Philosophy 143 Lecture Notes

The Physics of Epicurus

Epicurus developed his views in the period shortly after Aristotle, by which time the main lines of Greek physics had been well-developed. 

The Origins of Greek Physics

The primary objective of his predecessors had been to search for a unity underlying the multiplicity that is observed in the world of the senses.  The Milesian Thales (end of 6th century B.C.), who is credited with being the originator of these investigations of the physical world, had proclaimed that all is water.  What appears to the senses as being opposed to water, such as earth, which is dry, was on Thales's view only a way in which water exists.  The perception of difference masks an underlying unity which can be discovered by the philosopher.  Here is how Aristotle summed up the approach:

Most of those who first engaged in philosophy supposed that the only principles of things were to be found as material elements.  That of which all things consist, that from which they first arise and into which they finally vanish away, that of which the "basic being" (ousia) persists although the perceptible characteristics are changed,--this, they say, is the prime element and first-principle of all things.  Therein they hold that nothing either comes-to-be or is destroyed, since this kind of "basic nature" (physis) always persists.

As to the nature of what is fundamental, however, and even as to whether it is one or many, there was much disagreement.  Thales, the founder of this type of philosophy, declared the first-principle ot be water, and for that reason he also held that the earth rests upon water.  Probably the idea was suggested to him by the fact that the nutriment of everything contains moisture, and that heat itself is generated out of moisture and is kept alive by it.  For of course it is assumed that whatever something is generated out of must be its first-principle.  He drew his notion also from the fact that the seeds of everything have a moist nature; and of course the first-principle of moist things is water.  (Metaphysics 983b 7; cited in Wheelwright, The Presocratics, pp. 46-7.)


The Eleatic philosopher Parmenides took (early 5th century B.C.) the search for unity to its logical extreme.  On his view, multiplicity is unreal, so that reality consists of a single undifferentiated "being."  This is the first appearance (though not the last) in Western philosophy of what has come to be called metaphysical monism.  Aristotle described Parmenides's line of reasoning as follows:

On the ground that not-being, as contrasted with being, is nothing at all, Parmenides is forced to conclude that Being is one and that there is nothing else.  (Metaphysics 986b 29, cited in Wheelwright, p. 102).

That which is other than being is not; hence, by Parmenides' argument, it must follow that all things are Being, and hence One.  (Metaphysics 1001a 32, cited in Wheelwright, p. 103).

Aristotle also remarked at 986b that "Parmenides seems to have conceived of reality as one by definition."   The idea is that if there were something other than being, it could only be what is not being, i.e., not-being.  But as not-being is nothing, whatever might be other than being is also nothing, so there is nothing other than being.

Moreover, being is incapable of change.  Being itself could not come to be, as that would mean that it passes to what it is not, i.e., non-being .  As Parmenides is quoted as putting it:

"How could What Is be something of the future?  How could it come-to-be?  For if it were coming-to-be, or if it were going to be in the future, in either case there would be a time when it is not.  Thus coming-to-be is quenched, and [by similar reasoning] destruction is unthinkable." (Simplicius, Commentary on Aristotle's Physics, cited in Wheelwright, pp 97-98.)
By the same reasoning, no change is possible within being.  The final alternative, that being changes its place, is also ruled out, as Plato noted:
[Parmenides and his associates] maintain that everything is one and stationary and entirely-self-contained, since there is no empty place in which to move.  (Theaetetus 180A)
An empty space, or void, would be not-being, which therefore does not exist.  If there is no void, being cannot move, nor can anything move within it, as Aristotle noted:
Certain earlier thinkers maintained that What Is must necessarily be one and immovable: they argue that since the void does not exist What Is cannot be moved, and that there cannot be a plurality of things because there is no void to keep them apart.  (On Generation and Corruption, 325a 3, cited in Wheelwright, p. 103)
Zeno of Elea supported Parmenides's monism by arguing that change, where something becomes what it was not, is unreal.  His arguments centered around the possibility of motion, which the Greek physicists regarded as the primary source of change.  He propounded a series of paradoxes designed to show that the supposition of motion leads to a contradiction.

