Stanford University

Ancient Atomism (Stanford Encyclopedia of Philosophy)

1. Atomism before Leucippus?

Leucippus (5th c. BCE) is the earliest figure whose commitment to
atomism is well attested. He is usually credited with inventing
atomism. According to a passing remark by the geographer Strabo,
Posidonius (1st c. BCE Stoic philosopher) reported that
ancient Greek atomism can be traced back to a figure known as Moschus
or Mochus of Sidon, who lived at the time of the Trojan wars. This
report was given credence in the seventeenth century: the Cambridge
Platonist Henry More traced the origins of ancient atomism back, via
Pythagoras and Moschus, to Moses. This theologically motivated view
does not seem to claim much historical evidence, however.

In 1877, Tannéry argued that Zeno of Elea’s arguments about
divisibility must have been formulated in response to some early
Pythagoreans. Tannéry’s view, which was widely accepted in the
early twentieth century, is based on the claim that one of Zeno’s
paradoxes about the possibility of motion would best make sense if it
were attacking an atomist thesis, and thus that the Pythagoreans, who
are reported to have talked of monads or unit numbers, must have been
atomists of a sort. Tannery’s thesis has been thoroughly challenged
since then: most scholars instead consider atomism to be one of a
number of positions formulated in response to the arguments of
Parmenides and Zeno (first half of the fifth century). A
fourth-century Pythagorean, Ecphantus, interpreted the Pythagorean
monads as indivisible bodies: he is reported to have been sympathetic
to atomism of a kind similar to Democritus’. Plato’s discussion of the
composition of solids from plane surfaces is thought to be based on
fourth-century Pythagorean theories.

2. Leucippus and Democritus

Leucippus and Democritus are widely regarded as the first atomists
in the Greek tradition. Little is known about Leucippus, while the
ideas of his student Democritus—who is said to have taken over
and systematized his teacher’s theory—are known from a large
number of reports. These ancient atomists theorized that the two
fundamental and oppositely characterized constituents of the natural
world are indivisible bodies—atoms—and void. The latter is
described simply as nothing, or the negation of body. Atoms are by
their nature intrinsically unchangeable; they can only move about in
the void and combine into different clusters. Since the atoms are
separated by void, they cannot fuse, but must rather bounce off one
another when they collide. Because all macroscopic objects are in fact
combinations of atoms, everything in the macroscopic world is subject
to change, as their constituent atoms shift or move away. Thus, while
the atoms themselves persist through all time, everything in the world
of our experience is transitory and subject to dissolution.

According to Aristotle’s presentation (On Generation and
I 8), the motivation for the first postulation of
indivisible bodies is to answer a metaphysical puzzle about the
possibility of change and multiplicity. Parmenides had argued that any
differentiation or change in Being implies that ‘what is
not’ either is or comes to be. Although there are problems in
interpreting Parmenides’ precise meaning, he was understood to have
raised a problem about how change can be possible without something
coming from nothing. Several Presocratics formulated, in response,
philosophical systems in which change is not considered to require
something coming into being from complete nonexistence, but rather the
arrangement of preexisting elements into new combinations. The atomists
held that, like Being, as conceived by Parmenides, the atoms are
unchangeable and contain no internal differentiation of a sort that
would allow for division. But there are many Beings, not just one,
which are separated from another by nothing, i.e. by void.

By positing indivisible bodies, the atomists were also thought to be
answering Zeno’s paradoxes about the impossibility of motion. Zeno had
argued that, if magnitudes can be divided to infinity, it would be
impossible for motion to occur. The problem seems to be that a body
moving would have to traverse an infinite number of spaces in a finite
time. By supposing that the atoms form the lowest limit to division,
the atomists escape from this dilemma: a total space traversed has only
a finite number of parts. As it is unclear whether the earliest
atomists understood the atoms to be physically or theoretically
indivisible, they may not have made the distinction.

