At one point in the first Critique, Kant says:

But as regards the principles of ethics, of legislation, and of religion, spheres in which ideas alone render experience possible, although they never attain to full expression therein, [Plato] has vindicated for himself a position of peculiar merit, which is not appreciated only because it is judged by the very empirical rules, the validity of which as principles is destroyed by ideas. For as regards nature, experience presents us with rules and is the source of truth, but in relation to ethical laws experience is the parent of illusion, and it is in the highest degree reprehensible to limit or to deduce the laws which dictate what I ought to do, from what is done.

There's a lot going on here, packed together very finely, but the loudest echo of this sentiment that I can recall from the Kantian corpus is his account of hedonism and moral-sense theory in the Groundwork:

Empirical principles are wholly incapable of serving as a foundation for moral laws. For the universality with which these should hold for all rational beings without distinction, the unconditional practical necessity which is thereby imposed on them, is lost when their foundation is taken from the particular constitution of human nature, or the accidental circumstances in which it is placed. The principle of private happiness, however, is the most objectionable, not merely because it is false, and experience contradicts the supposition that prosperity is always proportioned to good conduct, nor yet merely because it contributes nothing to the establishment of morality—since it is quite a different thing to make a prosperous man and a good man, or to make one prudent and sharp-sighted for his own interests and to make him virtuous—but because the springs it provides for morality are such as rather undermine it and destroy its sublimity, since they put the motives to virtue and to vice in the same class and only teach us to make a better calculation, the specific difference between virtue and vice being entirely extinguished.

But it is one thing to be an empiricist about morality so as to think of moral experiences that justify perception-like statements, another to infer from the popularity of a behavior that that behavior is justified. To know what is popular requires empirical input; but so why is it as "reprehensible" to be the one kind of moral empiricist as the other? (I take "reprehensible in the highest degree" and "entirely extinguished" to extend as far as each other.) For if Kant goes on to say that moral-sense theory isn't as bad as hedonism, despite being an empiricism-minded thesis, then it does not quite seem enough to say that a moral theory is as bad as it can be just in case it's an empiricist one. Is Kant exaggerating (even in terms of his own system) the wrongness of appeals to moral popularity, the wrongness of moral appeals to experience, or both, or neither, or is something else going on/at stake, here? I ask this question because of late I have come to think that, given how much moral-information processing is subjective and relative, then moral popularity is constitutively involved in what is "morally real," so even when (or if) it is misguided to appeal to moral popularity to judge behavior, it cannot be misguided "in the highest degree."

  • I do not read the quotes as broadly as you do. "In the highest degree reprehensible" applies specifically to deducing "what I ought to do, from what is done", not to explanatory sentimentalism a la Hume. Indeed, Hume's guillotine itself objects to exactly such deducing. And "difference entirely extinguished" applies specifically to "most objectionable" moral arithmetic based on "private happiness", not to just any empirical input writ large. Kant seems to be vilifying utilitarianism with egoistic utility, more or less.
    – Conifold
    Commented May 8 at 6:07

2 Answers 2


<<spheres in which ideas alone render experience possible>> [my emphasis]

There is a long tradition of debate over the nature of morality, law and legislation: I cannot make that justice, I am not a historian, but I can try and mention the terms of the debate, essentially, in modern terms, the contrast between Roman law and Common law.

The basic principle underlying Roman law (which is the principle Kant is vouching for) is that laws must be "universal": namely, laws must be based on universal moral principles (which also entails the need for a specialist called "legislator"), as opposed to experience, or tradition, and even common sense or agreement, which are rather particular and arbitrary; in fact, laws must also be general, legislation must not be about particulars.

The basic principle underlying Common law is opposite: law is and can only be a matter of common sense, experience, and tradition, and the idea that there exist "universal moral principles" is rather not accepted. In this approach, laws are the result of a social evolutionary process where it is a "jury of peers", not even the judge, who is rather a guarantor of procedure, that is really converging to a common "sentiment".

Just please keep in mind I only mean that as a starting point for research: the two positions are more complex and more faceted than my sketch can make apparent. For example, take classic liberalism a la von Hayek: it is all for common law, but, on top of it, the observance and enforcement of law remain an essential and fundamental principle, as, without those, common law becomes pure power-based arbitrium.

  • While you have my +1 I would like to mention that the proper dichotomy is common law vs civil law or else English Law vs Roman Law
    – Rushi
    Commented May 8 at 12:01
  • @Rushi Thank you. In fact, I would agree with you as to common usage, except that I have decided for those two "links" as more representative of the two extremes of the opposition in question. But, just like in the notes I have mentioned von Heyek on the side of Common law, I could indeed have mentioned Civil law on the other side (which is less pure than Roman: it is Roman with French, i.e. with the principles of modern democracy [the details of which I'd rather not flesh out, it gets off topic]). -- Sounds better? I am thinking about integrating that in my answer. Commented May 8 at 12:51

OP: To know what is popular requires empirical input; but so why is it as "reprehensible" to be the one kind of moral empiricist as the other?

In Kant's system the personal rationalisation is more real than that whatever is derived from the popular simulacrum, so his 'moral' justice is rather more just—judged generally right—than the deduction from uncritically repeated experience or popular opinion. The moral aspect comes from the generality of the justice rather than the consensus.

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