Source: Prof Michael Sandel, Justice: ..., Episode 06: "MIND YOUR MOTIVE"
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.
I read the definitions of legislate and a priori, but what do they mean together?
Prof Sandel above isn't discussing any law. Google displays 6 pages of results which use this predicate (Is this the right linguistics term) in contexts outside philosophy as well.