For Guyau, the historian must study philosophies as living systems. The historian must reconstruct these systems, starting from their fundamental structures, or ‘key ideas’. Once those structures are identified and articulated, the historian has to follow their ‘path’ of development and growth, that is, the dynamic life and movement of those ‘key ideas’ in time. A special feature of Guyau’s account of the history of Western ethical thought is the quasi meta-historical place occupied by Epicureanism. In this regard, we could say of Guyau what Lampert says of Nietzsche when he claims that his recovery of Epicurus ‘forms a key component in his new history of philosophy’. To explain this pivotal role of Epicureanism in the history of thought, in the introduction to The Ethics of Epicurus Guyau tells us that:
It is Epicureanism […] that is born again with Helvétius, d’Holbach, Saint-Lambert. It is Epicureanism that inspires all the French writers of the eighteenth century […] Then it returns in England, gathering in Hobbes’ homeland even more numerous partisans. With Bentham and Mill, it assumes its definitive form, which… is not that different from its original source. Finally, with Spencer and Darwin it grows anew. To the more or less transformed moral system of Epicurus, a wide cosmological system is added: new Democritians provide modern Epicureans with the means to ground their ethics in the laws of the whole universe, encompassing man and the universe in a same conception (1878:13).
Guyau shows how Epicureanism reappears, here and there, in scientific discoveries and philosophical innovations. For him, as for Nietzsche, ‘Epicurus has been alive in all ages and he lives now’ (KSA 10.7 ). Nevertheless, if Epicureanism appears to Guyau as this fundamental force propelling the movement of thought, we must add that he does not describe this movement as a peaceful, conciliatory dialectics, but rather as an agonistic conflict between distinct forces. He explains:
Epicurus’ influence, which has remained considerable since the last century, is increasing despite the new Stoicism of Kant and his school. Everywhere, in theory and practice, we find [these] two moral philosophies [morales]… split philosophical thought and divide human beings. We could say that today the fierce half-a-millennium struggle between the Epicureans and the Stoics has rekindled and is burning anew (Guyau 1878:13).
For Guyau, Stoicism and Epicureanism are not just two ancient philosophical schools, but also two fundamental forces or philosophical passions in permanent opposition, elements of an unsolvable tension, which fuels the development of thought and morality. If, this recurring agon between Stoicism and Epicureanism characterizes our ways of thinking and practicing philosophy, Guyau’s approach is not neutral but partisan, situating himself on the side of Epicureanism.
The fierce opposition between these two moral doctrines only loses intensity through the intervention of a third force, that which Guyau calls ‘religious enthusiasm’.  In Guyau’s reconstruction, religious enthusiasm, as well as the superstition and fear it inspires, is the first target of the Epicurean project of liberation. In a strikingly modern way of considering the emancipatory potential of science, Guyau claims that Epicureanism – analogously to the positivism and utilitarianism of his time – would be a particularly strong tool in the human struggle against religious fears and mystified forms of authority:
[W]hen facing religion, Epicureanism’s force of resistance surpassed all other philosophies. As a matter of principle, Epicureanism rejected the miraculous and the supernatural… we can say that Epicurus and Lucretius embody the scientific and positivistic spirit of the modern utilitarians, which is why they remain strong (1878: 12).
Guyau explains that Epicureanism succeeded like no other philosophy in challenging religious ideas until the appearance of Christianity, which somehow triumphed over Epicurean materialism. According to Guyau, Epicureans ‘were weak when they faced Christianity because they consistently emphasized our ultimate annihilation… despite the fact that human beings desire to be immortal’. Nevertheless, for Guyau, if Epicureanism was defeated, it was not destroyed. He says that ‘after several centuries, when the enthusiasm for this new religion faded’, this life in this earth were again found to have value and to be worth taking seriously. Epicureanism re-emerges, therefore, with its attachment to the materiality of the earth, and with it a contemporary Stoicism resumes the battle that defines the history of moral thought.
For Guyau, the interpretation of Epicurus could be also understood a topos for philosophical experimentation, in the sense of a territory to formulate his own concepts, and to examine the implications of a way of life. In order to illustrate this, we will focus on the specific interpretation Guyau provides of the Epicurean notion of pleasure [plaisir], showing what we believe is innovative about his reading and the conceptuality he mobilizes to explain this ‘key idea’ of Epicureanism. We would also like to show how Guyau, like Nietzsche in his middle works, finds in Epicurus a very important philosophical attitude, consisting of the attempt to restore to philosophy its claim to be an art of existence, to be a form of self-cultivation and a way of life.