Consider, for example, the alleged motion of an object from point C to point D, where the distance between C and D can be whatever you please.  In order to complete the motion, the object would have to reach a point halfway between C and D.  Suppose it has done so.  Then to reach D, it would have to begin by traversing half the space from the halfway point to D itself.  And to pass over the remaining space, it would have to go halfway yet again.  If, as Zeno supposed, space is capable of infinite division, there would always be a space remaining between the object and the target.  And since the distance between C and D can be whatever you please, an object can never arrive at a destination point D from a starting point C, no matter how great or small the distance between them.


The Greek physicist Leucippus (late 6th century B.C.) who Epicurus allegedly claimed did not exist) tried to overcome the Parmenidean monism while conceding one of its main points.  He allowed that what is real contains no difference in it, but at the same time he postulated that there are many such undifferentiated beings.  What distinguishes these beings from one another is their position in the void, which he identified with non-being.  The beings and the void are therefore absolutely opposed to each other, just as being and non-being are opposed.

Leucippus held that a void exists because motion is real (which, of course, means that Zeno's paradoxes must somehow be overcome to give his position plausibility).  For an object to move, there must be empty space into what the object can move.  Parmenides held that because a void is non-being, there is no void and therefore no motion.  Leucippus agreed with him that a void is non-being, but because he clung the reality of motion, he refused to omit it from his physics. Again, Aristotle gave a detailed description of the situation:

Leucippus believed he had a theory which would be consistent with sense-perception, in that it did not abolish coming-to-be and perishing, nor movement, nor the multiplicity of things.  To that extent he accepted the testimony of appearances.  On the other hand he conceded to the monists that there could not be movement without a void, that a void is not-being, and that What Is cannot ever be What Is Not.  Accordingly he agrees that What is must be a complete plenum, absolutely and continuously full; with this qualification, however, that there is not just one plenum, there are numerous plena, in fact they are unlimited in number.  Because of their extremely small bulk they are invisible.  The move in the void--for there is a void--and by their coming together they cause coming-to-be, while by their getting loose from one another they cause perishing.  They act and are acted upon when they happen to come into contact; for they make contact without becoming one, and it is through contanct and hooking on to one another that they produce phenomena.  (On Generation and Corruption  325a 23, cited in Wheelwright, p. 178)
But this left him in an awkward position, in that he seemed to be asserting the existence of non-being, which Parmenides would find contradictory.  According to Simplicius,
Moreover, he affirmed that Not-Being exists quite as much as Being does, and that both of them together are the cause of why things occur.  The atoms, which eh called "being," in their individual character are compact and full; they move about in empty space, which he called "not-being," and which "is," he says, no less than being.  (Commentaries, cited in Wheelwright, p. 181)
[Note that Aristotle claimed to have solved the problem of motion without appealing to a void, which he thought was an impossibility.  According to him, motion can take place in totally full space (a plenum) insofar as bodies push what is ahead of them aside, initiating a chain-reaction which results in the displacement of bodies which reaches to the rear of the moving body. 
But there is no necessity for there being a void if there is movement. It is not in the least needed as a condition of movement in general, for a reason which, incidentally, escaped Melissus, viz. that the full can suffer qualitative change.

But not even movement in respect of place involves a void; for bodies may simultaneously make room for one another, though there is no interval separate and apart from the bodies that are in movement. And this is plain even in the rotation of continuous things, as in that of liquids. (Physics, Book IV, Chapter 7)

This type of motion is known as "antiperistasis." (If you have a hard time picturing this, think about the rotation of a tire, or what happens if you move your hand in a jar filled to the top with water.)  ]

According to Leucippus, the beings which make up the universe are atomic, which means litterally that they cannot be cut up. Atoms are absolutely indestructible, which is characteristic of Parmenidean being.  In combination, they make up the objects which appear to the senses.