The changes in the world of macroscopic objects are caused by
rearrangements of the atomic clusters. Atoms can differ in size, shape,
order and position (the way they are turned); they move about in the
void, and—depending on their shape—some can temporarily
bond with one another by means of tiny hooks and barbs on their
surfaces. Thus the shape of individual atoms affects the macroscopic
texture of clusters of atoms, which may be fluid and yielding or firm
and resistant, depending on the amount of void space between and the
coalescence of the atomic shapes. The texture of surfaces and the
relative density and fragility of different materials are also
accounted for by the same means.

The atomists accounted for perception by means of films of atoms
sloughed off from their surfaces by external objects, and entering and
impacting the sense organs. They tried to account for all sensible
effects by means of contact, and regarded all sense perceptions as
caused by the properties of the atoms making up the films acting on the
atoms of animals’ sense organs. Perceptions of color are caused by the
‘turning’ or position of the atoms; tastes are caused by
the texture of atoms on the tongue, e.g., bitter tastes by the tearing
caused by sharp atoms; feelings of heat are ascribed to friction.
Democritus was taken by Aristotle to have considered thought to be a
material process involving the local rearrangement of bodies, just as
much as is perception.

A famous quotation from Democritus distinguishes between perceived
properties like colors and tastes, which exist only ‘by
convention,’ in contrast to the reality, which is atoms and void.
However, he apparently recognized an epistemological problem for an
empiricist philosophy that nonetheless regards the objects of sense as
unreal. In another famous quotation, the senses accuse the mind of
overthrowing them, although mind is dependent on the senses. The
accusation is that, by developing an atomist theory that undermines the
basis for confidence in sense perception, thought has in effect
undercut its own foundation on knowledge gained through the senses.
Democritus sometimes seems to doubt or deny the possibility of

The early atomists try to account for the formation of the natural
world by means of their simple ontology of atoms and void alone.
Leucippus held that there are an infinite number of atoms moving for
all time in an infinite void, and that these can form into cosmic
systems or kosmoi by means of a whirling motion which randomly
establishes itself in a large enough cluster of atoms. It is
controversial whether atoms are thought to have weight as an intrinsic
property, causing them all to fall in some given direction, or whether
weight is simply a tendency for atoms (which otherwise move in any and
every direction, except when struck) to move towards the centre of a
system, created by the whirling of the cosmic vortices. When a vortex
is formed, it creates a membrane of atoms at its outer edge, and the
outer band of atoms catches fire, forming a sun and stars. These
kosmoi are impermanent, and are not accounted for by purpose
or design. The earth is described as a flat cylindrical drum at the
center of our cosmos.

Species are not regarded as permanent abstract forms, but as the
result of chance combinations of atoms. Living things are regarded as
having a psychê or principle of life; this is identified
with fiery atoms. Organisms are thought to reproduce by means of seed:
Democritus seems to have held that both parents produce seeds composed
of fragments from each organ of their body. Whichever of the parts
drawn from the relevant organ of the parents predominates in the new
mixture determines which characteristics are inherited by the
offspring. Democritus is reported to have given an account of the
origin of human beings from the earth. He is also said to be the
founder of a kind of cultural anthropology, since his account of the
origin of the cosmos includes an account of the origin of human
institutions, including language and social and political

A large group of reports about Democritus’ views concern ethical
maxims: some scholars have tried to regard these as systematic or
dependent on atomist physics, while others doubt the closeness of the
connection. Because several maxims stress the value of
‘cheerfulness,’ Democritus is sometimes portrayed as
‘the laughing philosopher.’

3. Plato and Platonists

Although the Greek term atomos is most commonly associated
with the philosophical system developed by Leucippus and Democritus,
involving solid and impenetrable bodies, Plato’s Timaeus
presents a different kind of physical theory based on indivisibles. The
dialogue elaborates an account of the world wherein the four different
basic kinds of matter—earth, air, fire, and water—are
regular solids composed from plane figures: isoceles and scalene
right-angled triangles. Because the same triangles can form into
different regular solids, the theory thus explains how some of the
elements can transform into one another, as was widely believed.

