Epicurus favoured a philosophy that would be easily intelligible and expressed in language which ordinary people could understand. Philosophy was divided into three areas: ethics, physics, and canonic. This last was essentially epistemological. It did not include formal logic, the usefulness of which was questioned. Both canonic and physics were seen as ancillary to ethics, the central part of Epicureanism. The aim was to achieve serenity, peace of mind, which is necessary for happiness.

Epicurus was a hedonist: in his view, only pleasure is good in itself. The goal of all action, the Epicurean life, was pleasure, which was held to be naturally and visibly pursued by all creatures and alone good in itself. But pleasure, in this theory, was virtually equivalent to the absence of pain, since there was no neutral state between the two. Epicurus sharply distinguished two types of pleasure: (1) the underlying harmonious state of body or of mind (catastematic pleasures); and (2) alterations of state, activities, etc. of body or of mind (kinetic pleasures). The former type was to be preferred, and indeed provided the only reliable basis for the expectation of the latter. Once the underlying state had been perfected and all pain removed, the there was no possibility of increasing pleasure, only of varying it by different activities. Some kinetic pleasures were to be rejected if they were a threat to one’s underlying well-being. The well-being of the mind, or freedom from disturbance (ataraxia), was particularly important. The virtues, including justice, were explained in terms of enlightened self-interest on hedonist principles; nobody was thought to be virtuous for any other end except the maximization of pleasure and minimization of pain.

The biggest threat to one’s underlying mental well-being was fear, above all fear of the gods and fear of death. The purpose of Epicurean physics is to understand the universe in such a way that these fears are dispelled. Atomic theory demanded that the soul, like all else, was composed of atoms and void –small round atoms which would easily disperse at death, so bringing an end to sensation. ‘Death is nothing to us.’ Likewise, sensations had to be the product of actual atoms reaching and affecting the mind, and though conglomeration of atoms could become distorted in transmission, recurrent sensations building up a clear picture in the mind must owe their origin to something existent. One such recurrent ‘sensation’ was our dream-images of the gods, superhuman creatures living in a blessed condition. The tenuous atomic images which built up such pictures of the gods came from another world far away, and their blissful condition meant that the gods had no concern for humans: such concerns would be painful. Thus we have nothing to frat from the gods any more than from death, and there is no divine providence or rationality governing the universe. Epicurean physics differed from Democritus’ by introducing the atomic swerve. Such a swerve seems to have been an occurrence of low probability, but was needed to explain how the atoms, originally falling uniformly though the void, should have come to collide in such a way as to generate interactions leading to the formation of a world. The swerve was also used to avoid an inevitable chain of causes and effects (i.e. fate), and so reconcile atomism with mankind’s clear concept of free will.

Epicurean epistemology emphasized the importance of three criteria of truth: the five sense, the anticipations or preconceptions (prolepseis), and the affections (pathe). All sensations were true, equally valid in their own way and all to be considered in devising the total picture of a thing; only in their interpretation did error arise, since the mind could add to and subtract from the content of an act of sensation when forming an opinion. The preconceptions were mental pictures of things built up naturally (hence without the intrusion of fallible opinion) from repeated acts of sensation. Since opinions, unlike sensations, may be true or false, a method was devised for distinguishing them. True, are those which have some confirmation (epimartyresis, confirmatory testimony) in our sensations and/or disconfirmation of the contrary (ouk antimartyresis). False, are those which have disconfirmation and/or confirmation of the contrary.

Epicurean social and political philosophy emphasized the amount of trouble that political and social commitments could bring with them, thus preventing us from achieving the goal of mental well-being. In opposition to the Stoics as well as to established Hellenistic ideals, it advocated the avoidance of political life, marriage and child-raising, having as a motto lathe biosas (get through life unnoticed).

Epicureanism became the second most influential of the Hellenistic philosophies, after Stoicism. Owing to the devotion of later Epicureans to their master, the basics of the philosophy remained more or less unchanged for centuries; but, above all, the theology and the hedonism provoked criticism among the ancients. The other schools were united in their opposition to the Epicureans from the time of Cicero, and it began to lose its appeal in later imperial times, with a change in religious and moral sensibilities.

Among the sources of our knowledge of ancient Epicureanism are the extant works of Epicurus himself, mainly through Diogenes Laertius, who preserves the Letter to Herodotus (on natural philosophy), the Letter to Menoeceus (on ethical theory), the Letter to Pythocles (on heavenly phenomena, of uncertain authenticity), and the forty Kyriae Doxae (Principal Doctrines), and through works by Lucretius, Cicero and Plutarch.