The concept of nature is front-loaded.  Nature is what things are in their origin.  Hence physis sometimes means “birth.”

Hence too Arius: If the Father is ungenerated and the Son begotten, then they must have distinct natures.

Athanasius and the Cappadocians deny the premise.  True, the Father is ungenerated in every sense; He is not even begotten.  The Son is ungenerated in the sense that He is eternal; but He is begotten.  He originates from the Father, while the Father originates from none.  Yet, they are the same nature, and same substance,  homoousios .  Origin does not determine nature.

Breaking the link between nature and origin, Trinitarian theology opened the possibility of a back-loaded, eschatological ontology.