In this theory, it is the elemental triangles composing the solids
that are regarded as indivisible, not the solids themselves. When
Aristotle discusses the hypothesis that the natural world is composed
of indivisibles, the two views he considers are Plato’s and
Democritus’, although he seems to have more respect for the latter
view. Aristotle criticizes both Plato’s and the fourth-century
Pythagorean attempts to construct natural bodies possessing weight from
indivisible mathematical abstractions, whether plane surfaces or

It has been suggested that Plato accepted time atoms, i.e.,
indivisible minima in time, but this is controversial. A report by
Aristotle suggests that the belief of Plato’s student Xenocrates in the
existence of indivisible lines was also shared by Plato; other
testimony suggests that points are really what Plato refers to as

In late antiquity, the Neoplatonist Proclus defended Plato’s account
against Aristotle’s objections; these arguments are preserved in
Simplicius’ commentary on Aristotle’s On the Heavens.
Simplicius credits the Pythagoreans as well as Plato with a theory
composing bodies from plane surfaces. Simplicius also compares
Pythagorean views to Democritean atomism, inasmuch as both theories
posit a cause for hot and cold, rather than taking these to be
fundamental principles, as the Aristotelians do.

4. Xenocrates

A treatise in the Aristotelian corpus probably not by Aristotle
himself (On Indivisible Lines) addresses and refutes a number
of arguments offered for the existence of indivisible lines, without
naming their author. Plato’s student Xenocrates (396–314 BCE), third
head of the Academy, is reported to believe in indivisible lines, and
he may well be the target of the Aristotelian treatise.

One of the arguments attacked addresses a Zenonian problem about
traversing or touching in succession an infinite series of parts. The
idea that there are indivisible lines offers an alternative to the view
that any extended magnitude must be divisible to infinity. Another
argument concerns Platonic Forms, and would only apply to those who
accepted their existence. It argues that the Form of a triangle
presupposes the existence of a Form of a line, and adds that this ideal
line cannot have parts, presumably because parts are taken to be prior
to the whole they compose and Forms need to have a kind of primacy to
be explanatory. A distinct argument also depends on the idea of
priority: it is argued that if the physical elements composing a body
are regarded as the ultimate parts prior to a whole, they cannot be
further divisible. Although this does not argue for indivisible lines
per se, it is used to suggest that the objects of sense as
well as those of thought must include things without parts.

A further argument depends on thinking that opposite properties must
have opposite characteristics: if ‘many’ or
‘large’ things have infinite parts, it is argued, then
‘few’ or ‘small’ things must have only a finite
number of parts. It is then concluded that there must be a magnitude
without parts, apparently so that it is not further divisible and thus
composed of an infinite number of parts. The last argument depends on
the idea that mathematicians talk of commensurable lines, and posit a
single unit of measurement: this would not be possible if the unit were
divisible, because the parts of the unit, if measured, would be
measured by the unit measure and it would then turn out to contain
multiple units within itself.

5. Minima Naturalia in Aristotle

An argument in Aristotle (Physics 1.4, 187b14–21) is
sometimes taken by later writers as evidence that Aristotle allowed
for the existence of minima in natural things. Aristotle writes that
there is a smallest size of material substrate on which it is possible
for the form of a given natural tissue to occur. Blood and bone, say,
are all materially composed of given proportions of earth, air, fire,
and water: there needs to be a certain minimal amount of these
material components present before the form of blood or bone can
occur. This doctrine, while it is surely compatible with the view that
the material components are nonetheless infinitely divisible, is
sometimes read, by some Neoplatonist commentators and later sources
interested in atomist theory, as evidence that Aristotle endorsed the
existence of minimal physical parts. In late antiquity, this debate
seems to have moved away from the radical solution of positing minimal
physical parts or atoms—a view that seems to have had few
advocates—into a puzzle about the possibilities of ‘bottom
up’ explanation or the need to regard emergent properties as
‘supervening’ and not mere products of the necessary
material base.