Democritus (late 5th century B.C.) developed the atomism of Leucippus.  Of particular interest is his claim that only the system of atoms can be genuinely known: whatever "knowledge" we gain through the senses is at best obscure.  Thus, Democritus took a skeptical stance toward the object of perception.  Sextus Empiricus stated that:

Plato and Democritus both maintain that only intelligibles are true--Democritus on the ground that nothing sense-perceptible has any natural existence, inasmuch as the atoms of which eveerything is composes have a nature which is completely devoid of any sense-perceptible quality.  (Against the Logicians II. 6, cited in Wheelwright, p. 196)
He claimed, in particular, that only the properties of the atoms, specifically their size and shape, are real.  The properties of perceptible objects, such as their color, are unreal.  Theophrastus summarizes Democritus's skeptical attitude neatly:
As for the other sensible qualities he argues that none has objective reality, bout that all of them are effects of our sensuous faculty as it undergoes alteration; it is a bodily alteration that pruduces images.  Nor does he regard hot and cold as having an objective nature; they are merely a matter of configuration [of the groupings of atoms].  What we experience as qualitative change within ourselves is the effect of incoming atomic configurations being massed together so as to produce intensity of effect.  What is massed together [on entering our sense organs] prevails [as a conscious experience], while what is widely diffused is imperceptible.

His proof of the unreality of sensory qualities is based on the fact that they do not appear the same to all: what is sweet to some is bitter to others, while to yet others it is sour or pungent or astringent; and sense-qualities of other types may be characterized similarly. (On the Senses I. 61)


Let us now turn to the atomism of Epicurus.  His system is laid out extensively in his Letter to Herodotus.  The letter begins with a distinction between what is evident to the senses and what is not evident to them.  The senses observe objects which have various properties such as size, shape, color and motion.   What is not evident to the senses is the unity underlying these objects.

The description of this underlying unity begins in a way very reminiscent of Parmenides, with the claim that nothing can come into being from what is not.  But while Parmenides had argued according to definitions that this is because being and non-being utterly exclude each other, Epicurus staked his claim on very different grounds.  Suppose that something came into being from what is not.  Then it would come into being without any of the preconditions needed to bring about its existence.  In the sensible world, a plant cannot come to be without a seed, but if a plant could come to be from nothing, there would be no reason why it could not be generated in another fashion.  As Epicurus put it, "everything would be coming into being from everything, without need for seeds" (A38).  But why not allow this as possible?  The only reason available to Epicurus is that it contradicts what is evident to the senses.  Parmenides, who considered the senses to be capable of generating only opinion and not knowledge, would never appeal to such a premise in his argument.

A second thesis that would be accepted by Parmenides is that being, when it is dissolved, is never destroyed into what is not.  The reason given for this claim is that if what is dissolved were destroyed, everything would be destroyed.  Again, unlike Parmenides, to make this claim stick, Epicurus would have to appeal to what is evident to the senses, i.e., that things do dissolve.

A third thesis is that the totality of things is now and will always  be just as it is.  This seems to contradict the evidence of the senses, until it is recognized that the thesis applies to the totality of things as a whole, not to its configuration.  In arguing for the truth of this claim, Epicurus notes that for the totality to change, there would have to be something that could "enter into" it to produce the change.  But the totality is all there is, so there is no such thing.  Again, this seems to be a thesis that Parmenides could accept, and even the argument for it purely conceptual, as are Parmenides's own arguments.

Having spoken of the totality as such, Epicurus breaks decisively from Parmenidean monism be dividing it into "bodies and void."  He appeals to sense-perception as proving the existence of bodies, and he repeats the argument of Leucippus that since perception also reveals that bodies move, and motion is possible only in a void, a void exists.  Unlike Leucippus, Epicurus recognized that to say that it exists requires that he deny that a void is non-being, and indeed he equates "void" with "space" and "intangible nature."  In this way, he could avoid making the apparently contradictory claim that non-being exists.

