"just as nothing is firmly established in speculative reason save by resolution to first indemonstrable principles, so nothing is firmly established in practical reason save through ordering to the ultimate end, which is the common good. What relates to [practical] reason in this way has the character of law."
"sicut nihil constat firmiter secundum rationem speculativam nisi per resolutionem ad prima principia indemonstrabilia, ita firmiter nihil constat per rationem practicam nisi per ordinationem ad ultimum finem, qui est bonum commune. Quod autem hoc modo ratione constat, legis rationem habet."
Thomas Aquinas, Summa theologiae I-II.90.2.ad 3, trans. McInerny. Latin from Corpus Thomisticum.