6. Diodorus Cronus

Diodorus Cronus (late 4th c. BCE), a member of the supposed
Dialectical School, is reported to have offered new arguments that
there must be partless bodies or magnitudes. Most reports suggest that
his focus was on logical arguments rather than on physical theory: he
used arguments that depend on positing mutually exhaustive

Perhaps drawing on an argument of Aristotle’s (Sens. 7,
449a20–31]), Diodorus apparently used the idea that there is a smallest
size at which an object at a given distance is visible as the basis for
an argument that there are indivisible magnitudes. His argument begins
from the idea that there is a difference in size between the smallest
size at which a given object is visible—presumably from a given
distance—and the largest size at which it is invisible. Unless we
concede that, at some magnitude, a body is both invisible and visible
(or neither), there cannot be any other magnitude intermediate between
these two magnitudes. Magnitudes must increase by discrete units.

Sextus Empiricus (AM 10.48ff) reports an argument of
Diodorus’ also concluding that magnitudes have discrete intervals. It
also denies the existence of moving bodies, insisting that bodies move
neither when they are in the place where they are, nor when they are in
the place where they are not. Since these alternatives are presented as
exhaustive, the conclusion must be that bodies are never moving.
However, rather than assert that everything is static, Diodorus took
the view that bodies must have moved without ever
being in motion: they are simply at one place at one moment,
and at another place at another moment.

As well as postulating the existence of indivisible smallest bodies
and magnitudes, Diodorus seems to have supposed that there are
indivisible smallest units of time. The argument about motion does not
quite make it explicit that this is what he is committed to, but it is
a reasonable inference: given his insistence that bodies are always at
one place or another at any given time, he might well suppose that
infinite divisibility of time would open up the threatening possibility
of indeterminacy as to whether the change of place has taken place.

For those who posit indivisibles as a way to escape paradoxes about
infinite divisibility, parallel arguments might equally well have been
applied to the problem of completing tasks in an infinitely divisible
time. Sextus Empiricus reports that the Aristotelian Strato of
Lampsacus (d. 268/70 BCE) argued for time atoms, although this is
contradicted by other sources. Sorabji 1983 suggests that Strato merely
countenanced the possibility that time could be discrete while
space and motion are continuous, without endorsing this position.

7. Epicurean Atomism

Democritus’ atomism was revived in the early Hellenistic period, and
an atomist school founded in Athens about 306, by Epicurus (341–270
BCE). The Epicureans formed more of a closed community than other
schools, and promoted a philosophy of a simple, pleasant life lived
with friends. The community included women, and some of its members
raised children. The works of the founder were revered and some of them
were memorized, a practice that may have discouraged philosophical
innovation by later members of the school.

Epicurus seems to have learned of atomist doctrine through
Democritus’ follower Nausiphanes. Because Epicurus made some
significant changes in atomist theory, it is often thought that his
reformulation of the physical theory is an attempt to respond to
Aristotle’s criticisms of Democritus. Even more significant, however,
is the increasing centrality of ethical concerns to Epicurus’ atomism,
and the importance of the view that belief in an atomist physical
theory helps us live better lives.

Epicurus takes to heart a problem Democritus himself recognized (see
2. above), which is that atomist theory threatens to undermine itself
if it removes any trust we can place in the evidence of the senses, by
claiming that colors, etc. are unreal. He notoriously said that
‘all perception is true,’ apparently distinguishing between
the causal processes which impact our senses, all of which originate
with the films of atoms sloughed off by objects, and the judgments we
make on the basis of them, which may be false. Reasoning to truths
about things that are not apparent—like the existence of
atoms—depends on the evidence of the senses, which is always true
in that it consists of impacts from actually existing films. For
particular phenomena, like meteorological events, Epicurus endorses the
existence of multiple valid explanations, acknowledging that we may
have no evidence for preferring one explanation over another.

