The study of nature as an object of scientific interest matured through the investigations of Presocratic philosophers on the observable world. Herodotus is in dialogue with those expanding its domain into the spheres of natural science and the human. Physis embraces the interior and exterior regularities of subjects as diverse as landmasses, rivers, seas, elements, animals, and men. Unique to Herodotus, however, is the use of nature as a category of historical explanation; it is a standard of measurement that permits historical inference.