Epicurus regarded the universe as infinite and eternal and as consisting only of bodies and space. Of the bodies, some are compound and some are atoms, or indivisible, stable elements of which the compounds are formed. The world, as seen through the human eye, is produced by the whirlings, collisions, and aggregations of these atoms, which individually possess only shape, size, and weight. In biology, Epicurus anticipated the modern doctrine of natural selection. He postulated that natural forces give rise to organisms of different types and that only the types able to support and propagate themselves have survived.
Epicurean psychology is thoroughly materialistic.
The soul is regarded as being composed of fine particles distributed throughout the body. The cardinal virtues in the Epicurean system of ethics are justice, honesty, and prudence, or the balancing of pleasure and pain. Epicurus preferred friendship to love, as being less disquieting. His personal hedonism taught that only through self-restraint, moderation, and detachment can one achieve the kind of tranquillity that is true happiness. Despite his materialism, Epicurus believed in the freedom of the will.