It may be that Epicurus was less troubled by any such
epistemological uncertainties because of his emphasis on the value of
atomist theory for teaching us how to live the untroubled and tranquil
life. Denying any divine sanction for morality, and holding that the
experience of pleasure and pain are the source of all value, Epicurus
thought we can learn from atomist philosophy that pursuing natural and
necessary pleasures—rather than the misleading desires inculcated
by society—will make pleasure readily attainable. At the same
time, we will avoid the pains brought on by pursuing unnatural and
unnecessary pleasures. Understanding, on the basis of the atomist
theory, that our fears of the gods and of death are groundless will
free us from our chief mental pains.

Epicurus made significant changes to atomist physical theory, and
some of these have been traced to Aristotle’s criticisms of Democritus.
It seems that Democritus did not properly distinguish between the
thesis of the physical uncuttability of atoms and that of their
conceptual indivisibility: this raises a problem about how atoms can
have parts, as evidenced by their variations in shape or their ability
to compose a magnitude, touching one another in a series on different
sides. Epicurus distinguished the two, holding that uncuttable atoms
did have conceptually distinct parts, but that there was a lowest limit
to these.

Epicurus’ view of the motion of atoms also differs from Democritus’.
Rather than talking of a motion towards the center of a given cosmos,
possibly created by the cosmic vortex, Epicurus grants to atoms an
innate tendency to downward motion through the infinite cosmos. The
downward direction is simply the original direction of atomic fall .
This may be in response to Aristotelian criticisms that Democritus does
not show why atomic motion exists, merely saying that it is eternal and
that it is perpetuated by collisions. Moreover, although this is not
attested in the surviving writings of Epicurus, authoritative later
sources attribute to him the idea that it belongs to the nature of
atoms occasionally to exhibit a slight, otherwise uncaused swerve from
their downward path. This is thought to explain why atoms have from
infinite time entered into collisions instead of falling in parallel
paths: it is also said, by Lucretius, to enter into the account of
action and responsibility. Scholars have proposed a number of
alternative interpretations as to how this is thought to work.

Epicurus seems to have taken a different view on the nature of
properties, denying Democritus’ claim that perceived properties only
exist ‘by convention’. His successor Polystratus further
defended and elaborated a claim about the reality of properties,
including relational properties. Moreover, with the recovery of new
papyrological evidence, controversy has arisen about the extent to
which Epicurus rejected Democritus’ attempt to account for all causal
processes by the properties of the atoms and void alone. Although
Epicurus’ ideas have long been known from three surviving letters
preserved in the biography by Diogenes Laertius, no copy of his longer
work On Nature had been available. However, following
excavation of the Epicurean library at Herculaneum that was buried by a
volcanic eruption, some parts of this work are being recovered. Many of
the scrolls found are badly damaged, however, and interpretation of
this newly recovered material is ongoing.

The Herculaneum library contains much work of the Epicurean
Philodemus (1st c. BCE). Philodemus wrote extensively, including on the
history of philosophy, ethics, music, poetry, rhetoric and the
emotions. He wrote a treatise on the theory of signs: because they are
empiricists, believing that all knowledge comes from our sense
experience, later Epicureans were concerned about the basis for our
knowledge of imperceptibles like the atoms, and engaged in an extensive
debate with the Stoics about the grounds for inferences to
imperceptible entities.

Although Epicurus’ doctrines teach the value of a quiet life in a
specially constructed Epicurean community and decry the search for
fame, atomist theory is also regarded as a cure for the troubles
afflicting others outside the community, and there are certainly
Epicurean texts written for a wider audience. Besides the letters by
Epicurus himself summarizing his doctrines, the Epicurean philosopher
Lucretius (d. c. 50 BCE) wrote a long Latin poem advocating Epicurus’
ideas to Roman audiences. Lucretius makes clear his close allegiance to
Epicurus’ own views, and provides more detail on some topics than has
survived from Epicurus’ own work, such as an extended account of the
origins of human society and institutions. A less sympathetic
contemporary of Lucretius, Cicero, also wrote a number of Latin works
in which an Epicurean spokesman presents the doctrines of the school.
Diogenes of Oenoanda propagated Epicurean doctrines in Asia Minor,
inscribing them on the wall of a Stoa in his home town. Excavation of
these since the nineteenth century has also produced new texts, aimed
at converting passersby to Epicurean theory. Smith 1993, in his latest
edition of the text of the inscriptions, dates them to the early second
century CE.

