This thesis looks into Epicurus regardless of the Epicureans, in order to emphasize (part I) the fact that he did not claim to propound a scientific theory of nature, but to make it possible by building upon our practical relation to the world : we think the living in so far as we ourselves are living in contact with the surrounding reality by way of need. This one makes us understand our being in the world in the whole form of the prolepsis which becomes a reality throughout successive patterns, from the whole, the atoms and the void, to the casual realities among which we live. In a periodeia found in the Letter to Herodotus and Book XXV of the treatise On nature, and then on the scale of the three Letters, the Key Doctrines and the Vatican Sayings, we move from the tropos pathologikos to the tropos aitiologikos, from the indestructibility of the whole, which provides our non-contradictory relation to the world, to the indestructibility of the philia which grants us the divine blessedness inasmuch as we do not loss sight of our practical reality. The fragments of Book XXV (part II) make it possible to understand the qualitative leap from the necessities which constitute our generated reality (apogegennemenon), to a soul which freely reacts upon the necessities of our present reality about to be generated (apogegennemenon) : we perceive ourselves, in that henceforth aetiological discourse, as causes among causes, without necessity transcending these necessities, and without clinamen. Part III investigates the reasons (be it historical circumstances, the stoic polemic, or other) why this materialism, grounded on practice, which gives the Hellenistic oikoumene a political link in the form of a genuine philia, was lost by the Epicureans, for the profit of dogmatic and so called scientific tendencies (Lucretius), and even the philosophical diversion of the indestructible philia into the restricted circle of friends, among others.