bʰew-sis-
“nature, growth, being”physics, physiology, metaphysics
Abstract from *bʰew- giving Greek physis, English physics, physiology, metaphysics.
Discussion
The PIE form *bʰew-sis- (nature, growth, being) is an abstract noun from the root *bʰew- (to be, to become, to grow), formed with the suffix *-sis- that creates nouns of process or state. The literal meaning is "the process of becoming" — nature not as a thing but as an activity, a continuous coming-into-being.
Greek phýsis (φύσις, "nature, constitution, natural form") is the direct continuation and one of the most consequential words in Western intellectual history. When the pre-Socratic philosophers asked "what is the phýsis of things?" they were asking what everything is in the process of becoming — the fundamental growth-principle underlying all change. This single question, framed by this single PIE derivative, inaugurated natural philosophy and, eventually, natural science.
The Greek derivatives entered English in massive quantities: physics (the study of nature), physical (of or relating to nature/body), physician (one who studies nature, hence a healer), physiology (the study of nature's processes), metaphysics (beyond physics, Aristotle's treatise on first principles — named by his editor Andronicus of Rhodes because it came after the Physics in the collected works, not because the subject matter was "beyond" nature). Physique, physiognomy, and physiotherapy extend the family further.
The verbal root *bʰew- is itself one of the most important in PIE. It gave the English verb be (via Old English bēon, from PGmc *beuną) in its "become/exist" sense — distinct from the copular *h₁es- (am/is) that provides the present tense forms. The distinction between *bʰew- (to become, to grow into) and *h₁es- (to be, to exist statively) is one of the most fundamental semantic contrasts in PIE verb morphology, and English preserves it in the suppletive paradigm: I am (*h₁es-) but I will be (*bʰew-).
Latin fuī (I was, I have been) continues the root in the perfect system, while the future tense forms (erō, eris, erit) may also reflect *bʰew- derivations. The Latin compound fīō (I become, I am made) preserves the "becoming" sense transparently.
Sanskrit bhávati (he becomes, he is) and the noun bhāva- (becoming, state of being, existence) continue the root in the Indo-Iranian branch. The Buddhist philosophical term bhāva (existence, becoming — one of the twelve links of dependent origination) shows the root operating at the highest level of abstract thought in a tradition entirely independent of Greek philosophy.
Old Irish buith (to be) and Lithuanian bū́ti (to be) confirm the root across Celtic and Baltic. The universality of *bʰew- — attested in every major IE branch with the same core meaning — makes it one of the most securely reconstructed PIE roots and one of the most philosophically productive words in human language.