8. Atomism and Particle Theories in the Sciences

Some figures concerned with the natural sciences, especially
medicine, are thought to have regarded organic bodies as made of some
kind of particles. The details of these views are often obscure. Galen,
in On the Natural Faculties, divides medical theorists into
two groups, following the division of natural philosophers. On the one
side are continuum theorists, who hold that all matter is infinitely
divisible but that all the matter in things subject to generation and
corruption is susceptible to qualitative alteration. On the other are
those who suppose that matter is composed of tiny, unchangeable
particles separated by void spaces, and explain qualitative change as
produced only in compound bodies, by rearrangement of the particles
alone. In Galen’s view, qualitative alteration is needed to produce the
powers whereby beneficent Nature directs change: Galen credits the
first group with asserting the priority of Nature and its beneficent
order, and the latter with denying this.

Although ancient natural philosophers tend to fall on either side of
Galen’s divide—continuum theory plus beneficent teleology, vs.
atomism plus blind necessity— there is a danger in taking this
dichotomy to be exhaustive or exclusive of possible natural
philosophies. Inasmuch as the view Plato develops in Timaeus
is atomistic and also endorses teleological explanation, for example,
his position complicates the picture, and other theories of natural
philosophy in the Hellenistic period do not divide so neatly onto one
side or the other. Galen has polemical interests in discrediting those
who deny the need for qualitatively irreducible faculties or powers
employed by Nature to produce beneficial results. In cases where we
have only scattered reports and secondhand information, it is
difficult to know which views should be counted as atomistic. A
prevailing tendency in modern scholarship to identify atomist
tendencies with ‘mechanistic’ thinking is not
characteristic of ancient Greek atomism: the identification was made
in the work of Henry More and Robert Boyle in the 17th century. Galen
elsewhere explicitly contrasts atomist thought with the schools who
appeal to ideas from mechanics.

The theories of Heracleides of Pontus (4th c. BCE) and Asclepiades
of Bythnia (2nd c. BCE) are sometimes likened to atomism. Both—a
pupil of Plato, and a medical theorist—are said to have posited
the existence of corpuscles they call anarmoi onkoi, i.e. some
kind of ‘masses’, but the precise meaning is disputed.
Although the theories of Asclepiades in particular are often
assimilated to atomism, there is reason to think that Galen’s
identification of his view as atomistic is polemical, and that
Asclepiades’ particles are capable of division into infinitely many
pieces. Erasistratus of Ceos, one of the great anatomists of the third
century BCE, is another of those whom Galen suggests may have been on
the atomist side, despite his acceptance of design in nature.
Erasistratus had posited that the tissues of the body are composed of a
triple braid of vein, artery and nerve: Galen reports that even the
tissue of the nerve is made up of this tiny braid. He claims that the
Erasistrateans are divided as to whether the elemental nerve tissue is
a continuous mass or is composed of small particles like those of the

One of the most prominent writers on mechanics in antiquity, Hero of
Alexandria (1st c. CE), has been regarded, following Hermann Diels, as
an atomist. In the introduction to his Pneumatica, he
describes matter as made up of particles with spaces between
them. However, Hero’s account of pneumatic effects involving the
compression of air—discovered by Ctesibius—seems to depend
on the deformation of elastic particles which can be compressed
artificially but will spring back to their original shape quite
vehemently. If so, his account denies a fundamental tenet of classical
atomism, that atoms do not change in their intrinsic properties like

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