Next, Epicurus notes that bodies are either compound or are the components of compound bodies.  The ultimate components he called atomic, i.e., indivisible, and unchangeable.  Here he exploited his fundamental thesis that being cannot be destroyed: he calls the atoms "full by nature and not being subject to dissolution in any way or fashion."  The idea is that compound bodies dissolve into their components but the components, being unitary, cannot dissolve themselves, and moreover cannot become non-being, since otherwise they all would be destroyed.

The totality is said to be unlimited.  The argument is that a limitation of the totality would require that it have an end-point or extreme.  An end-point can exist only as the beginning of something else.  But there is nothing other than the totality (as had already been established), so the totality has no extreme and consequently is unlimited.  This lack of limitation applies both to bodies and the void.  If the void were limited and bodies were unlimited (in number), there would be no place for all the bodies.  On the other hand, if the bodies were limited and the void unlimited, there would be so much space available for bodies that they would not meet with one another to form anything stable, but instead move in a scattered fashion throughout the universe.  Note that once again, appeal must be made to sense-perception for the datum that that there are stable bodies which are not in motion (a claim that modern science has since overthrown).  This view flies against that of Parmenides, Plato and Aristotle.  In particular, Aristotle believe that the totality of bodies can be limited by something which is not itself body and which lies outside the cosmos, namely the prime mover (Physics, VIII, 10), which he identified with God (Metaphysics XII, 7).

Epicurus then turns to the details of his atomistic picture of the underlying reality. The properties of the atoms are shape, weight, size, and whatever properties accompany shape.  These are the properties which are preserved through all dissolutions.  That something must be preserved is the basic point of atomism: that change cannot be a passage from being to not-being.  "For every quality changes, while the atoms do not change in any respect; for it is necessary that during the dissolution of compounds something should remain solid and undissolved, which will guarantee that the changes ar enot into what is not nor from what is not . . ." (I-2 54).

The atoms come in a vast, though not unlimited (merely "ungraspable") number of shapes, and there is an unlimited number of atoms having each specific shape.  They come in several sizes, but not in all sizes.  The reason for this is that if they were big enough, atoms would have to be visible, but sense-perception does not reveal them.  (Also, Epicurus's theory of vision, to be examined later, is incompatible with the visiblity of atoms.)  Atoms do not possess such properties as color, taste, smell, etc., a claim which we will also examine later.

Atoms move all the time, rebounding when they collide with other atoms, and vibrating when they are components of a stable compound body.  The reason they are always in motion is that they are always surrounded by void, which gives no resistance to the motion they already have.  Here Epicurus seems to presuppose a form of inertia, according to which  bodies in motion tend to stay in motion unless something interferes with it.  As the void cannot interfere, bodies will continue moving through it.

A related point concerns the natural motion (as opposed to rebound motion) of atoms.  In Lucretius's vesion of Epicurean physics, all bodies move straight downward through the void as a result of their weight.  If there were no deviation from this downward motion, there would be no rebound of atoms, and hence no compound bodies would be formed.  So Lucretius states that "at quite uncertain times and places they veer a little form their course, just enough to cause a change of motion."  This is called (derisively) the "swerve," and it was heavily criticized by Cicero and others on the grounds that it is a change without a reason.  We will return to the issue of the swerve shortly.

The number of universes ("cosmoi") is unlimited.  The reason is that the number of atoms is unlimited, as is the space in which they might be configured to form a universe.  Epicurus makes the further claim that some of these cosmoi are like our own and some different from it.  The reasoning here is that there are so many atoms that there could not help but be similarities in their configurations.  Epicurus concludes that "there is no obstacle to the unlimitedness of worlds."  It must be noted, however, that this negative claim cannot establish that there are any similarities, since there might be infinitely many dissimilar configurations of universes.

