Philosophy was, for Epicurus, the art of living, and it aimed at the same time both to assure happiness and to supply means to achieve it. As for science, Epicurus was concerned only with the practical end in view. If possible, he would have done without it. “If we were not troubled by our suspicions of the phenomena of the sky and about death,” he wrote, “and also by our failure to grasp the limits of pain and desires, we should have no need of natural science.” But this science requires a principle that guarantees its possibilities and its certainty and a method of constructing it. This principle and this method are the object of the On the Criterion, or Canon. Since he made the Canon an integral introduction to the Physics, however, his philosophy falls into two parts, the Physics and the Ethics.

The Canon held that all sensations and representations are true and serve as criteria. The same holds for pleasure and pain, the basic feelings to which all others can be traced. Also true, and included among the criteria, are what may be called concepts (prolēpsis), which consist of “a recollection of what has often been presented from without …” Therefore, one must always cling to that “which was originally thought” in relation to every single “term” and which constitutes its background. Since the truth attested by each of the criteria is reflected in the phainomena, one must cling to these, employing them as “signs,” and must “conjecture” whatever “does not appear.” With the use of signs and conjecture, however, the level of judgment is reached, and thought is well advanced into that sphere in which error is possible, a state that begins as soon as single terms are tied into a proposition. Error, which consists of what “our judgment adds” to the evidence, can be of two types, one relative to what is not an object of experience, the other relative to what is such an object but for which the evidence is dubious. Each type has its own method of proof. Following the principles and methods of the Canon, Epicurus arrived at an atomism that, like that of the ancient naturalist Democritus, taught that the atoms, the void space in which they move, and the worlds are all infinite. But in contrast to Democritus, who had followed the deductive route of the intellect, considering the knowledge of the senses to be spurious, Epicurus, following an inductive route, assigned truth to sensation and reduced the intellect to it. On the basis of the totality of problems as Aristotle posed them in his Physics, Epicurus modified entirely the mechanical theory of causes and of motion found in Democritus and added the concept of a natural necessity, which he called nature, and that of free causality, which alone could explain the freedom of motion of humans and animals. For this purpose he distinguished three forms of motion in the atoms: a natural one of falling in a straight line, owing to their weight; a forced one due to impacts; and a free motion of declination, or swerving from a straight line. Secondly, he made finite the number of forms of the atoms in order to limit the number of sensible qualities, since each form begets a distinctive quality, and he taught a mathematical as well as a physical atomism. Lest an infinity of sensible qualities be generated, however, by an infinity of aggregations (if not of atomic kinds), Epicurus developed, from just this concept of infinity, the law of universal equilibrium of all the forces, or “isonomy.” Upon it, enclosing the events in a circle, he founded a theory of cyclic returns.

As part of his Physics, Epicurus’s psychology held that the soul must be a body. It is made of very thin atoms of four different species—motile, quiescent, igneous, and ethereal—the last, thinnest and the most mobile of all, serving to explain sensitivity and thought. Thus constituted, the soul is, from another perspective, bipartite: in part distributed throughout the entire body and in part collected in the chest. The first part is the locus of sensations and of the physical affects of pain and pleasure; the second (entirely dissociated from the first) is the psychē par excellence—the seat of thought, emotions, and will. Thought is due not to the transmission of sense motion but to the perception of images constituted by films that continuously issue from all bodies and, retaining their form, arrive at the psychē through the pores. The full autonomy and freedom of the psychē is assured, as, with an act of apprehension, it seizes at every moment the images it needs, meanwhile remaining master of its own feelings.

The object of ethics is to determine the end and the means necessary to reach it. Taking his cue from experience, Epicurus looked to the animal kingdom for his answer. He concluded that the chief end is pleasure. He distinguished two kinds—a “kinetic” pleasure of sense and a “static” pleasure, consisting in the absence of pain—and taught that the pleasure of sense is good, though it is not good merely as motion but rather as a motion favourable to the nature of the receiving sense organ. In essence, pleasure is the equilibrium of the being with itself, existing wherever there is no pain.

Epicurus concluded that “freedom from pain in the body and from trouble in the mind” is the ultimate aim of a happy life. The damages and the advantages following the realization of any desire must be measured in a calculus in which even pain must be faced with courage if the consequent pleasure will be of longer duration.

Having thus given order to his life, however, the wise person must also provide himself with security. This he achieves in two ways—by reducing his needs to a minimum and withdrawing, far from human competition and from the noise of the world, to “live hidden”; and by adding the private compact of friendship to the public compact from which laws arise. To be sure, friendship stems from utility; but, once born, it is desirable in itself. Epicurus then added that “for love of friendship one has even to put in jeopardy love itself”; for every existence, being alone, needs the other. “To eat and drink without a friend,” he wrote, “is to devour like the lion and the wolf.” Thus, the utility sublimates itself and changes into love. But as every love is intrepid, the wise person, “if his friend is put to torture, suffers as if he himself were there” and, if necessary, “will die for his friend.” Thus, into the bloody world of his time, Epicurus could launch the cry: “Friendship runs dancing through the world bringing to us all the summons to wake and sing its praises.”

If human unhappiness stemmed only from vain desires and worldly dangers, this wisdom, founded upon prudence alone, would suffice. But besides these sources of unhappiness there are two great fears, fear of death and fear of the gods. If science, however, is effective in revealing the bounds of desire and (as already seen) in quelling the fear of the gods, it can also allay the fear of death. Regarding the soul as a body within another body, science envisions it as dissolving when the body dissolves. Death, then, “is nothing to us, so long as we exist, death is not with us; but when death comes, then we do not exist.” But death is feared not only for what may be waiting in the beyond but also for itself. “I am not afraid of being dead,” said the comic Epicharmus of Cos; “I just do not want to die.” The very idea of not existing instills a fear that Epicurus considered to be the cause of all the passions that pain the soul and disorder human lives. Against it Epicurus argued that if pleasure is perfect within each instant and “infinite time contains no greater pleasure than limited time, if one measures by reason the limits of pleasure,” then all desire of immortality is vain. Thus, Epicurus’s most distinguished pupil, Metrodorus of Lampsacus, could exclaim, “bebiōtai” (“I have lived”), and this would be quite enough. He who has conquered the fear of death can also despise pain, which “if it is long lasting is light, and if it is intense is short” and brings death nearer. The wise person has only to replace the image of pain present in the flesh with that of blessings enjoyed, and he can be happy even “inside the bull of Phalaris.” The most beautiful example was set by Epicurus at the moment of his death:

A happy day is this on which I write to you. …The pains which I feel…could not be greater. But all of this is opposed by the happiness which the soul experiences, remembering our conversations of a bygone time.

The ultimate concentration of all his wisdom is the Tetrapharmacon, preserved by Philodemus: “The gods are not to be feared. Death is not a thing that one must fear. Good is easy to obtain. Evil is easy to tolerate.”