In the second Critique, when Kant brings up the question, "What if everyone did that?" he is to an extent presenting the question as a "commoner's" version of the more elaborate thesis he states:
Thus, people say: "If everyone permitted himself..." [emphasis added]
He uses a word (translated as) "commonest" ten times in that document, e.g. "The commonest intelligence can easily and without hesitation see what, on the principle of autonomy of the will, requires to be done..." In this, he is indirectly echoing his own startled reaction to J. Rousseau's ethical writings:
Kant's biographers tell a legend that Kant, who never interrupted his daily routine, gave up his daily walk in order to continue his study of Rousseau's Emile. The only picture in Kant's otherwise sparsely decorated home was a portrait of Rousseau that he hung over his writing desk. Rousseau’s influence helped give rise to important Kantian themes such as the dignity of common humanity, the importance of autonomy, the centality of virtue, and the nature of proper theodicy. [emphasis added]
And again, regarding the happiness/virtue tension in theories of his own time:
[The conflation of happiness and virtue] can only, indeed, be maintained in the perplexing speculations of the schools...
So, for all that, Kant's theory is not quite so "simple" as we might think the above is meant to have it be. First, since maxims are the initial "units" of moral judgment, there are complications regarding what maxims are (what "maxim" refers to/is defined as). The four overarching categories of deficient moral theories, as glossed in the Groundwork, correspond to the four (meta)categories of principles-of-the-understanding in the first Critique:
- Hedonism → axioms of intuition
- Moral-sense theory → anticipations of perception
- Metaphysical perfectionism → analogies of experience
- Divine-command theory → postulates of empirical thought
At least some maxims, then, are roughly "on the same level as" principles-of-the-understanding. The unity that theoretical reason brings to empirical understanding ex post facto is reflected by a unity of practical reason that precedes moral understanding.
Further complications arise with respect to the more detailed picture of categories of freedom and the "type" of the moral law. I haven't confirmed this ever, but Kant might be using the word "type," here, based on the word's Christian history, where types are paired with antitypes, which are particulars that yet generalize over the generality of types. The laws-of-nature version of the categorical imperative is a use of the theoretical concept of laws-of-nature, relative to practical concepts, according to this type-antitype scheme (Kant says something ambivalent about "the scheme of the law," here). Per the categories of freedom as the "components" of the typical representation, Kant says:
In this way we survey the whole plan of what has to be done, every question of practical philosophy that has to be answered, and also the order that is to be followed.
Finally (for present purposes), Kant closer to the second Critique's end says:
To set before children, as a pattern, actions that are called noble, magnanimous, meritorious, with the notion of captivating them by infusing enthusiasm for such actions, is to defeat our end. For as they are still so backward in the observance of the commonest duty, and even in the correct estimation of it, this means simply to make them fantastical romancers betimes. But, even with the instructed and experienced part of mankind, this supposed spring has, if not an injurious, at least no genuine, moral effect on the heart, which, however, is what it was desired to produce.
I.e. formalizing supererogation in our deontic understanding appears to bring in enormous complications:
[Going beyond or "below" duty's call is] plainly not representable in [standard deontic logic]... As a measure of the increased expressive power, the three-fold partition of normative positions for SDL is extended to a twelve-fold partition; with an extension to accommodate praiseworthiness and blameworthiness (McNamara 2011a,b), an eighty-four fold partition results. [emphasis added]
Another way to introduce a supererogation operator in DL also highlights the added complexity:
Let's define the basic operator for the word, "Do," as in, "Do this." Let's have the symbol be ±, so ±A means "Do A." Then ±(¬A) goes to, "Do not A" = -A. So we have ±A and -A. In natural language we also have expressions like, "Do be nice," which fact allows us to have any imperative represented with the ±/- operators.
... J:±A [means], "J is why do A." With this in mind, let's correlate simple logical forms of imperatives with deontic operators:
- J:±A = A is obligatory (there is a reason to A).
- J:-A = A is forbidden (there is a reason not to A).
- ¬J:±A & ¬J:-A (there is no reason to do A and no reason to not do A) = A is indifferently optional.
- J:±(A v ¬A) (there is a reason to do (A or not A)) = A is differently optional.
But on this kind of scheme, supererogation apparently would be:
- J:±(A v (A & B) v (A & B & C) v ... v (A & ... & N))
In summary, then, Kant thinks that everyone should be able to know and understand their moral responsibilities, so there must be some kind of practical limit on the complexity of moral questions. There are no "moral experts", or even if there are, this is not on account of greater intellectual capacity.
EDIT: perhaps we need to differentiate between:
- Metaethics is as simple as deriving the CI.
- Ethics is as simple as applying the CI.
- Applying the CI is as simple as...?