In Lucretius we find the claim that the gods live in the regions between the cosmoi, suitably distant from interference in human affairs.  Aristotle had situated his god, the prime mover, outside the cosmos as well, but allowed for its ability to influence the world causally (as an end, or "final cause.")  It was Epicurus's position that the blessedness of the gods precludes their interest in human affairs.


We are told by Cicero that Epicurus introduced the swerve to solve a problem only directly related to that of the motion of bodies: "the necessity of fate" (On Fate, 22). Lucretius describes the reasoning involved:

If every motion is always linked to another, and new motions always arise from the old in definite order, and the atoms do not produce by swerving a starting point for motion which can break the bonds of fate and prevent one cause from following from another from infinity, where does this free will which living things throughout the world have, where, I say, does this will torn from the grasp of the fates come from? Through this we all go where each one's pleasure leads and swerve from our paths at undetermined times and places, just as our minds incline to do. For it is far from doubtful that everyone's own will provides the starting point for these things and that this is the source of motion in our limbs. (On the Nature of Things, 2.251)
The starting point of this argument is the observation that the motions of our bodies respond to our will, and that our will is indeterminate, in that it is "our pleasure" or "our inclination" which brings about the motion. Lucretius seems to assume that this indeterminacy must be given a basis in our physical constitution, since the soul itself is composed of atoms. If the motion of the atoms is determinate, there would be no accounting for the indeterminacy of the will, which would then be in the "grasp of the fates." The swerving of the atoms would "break the bods of fate" because it interrupts the casual chain.

Cicero describes the swerve as being (though not in Epicurus's own words) uncaused.

Epicurus introduced this line of reasoning because he was afraid that if an atom always moved by its natural and necessary heaviness, we would have no freedom, since our mind would be moved in such a way that it would be compelled by the motion of atoms. Democritus, the founder of atomism, preferred to accept that all things happened by necessity than to tear from the atomic bodies their natural motions. (On Fate, 23)
That some motions of atoms are uncaused is a thesis that would be "mocked by the physicists" (On Fate, 25.
What new cause, then, is there in nature which would make the atom swerve? Or surely you don't mean that they draw lots with each other to see whch ones will swerve and which not/ Or why do they swerve by one minimal interval, and not by two or three? This is wishful thinking, not argument. On Fate, 46)
And because the uncaused motion of atoms cannot be allowed, if Epicurus can only defend freedom of the will on this basis, there is no one "who does more to confirm, not just fate, but even a powerful necessity governing all things, or who has more effectively abolished voluntary motions of the mind, than [Epicurus], who concedes that eh could not have resisted fate in any other way than by taking refuge in these fictitious swerves" (One Fate, 48).

Cicero proposed as a solution to the problem a suggestion of the Academic skeptic Carneades. The key move here is to distinguish between "external and antecedent" causes from causes in general. Thus, an atom's natural motion is due to its weight, which serves as a cause. But if an atom is deflected from its natural path by collision, the cause (i.e., the motion of the other atom) is external to the deflected atom and occurs antecedently to its motion. The mind can then be said to be moved without an external and antecedent cause: "for voluntary motion itself contains within it a nature such that it is in our power and obeys us, bout not without a cause. Its very nature is the cause of this fact" (On Fate, 37). This kind of solution was revived in the seventeenth century by the Jewish philosopher Spinoza.

Truth-Value Gaps

The indeterminacy of atoms creates another problem for Epicurus. If it is really undetermined now how an atom (or someone's will) will move at some future point, it seems that statements such as "that atom will move in a straight line at noon today," or "Cicero will mock Epicurus at noon today," uttered in the morning, have no truth value. According to Cicero, this consequence was embraced by Epicurus.

The problem here goes back to Aristotle, who discussed such "future contingents" in his book On Interpretation, Chapter 9. At issue is whether all propositions are either true or false, and this seems to hold in the case of what has already taken place.

