In his essay, written in 1939 and published in 1960, Heidegger claims that Aristotle’s Physics is the hidden and therefore never adequately studied foundational book of philosophy. Aristotle uses the word φύσις in two fundamental meanings. It designates the being (οὐσία) of entities in the whole, and the being of entities that have, in themselves, the course of their movement. Aristotle’s Physics is the transition between original Greek thinking and metaphysics.
In Physics, we learn that φύσις is the principle, ἀρχή, of movement. The goal of this movement is the τέλος of the entity. Movement ceases when an entity reaches its end and for this reason repose is the culmination of movement. When entities reach their end, they present themselves in unconcealment.
Aristotle explains the movement of entities as the interplay between form, μορφή, and matter, ὕλη. Form poses an entity in its appearance, εἶδος, as it is. This process always involves a privation, στέρεσις, since the coming to presence of an entity (like a fruit) is, at the same time, the disappearance of what it was before (the blossom). Form is the essence of φύσις as principle and principle is the essence of φύσις as form.
Physis is the name Heraclitus gave to the unconcealment (ἀλήθεια) of being. Entities presence out of concealment. In Aristotle’s conception of φύσις, the negativity inherent in truth disappears. Truth is no longer unconcealment, but resides now in the assertion and consists in the correspondence of the intellect and its object.
Vom Wesen und Begriff der φύσις. Aristoteles, Physik B, 1 (GA 9)