Physis — Nature
FOO-sis · Greek · φύσις
Physis means nature — the essential character of a thing, what makes it what it is. In Christian theology, it became the key term for expressing what Christ has two of: a divine nature and a human nature, united in one Person.
The Greek word physis refers to the essential character or constitution of a thing — its nature in the deepest sense. A thing's physis determines what it can do, how it acts, and what it fundamentally is.
In philosophy, physis stands close to ousia (essence), but with a slightly different emphasis. Where ousia asks "what is this?", physis asks "what is it like? — what are its capacities, its properties, its way of being?"
The Council of Chalcedon (451 AD) defined that Christ possesses two physeis — two natures — divine and human, united in one hypostasis (person). These natures are "without confusion, without change, without division, without separation."
This means Christ's divinity is not diluted by his humanity, and his humanity is not absorbed by his divinity. Each nature retains its full reality. Christ truly thinks human thoughts, feels human emotions, and suffers human pain — while simultaneously being the eternal Word through whom the universe was made.
The importance of physis in this definition cannot be overstated. Without it, the Hypostatic Union cannot be articulated: one Person, two natures.
From the perspective of this site, physis completes a trilogy of terms that make Chalcedonian Christology possible: ousia (what God is in essence), hypostasis (who Christ is as a person), and physis (what Christ has two of — divine and human). Together, these three words allow the Church to say what it most needs to say: that in Christ, God has truly entered human life without ceasing to be God.