In science, a principle is a general statement, unproved, but justified a posteriori by verification of its consequences.  In loose terms a principle is a theory that has solidified to a good amount. In the 19th centuries, the first law and second law were referred to as the first main principle and second main principle.
“Of those who hold that the first principle is one, moving, and infinite, Anaximander, son of Praxiades, a Milesian, who was a successor and pupil of Thales, said that the infinite is principle and element of the things that exist. He was the first to introduce this word ‘principle’ <??>. He says that it is neither water nor any other of the so-called elements but some different infinite nature, from which all the heavens and the worlds in them come into being.”— Simplicius (c.550), Commentary on Aristotle’s Physics (24.13) (Ѻ)