The natural law philosophy is grounded in metaphysics. That is, it is grounded in something beyond the physical; it is not accessible to the senses directly. No one can see, hear, taste, feel (tactilely), or smell natural laws. If they are real, their reality is vouchsafed in some fashion other than through direct sensual contact. Their reality should not be understood as a becoming, either, as made up of ideals which may be fulfilled in the course of time. The founders of these United States were not idealists in this sense; they did not conceive of natural laws as something it would be desirable to see established. on the contrary, they were understood as being already everywhere established, inviolable, and finished.
See The Founding of the American Republic: The Enlightenment Impetus