George is right: we must go before Aristotle...
If we wish to deal with PHYSIS in the sense of NATURE (or related meanings), we must fo to the very beginning of philosophy: Anaximander of Miletus.
He is the dividing line between the age of myth and the age of reason. For the age of myth, one need to read only the Iliad or the Bible, for the stories in these book are accounts of the causes of the world, wordly things and events, and the happenings caused unto humans. All the way to and including the sage Thales, people believed that "the world is full of gods." And what are gods? Ultimately and before anthropomorphisms, the gods were operating overpowers that are immortal. (The gods were as evident as the thundering sky, the stormy sea, the erupting volcanos, etc. etc. These powers were the gods by these or any other names.)
What Anaximander did and all philosophers have done is well expressed in the words of the Renaissance Telesio: they are speakers of reality "iuxta propria principia:" according to its own principles... rather than outside agents or overpowers. Fundamentaly, philosophy is aetiology (aitiologia).
Only a few fragments remain of Anaximander's "Peri Physeos." His subject is precisely PHYSIS -- the process of arising, emerging, or growing [like the rosy fingered dawn], that is, beginning to emerge and increase in its being.
But, in thinking of HOW anything is born [comes into being], his mind is directed also to the end of things. So, in an isolated quoatation by others, he says, It is only just that what is born should die (or what emerges should perish)... according to Necessity [Moira}." He does not know yet HOW anything comes into being or perishes, but sees the justice of what happens, according to the ancient intuition of Moira.
Since our topic is not Physics (the physics of the pre-Socratics, of Aristotle, etc.), I will say ony a few more words about "physis," as to how it was translated into Latin. They employed the verb "to be born" (nascor) and coined a strange word, NATURA, like "nat--urum" in the plural: the things that are about to be born (or the things which are in the process of being born). So, Anaximander's title would be translated as "De Natura." Anyway, the concept of process was soon lost, because "natura" came to designate either the world of born things [the actualized physical world], or the character (eidos; essence) of something. I understand that "physis" was also used in Greek to designate the character (essence) of this or that being.
"Physis" is from the verb "phyO" [= I grow, etc.], but Latin has the corresponding verb "fio" (I become, change into, etc.) Undoubtedly, the Greek etym, "phy" took on the value of either FI (as in fio), or of FU (as in fulmen).
Fulmen (= lightning, lightning-flash) = fu + lumen, namely a light which appears and quickly disappears. In other words, FU or PHY names appearing and disappearing, being born and dying, emerging and perishing, and that is precisely what Axamimander was dealing with on a cosmic scale. (The Romans could have made a different strange translation of physis as fuentia, even though this word still lacks the process-expressivenes of the "-sis" of physis. "Facturia" would have been a horrible word and, anyway, would have soon implied that which is already made, rather than what is in the making, "in fieri." The Flux of Herclitus is "That Which Is In Fieri," "The Becoming.")