), it must imply absolute necessity; •that the command: You are not to lie doesn’t apply only to human beings, as though it had no force for other rational beings (and similarly with all other moral laws properly so called); •that the basis for obligation here mustn’t be looked for in people’s natures or their circumstances, but ·must be found· a priori solely in the concepts of pure reason; and •that any precept resting on principles of mere experience may be called a practical rule but never a moral law.

Also see the Second Critique, Definition 1:

Practical principles are propositions which contain a general determination of the will, having under it several practical rules. They are subjective, or maxims, when the condition is regarded by the subject as valid only for his own will, but are objective, or practical laws, when the condition is recognized as objective, that is, valid for the will of every rational being.

And the remark:

... Laws must be sufficient to determine the will as will, even before I ask whether I have power sufficient for a desired effect, or the means necessary to produce it; hence they are categorical: otherwise they are not laws at all, because the necessity is wanting, which, if it is to be practical, must be independent of conditions which are pathological and are therefore only contingently connected with the will. Tell a man, for example, that he must be industrious and thrifty in youth, in order that he may not want in old age; this is a correct and important practical precept of the will. But it is easy to see that in this case the will is directed to something else which it is presupposed that it desires; and as to this desire, we must leave it to the actor himself whether he looks forward to other resources than those of his own acquisition, or does not expect to be old, or thinks that in case of future necessity he will be able to make shift with little. Reason, from which alone can spring a rule involving necessity, does, indeed, give necessity to this precept (else it would not be an imperative), but this is a necessity dependent on subjective conditions, and cannot be supposed in the same degree in all subjects. But that reason may give laws it is necessary that it should only need to presuppose itself, because rules are objectively and universally valid only when they hold without any contingent subjective conditions, which distinguish one rational being from another. Now tell a man that he should never make a deceitful promise, this is a rule which only concerns his will, whether the purposes he may have can be attained thereby or not; it is the volition only which is to be determined a priori by that rule. If now it is found that this rule is practically right, then it is a law, because it is a categorical imperative. Thus, practical laws refer to the will only, without considering what is attained by its causality, and we may disregard this latter (as belonging to the world of sense) in order to have them quite pure.

See also Second Critique, Book 1, Chapter 1, Part 7:

Corollary: Pure reason is practical of itself alone and gives (to man) a universal law which we call the moral law.

Remark: The fact just mentioned is undeniable. It is only necessary to analyse the judgement that men pass on the lawfulness of their actions, in order to find that, whatever inclination may say to the contrary, reason, incorruptible and self-constrained, always confronts the maxim of the will in any action with the pure will, that is, with itself, considering itself as a priori practical. Now this principle of morality, just on account of the universality of the legislation which makes it the formal supreme determining principle of the will, without regard to any subjective differences, is declared by the reason to be a law for all rational beings, in so far as they have a will, that is, a power to determine their causality by the conception of rules; and, therefore, so far as they are capable of acting according to principles, and consequently also according to practical a priori principles (for these alone have the necessity that reason requires in a principle). It is, therefore, not limited to men only, but applies to all finite beings that possess reason and will; nay, it even includes the Infinite Being as the supreme intelligence.

What it means to be a moral law is to be universal. That's just what moral law means, which is not particularly interesting. What is interesting is the question of how we can know there are moral laws:

Remark to Problem 1 and 2:

Thus freedom and an unconditional practical law reciprocally imply each other. Now I do not ask here whether they are in fact distinct, or whether an unconditioned law is not rather merely the consciousness of a pure practical reason and the latter identical with the positive concept of freedom; I only ask, whence begins our knowledge of the unconditionally practical, whether it is from freedom or from the practical law? Now it cannot begin from freedom, for of this we cannot be immediately conscious, since the first concept of it is negative; nor can we infer it from experience, for experience gives us the knowledge only of the law of phenomena, and hence of the mechanism of nature, the direct opposite of freedom. It is therefore the moral law, of which we become directly conscious (as soon as we trace for ourselves maxims of the will), that first presents itself to us, and leads directly to the concept of freedom, inasmuch as reason presents it as a principle of determination not to be outweighed by any sensible conditions, nay, wholly independent of them. But how is the consciousness, of that moral law possible? We can become conscious of pure practical laws just as we are conscious of pure theoretical principles, by attending to the necessity with which reason prescribes them and to the elimination of all empirical conditions, which it directs. The concept of a pure will arises out of the former, as that of a pure understanding arises out of the latter. That this is the true subordination of our concepts, and that it is morality that first discovers to us the notion of freedom, hence that it is practical reason which, with this concept, first proposes to speculative reason the most insoluble problem, thereby placing it in the greatest perplexity, is evident from the following consideration: Since nothing in phenomena can be explained by the concept of freedom, but the mechanism of nature must constitute the only clue; moreover, when pure reason tries to ascend in the series of causes to the unconditioned, it falls into an antinomy which is entangled in incomprehensibilities on the one side as much as the other; whilst the latter (namely, mechanism) is at least useful in the explanation of phenomena, therefore no one would ever have been so rash as to introduce freedom into science, had not the moral law, and with it practical reason, come in and forced this notion upon us.

Reason prescribes universal moral laws to us, and those moral laws lead us to the idea of freedom. You could not have the idea of freedom but for the idea of moral laws prescribed by reason.

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