Aquinas' Authority
In Natural Law Theory, as systematized by Thomas Aquinas in the Summa Theologica, the criteria for a law to be just include that "the law does not exceed the power of the lawmaker." This brings me to wonder where the lawmaker received his power. If we assume that the lawmaker must be designated by law, this criterion seems paradoxical because it would require the presence of a positive law which is unjust, since there existed no lawmaker with the legal authority to create that particular law. This unjust law would then be used to justify the authority of the lawmaker who created laws that justify the power of future lawmakers. The system of just laws is necessarily incomplete.
However, there is a strain of conditioned democratic thought here: one could easily contest the paradox by pointing out that the rulers of Aquinas' time were not necessarily subject to law. The kings of medieval Europe were not required to obey their own promulgations, which makes sense: only the divided man can be his own subject. Early positivist thinkers, such as John Austin, found this answer suitable. Austin reformulated the "just promulgator" clause of Summa Theologica in such a way that his theory explicitly excludes the lawmaker from any legal bindings. Rather, the sovereign of Austin's positive law "receives habitual obedience from the bulk of the population, but... does not habitually obey any other (earthly) person or institution" (Stanford Encyclopedia - John Austin).
It is interesting to note, in this context, the use of "(earthly)" as an aside. I have yet to read Austin's writing on the subject, however, in light of the fact that Austin saw himself as furthering Aquinas' theory, it has interesting implications for the question at hand. It may, in fact, solve the query. If we import this notion, that the sovereign need not hold allegiance to an earthly law, into Aquinas' theory, it seems to indicate that this original authority is of divine origin. Presumably, the only evidence one might find for whether the lawmaker was "authorized" would be to compare his laws to natural law... but this seems to break down the driving line of legal positivism, the separation thesis. I'm not certain of this analysis, and will get back to it, but I do think it's interesting.