데모크리토스, 피타고라스를 포함한 원자론자들에 의하면 신체 주변을 에워싼 공기가 생물들의 신체를 압축하여 원자들을 이끌어냄으로써 결코 정지하지 않는 생물들의 운동을 유발하며, 운동을 유발하는 데 부족한 원자들은 호흡을 통해 외부로부터 유입된 같은 종류의 다른 원자들에 의해 보충된다. 왜냐하면 외부로부터의 원자들은 압축과 결속작용을 통해 생물들의 내부에 있는 원자들이 빠져나가는 것을 방지하기 때문이다. 생물들은 이러한 작용을 할 수 있는 한 살아 있게 된다.
That is why, further, they regard respiration as the characteristic mark of life; as the environment compresses the bodies of animals, and tends to extrude those atoms which impart movement to them, because they themselves are never at rest, there must be a reinforcement of these by similar atoms coming in from without in the act of respiration; for they prevent the extrusion of those which are already within by counteracting the compressing and consolidating force of the environment; and animals continue to live only so long as they are able to maintain this resistance.
<Aristotle's De Anima 403b:11-17>
That is why, further, they regard respiration as the characteristic mark of life; as the environment compresses the bodies of animals, and tends to extrude those atoms which impart movement to them, because they themselves are never at rest, there must be a reinforcement of these by similar atoms coming in from without in the act of respiration; for they prevent the extrusion of those which are already within by counteracting the compressing and consolidating force of the environment; and animals continue to live only so long as they are able to maintain this resistance.
<Aristotle's De Anima 403b:11-17> 
