Epicurean Physics

Physics is the first of the three parts of the outline of Epicurean philosophy: the study of nature, which shows that the universe is entirely natural, made of atoms moving through void, with no gods over it and nothing to fear in death.

The doctrines of Physics

1. The Universe Operates Naturally Through Atoms and Space
2. The Universe Is Infinite In Size And Eternal In Time And Has No Gods Over It
3. True Divinity Is Incorruptible And Blessed
4. Death Is Nothing To Us
5. There Is No Necessity To Live Under the Control of Necessity

Key terms: Atom (Atomos) · Void (Kenon) · Universe (To Pan) · Swerve (Clinamen) · Worlds (Kosmoi) · Images (Eidola) · Soul (Psyche) · Celestial Phenomena (Meteora) · Study of Nature (Physiologia) · Necessity and Fate (Ananke and Heimarmene) · Equal Distribution (Isonomia)

1. Introduction To Epicurean Physics

Epicurean physics lays the groundwork for the whole philosophy: what we can know, and how we should live, both depend on understanding what the universe is and how it works. Epicurus' own list of the fundamental principles of physics has not survived, but the most important points can be recovered by comparing his Letter to Herodotus, Lucretius' On the Nature of Things, and the inscription of Diogenes of Oinoanda.

Diskin Clay compared the elementary principles set out in the Letter to Herodotus and in Lucretius in his article “Epicurus' Last Will and Testament”:

• Nothing comes into being out of nothing. Letter to Herodotus 38.8–39.1; Lucretius I 145–150; Lucretius I 159–160
• Nothing is reduced to nothing. Letter to Herodotus 39.1–2; Lucretius I 215–218; Lucretius I 237
• The universe always was as it is and always will be. Letter to Herodotus 39.2–5; Lucretius II 294–307; Lucretius V 359–363
• The universe is made up of bodies and void. Letter to Herodotus 39.6–40.2; Lucretius I 418–428
• Bodies are atoms and their compounds. Letter to Herodotus 40.7–9; Lucretius I 483–486
• The universe is infinite. Letter to Herodotus 41.6–10; Lucretius I 958–964; Lucretius I 1001
• Atoms are infinite in number and space extends without limit. Letter to Herodotus 41.11–42.4; Lucretius I 1008–1020
• Atoms of similar shape are infinite in number, but the variety of their shapes is indefinite, not infinite. Letter to Herodotus 42.10–43.4; Lucretius II 522–527
• Atomic motion is constant and of two kinds. Letter to Herodotus 43.5–44.1; Lucretius II 95–102; Lucretius I 952
• Atoms share only three of the characteristics of sensible things: shape, weight, and size. Letter to Herodotus 54.3–6; Lucretius II 748–752

2. Elementary Principles: Atoms, Void, Infinity, and Eternity

• Matter cannot be created. Nothing comes into being out of nothing. This rules out creation by a god: the universe has always existed in some form.
• Matter cannot be destroyed. Nothing is reduced to nothing. The changes we see in nature are rearrangements of matter, never its annihilation.
• The universe consists of bodies and void. Everything is either solid, indivisible particles, the atoms, or the empty space through which they move, the void. There is no third kind of thing.
• Bodies are either atoms or compounds of atoms. Atoms are the simplest form of matter; everything else is made of them.
• The number of atoms is infinite. Matter is never exhausted in an infinite universe.
• The void is infinite in extent. Space has no boundary, so the universe as a whole has no edge and nothing outside it.

3. Atomic Motion

• Atoms are always in motion. Their motion never began and never ends.
• In the void all atoms move at the same speed, whatever their size or weight, unless something collides with them.
• Motion is straight in the void and vibrating in compounds. Free atoms travel in straight lines until they collide; atoms bound in a compound vibrate in place, holding it together.
• Atoms can swerve slightly. The swerve, Lucretius' clinamen, a minimal deviation at no fixed time or place, explains how atoms come to collide and breaks the chain of necessity that would otherwise rule out free will (see There Is No Necessity To Live Under the Control of Necessity).

4. Qualities and Interactions of Atoms

• Atoms have only shape, weight, and size. Every other quality we perceive, such as color, taste, or warmth, belongs to compounds and arises from how atoms are arranged.
• The number of different shapes is not infinite, merely innumerable. The variety of atomic shapes is vast but limited, which is why nature produces stable kinds of things.

5. Bodies

Bodies are combinations of atoms and void. They have properties and qualities that no single atom has, and they give rise to events; their qualities are real for as long as the combination lasts (see Letter to Herodotus 68–73).

6. Properties and Accidents

Epicurus distinguished permanent properties, which a thing has for as long as it exists, from accidents, which come and go without destroying it. This explained the nature of bodies and their changes without appealing to incorporeal entities such as Plato's forms.

7. Causation

Three causes account for the motion of atoms: their weight, the blows of collision, and the swerve. Together they leave room for things that happen by necessity, things that happen by chance, and things that depend on us.

8. Nature Has No Gods Over Her

• There are no eternal or supernatural forms, essences, or other natures (see Platonic Ideal Forms, Aristotelian Essences, And Divine Revelation Do Not Exist).
• There is nothing above or outside the universe (see The Universe Is Infinite In Size And Eternal In Time And Has No Gods Over It).
• There are no supernatural gods, but there are natural gods; assign to them nothing inconsistent with incorruption and blessedness (see True Divinity Is Incorruptible And Blessed).
• The human soul is natural and is born with the body, and it does not survive the death of the body (see Death Is Nothing To Us).
• Images are a flow of fine particles from the surfaces of bodies.
• There are infinitely many worlds, forming and dissolving.

9. Perception and Sensation

Sensations are our direct contact with the physical world and the foundation of all knowledge. Errors arise not from the sensations but from the opinions we add to them. Epicurean physics and canonics are therefore inseparable (see Epicurean Canonics and All Sensations Are True).

10. Why Study Nature

Epicurus did not value the study of nature for its own sake. If we were not troubled by fears about celestial phenomena and death, and by not understanding the limits of pains and desires, he says, we would have no need of natural science (Principal Doctrine 11); and no one can be free of fear about the most important things without knowing the nature of the universe (Principal Doctrine 12).

By showing that the universe runs by nature, that the gods neither reward nor punish, and that death is nothing to us, physics removes the greatest obstacles to a pleasant life, and it grounds ethics in the feelings of pleasure and pain that nature gives every living thing (see Epicurean Ethics).