Source: Prof Michael Sandel, Justice: ..., Episode 06: "MIND YOUR MOTIVE" [in this episode, Prof Sandel introduces Kant.]
25:35: How many moral laws from Kant’s point of view are there in this room, are there a thousand or is there one?
25:45: He thinks there's one, which in a way does go back to this question: all right, what is the moral law, what does it tell us? So what guarantees, it sounds like … to act autonomously, is to act according to one's conscience, according to a law one gives oneself.
26:04: But what guarantees that ... if we all exercise our reason, we will come up with one and the same moral law? That's what Ahmady [the student whose question asked Prof Sandel to explain all this] wants to know.
26:15: Here's Kant's answer: The reason that leads us to the law we give ourselves, as autonomous beings, is a reason.
26:33: It's a kind of practical reason that we share as human beings. It's not idiosyncratic. The reason we need to respect the dignity of persons, is that we're all rational beings. We all have the capacity for reason. And it's the exercise of that capacity for a reason which exist undifferentiated in all of us, that makes us worthy of dignity, all of us.
27:06: And since it's the same capacity for reason, unqualified by particular autobiographies and circumstances, it's the same universal capacity for reason that delivers the moral law. It turns out that to act autonomously, is to act according to a law we give ourselves exercising our reason. But it's the reason, we share with everyone as rational beings. Not the particular reason we have, given our upbringing, our particular values, our particular interests.
27:46: It's pure practical reason in Kant's terms, which legislates a priori, regardless of any particular contingent or empirical ends.
How does everyone have the same capacity for reason, unqualified by particular autobiographies and circumstances? What about the mentally ill, whose capacities for reason ARE limited by their unfortunate medical circumstances? And what of babies?