When the subject . . . is individual, and that which is predicated of it relates to the future, the case is altered. For if all propositions whether positive or negative are either true or false, then every given predicate must either belong to the subject or not, so that if one man affirms that an event of a given character will take place and antoher denies it, it is plain that the satement of the one will correspoind with reality and that of the other will not. For the predicate cannot both belong and not belong to the subject at the same time with regard to the future.
The consequence of this view seems to be that nothing takes place fortuitously, since "the meaning of the word 'fortuitous' with regard the the present or future events is that reality is so constituted that it may issue in either of two opposite directions." This means hat "All, then, that is about to be must of necessity take place. It results from this that nothing is uncertain or fortuitous, for it it were fortuitous it would not be necessary."

The problem of necessity cannot be avoided, Aristotle maintained, simply by denying the claim that one or the other of the events will take place: "If an event is neither to take place nor not to take place the next day, the element of chance will be eliminated. For example, it would be necessary that a sea-fight should neither take place place nor fail to take place on the next day."

If necessity cannot be avoided, "there would be no need to deliberate or take trouble, on the supposition that if we should adopt a certain course, a certain result would follow, while if we did not , the result would not follow." Aristotle asserted that "this view leads to an impossible conclusion," as there are "many obvious instances" of events which may potentially unfold in either direction, such as a coat's being destroyed either by being cut in half or by wearing out.

Aristotle's ultimate suggestion is to allow that while it is true that "everything must either be or not be, whether in the present or in the future," "it is not always possible to distinguish and state determinately which of these alternatives must necessarily come about." This does not seem to be a very satisfactory solution, as it appears to be an admission of the necessity of event, accompanied by a denial of our ability to say which events are necessary. In the following remarks, Aristotle states that "when in future events there is a real alternative, and a potentiality in contrary directions, the corresponding affirmation and denial have the same character." What character? Aristotle descirbes it confusingly: "One of the two propositions in such instances must be true and the other false, but we cannot say determinately that this or that is false, but must leave the alternative undecided." This sounds like his orignial solution. Then he follows this by saying something much stronger:

One may indeed be more likely to be true than the other, but it cannot be either actually true or actually false. It is therefore plain that it is not necessary that of the affirmation and a denial one should be true and the other false. For in the case of that which exists potentially, but not actually, the rule which applies to that which exists actually does not hold good. The case is rather as we have indicated.
This appears to be the position attributed to Epicurus by Cicero. It is a denial of the "law of bivalence," according to which every proposition is (at least actually) either true or false.

It is interesting that Cicero himself asserted the truth or falsehood of statements about the future but denied the necessity of the events when those events are "not in virtue of eternal causes derived from a necessity of nature." In other words, the "necessity" that is important in the case of human action is a causal necessity. As we saw above, Cicero thought that causal necessity of an "external and antecedent" kind is the only sort of necessity to threaten human freedom. Interestingly, Cicero also stated that if the denial of the present truth or falsehood of future events were the only alternative to the necessity of fate, he would "rather accept that blow," since the claim about truth and falsehood "is at least subject to debate," but the doctrine of fate is "intolerable."

Still, Cicero was sure of how the debate would come out. He pointed out that the Epicureans were ashamed to deny that the truth of the disjunction, that either one of two alternative future events will come to be. (This is known now as the law of the excluded middle.") But he went on to ask how one could uphold this principle if neither of the disjuncts has a truth-value.

What an amazing audacity and ignorance of logic! For if in speech there is something which is neither true nor false, certainly it is not true. But how can what is not true not be false? Or how can what is not false not be true? (On Fate, 37
Epicurus would have to reply that there are "truth-value gaps," such that what is not true could be either true or indeterminate, the same applying to what is not false. Indeterminacy would be a third "truth-value." This suggestion is not so outrageous that it would not be developed by sober logicians in the twentieth century. (See Nicholas Rescher's 1969 survey of such developments, Many-Valued Logic.)
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