It was subsequently alluded to in the Bible, and was then developed in the Middle Ages by Catholic philosophers such as Albert the Great and his pupil Thomas Aquinas. During the Age of Enlightenment, the concept of natural laws was used to challenge the divine right of kings, and became an alternative justification for the establishment of a social contract, positive law, and government – and thus legal rights – in the form of classical republicanism. Conversely, the concept of natural rights is used by others to challenge the legitimacy of all such establishments.