How we jump from "is" to "ought"--from historical facts to the Law, and its current force on us--is, as always, harder. I admit a gap where it comes to the question of whether G-d spoke to Moshe Rabbeinu and said such words, or whether the Jewish people, astonished at the miracle of their liberation that had befallen them, and at the formidable evidence of G-d they saw at the mountain, took upon themselves a code of law as a testament to their love and fear of this G-d. After years of pondering this question, I eventually moved from a quite firm conviction of the latter to a far firmer conviction of the former, mostly as a result of newfound humility through learning. In any case, the upshot is the same--and arguably, subtly, the cause is the same, too. If one believes that a single G-d moves the world, including its humans, then one believes that the Law began as a reciprocal covenant between humans and G-d.