During the progress of the constitutional conflict, he was absorbed in research, publishing "De jure naturali et gentium juxta disciplinam Ebraeorum" in 1640. It was a contribution to the theorising of the period on natural law. In the words of John Milton, this "volume of naturall & national laws proves, not only by great authorities brought together, but by exquisite reasons and theorems almost mathematically demonstrative, that all opinions, yea errors, known, read, and collated, are of main service & assistance toward the speedy attainment of what is truest." It develops into a theory of international law, taking as its basis the Seven Laws of Noah.