ceaseless motion, traveling at a constant and incredibly high speed. Compound bodies and
the worlds of which they are a part are transient, coming into being and passing away. Only
atoms and the void are eternal and indestructible, having no beginning and no end. Epicu-
rus also posited several physical theories criticized by later ancient philosophers, including
that the Earth is flat and rests on a gradually less and less dense foundation, and that the
Sun and stars are very small, in fact about the size that they appear to us.
Like all compound bodies, humans consist of atoms. Epicurus taught that both body and
soul were corporeal: the body was made of relatively large, dense atoms, and the soul,
responsible for sensation and thinking, of several types of small, light, and mobile atoms.
Perceptions arise when images (eido ̄la) flow off of physical objects and strike the sense
organs. Sight, for instance, occurs when thin, swift moving images fly off of objects and
strike the eyes. Thought is caused by even thinner images directly striking the mind, which
Epicurus located in the chest near the heart. He held that there are an almost infinite
number of different images flying around us at any time on which our thoughts can focus.
The process of thinking is thus a focusing of the mind on one external image after another.
At death, the soul atoms escape from the body and disperse. Epicurus taught that there is no
afterlife, since the soul does not survive after death, and held that therefore we should fear
neither death nor punishment in the afterlife. Although a strict materialist, Epicurus was not
an atheist. He held that the gods existed, but were completely blessed creatures who lived
lives of perfect pleasure and had nothing to do with our world.
- Scientific Method. Epicurus shunned traditional logic, substituting what he labeled
“canonic” (from the Greek word kano ̄n, “rule, standard”), his term for his theory of knowl-
edge that he connected closely to physics. Epicurus was an empiricist, teaching that knowl-
edge was possible and derived from sensation. He held that there were three criteria of
truth: sensation, general concepts, and feelings. Sensation was the primary criterion of
truth. He said “all sensations are true,” a claim which at first sight appears implausible.
Epicurus, though, carefully distinguished sensations themselves from the judgments that
people make about them. In the case of an optical illusion like an oar appearing bent when
partially submerged in water, Epicurus would say that the image of the oar that reaches our
eyes is true: we see an image made up of certain sizes, shapes, and colors. Error occurs
when we add false judgments to our perceptions, such as “this oar is bent.” Sensation has
not fooled us, but our interpretation of the sensation that has reached our eyes. Our know-
ledge of the world is ultimately based on sensations, and the judgments we make on the
basis of sensation must be scrutinized for possible error. An important way to avoid making
errors of judgment and attain knowledge is by attending to “general concepts” (prole ̄pseis).
Epicurus maintained that general concepts could function as a criterion of truth. He
believed that humans form general concepts by generalizing from their sensations. From
such general concepts, people make statements that are true and false about objects in the
world. Epicurus’ third criterion of truth was “feelings” (pathe ̄). He taught that our actions
must be judged by the primary feelings of pleasure and pain, and took them to be the
criterion of ethical truth. All our actions must be directed to maximizing our pleasure and
minimizing our pain in the long run.
Relying on these criteria of truth, Epicurus argued that we could gain knowledge not
only of the visible world, but also of the microscopic world of atoms and the distant
movements of the heavens. When we are investigating the visible world directly accessible
to us, Epicurus taught that we should accept as true things verifiable by direct and
clear observation, and false what we cannot so verify. But when we are investigating the
EPICURUS OF SAMOS