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Dictionary of the History of Ideas

Studies of Selected Pivotal Ideas
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The aim of this study is to examine in what way the
different Greek and medieval conceptions of nature
have contributed to paving the way for the scientific
theories of modern times. Within this general frame-
work we shall pay special attention to the classical
atomic theory in its different variants, because the
development of this theory illustrates to what extent
modern science, with its underlying conception of
nature, can be traced to the speculations of Greek

The very fact that the atomic theory has a long
history does not make it easy to define its content in
such a way as to comprehend its different variations.
In its widest sense it may be defined, however, as the
theory that nature is composed of relatively simple and
relatively unchangeable minute particles, which are
too small to be directly observable. The observable
changes in nature can be explained, however, by their
reduction to changes in the configuration of the parti-
cles. The observable multiplicity of the existing forms
in nature must likewise be based upon differences of
forms and of configuration of the particles.

Within this general atomistic conception a distinc-
tion can be made between atomism in the strict sense,
and other forms. Atomism in the strict sense, pro-
pounded by Leucippus and Democritus (fifth century
B.C.), is the doctrine that the particles, of which the
material things are composed, are simple and un-
changeable in an absolute sense (hence the name
“atoms,” which means indivisible). Whatever may be
the changes in configuration of these atoms, the atoms
themselves remain intrinsically unchanged. The ob-
servable differences in nature, however, are not exclu-
sively based upon differences in configuration of the
atoms. Another ground of difference is to be found in
the atoms themselves. Although the atoms do not differ
qualitatively, they are supposed to have different sizes
and shapes, even in an infinite multitude. Consequently,
the infinite variety of observable things can be ex-
plained by these different sizes and shapes and by the
different ways in which the atoms can be combined.

As to the question of what causes the atoms to
combine and recombine, Democritus points to motion
as a primitive property of atoms. This motion itself
is not caused by an outside force; it is an inherent
property of the atoms. Like the atoms themselves, their
motion is eternal and incorruptible.

Other forms of Greek atomism differ from the atom-
ism of Democritus mainly in three points. First, they
do not restrict the differences between the atoms to
purely quantitative ones, but also accept differences
in quality. Anaxagoras (fifth century B.C.) assumed even
as many qualitatively different atoms as there are
different natural kinds of substances. As a rule, how-
ever, the more “liberal” forms of atomism accept only
a limited number of types of primitive atoms. Their
qualitative differences are usually based upon the
widespread doctrine of the four elements: earth, water,
air, and fire (Empedocles, fifth century B.C.).

A second point of difference concerns the indivisi-
of the atoms. Whereas Democritus holds that this
indivisibility has to be taken in a strict sense, other
forms of “atomism” speak of an indivisibility only in
a relative sense. The smallest particles of a certain
substance could be divided, but then they change their
nature. Here again Anaxagoras has a different opinion.
In his conception the particles could infinitely be di-


vided without losing their specific nature. In view of
its importance in the Middle Ages the most interesting
doctrine is that which found its origin in the circles
of Aristotle's commentators. Finally, the most general
characteristic which distinguishes Democritus' atomism
from other doctrines is probably its conception of
nature. Whatever forms may be found in nature, their
origin has to be sought exclusively in the inherent
properties of the atoms: their number, size, shape, and
their motion. Other doctrines assume, in one way or
another, some fundamental nonmaterial principle as
the source of the order in nature.

The best way to understand the specific features of
Democritus' atomism is to examine the problems for
which he sought a solution. These were not the kind
of problems in which modern scientific atomic theory
is engaged. This is not to say that there is no connection
at all, for there is an unmistakable historical connec-
tion, but the intellectual climate of the fifth century
B.C. differed considerably from ours. Not only are
modern scientific problems posed within the context
of already existing scientific theories and hypotheses,
but they also presuppose an intellectual tradition. The
first Greek philosophers, however, had to find their way
without such a tradition; their task was the very crea-
tion of an intellectual climate in which scientific the-
ories could be formulated.

Shortly before Democritus, Parmenides and Hera-
clitus had already considerably contributed to the cre-
ation of such a climate. Parmenides (about 500 B.C.)
was greatly impressed by the cognitive power, pos-
sessed by the human intellect: only that can exist which
can be understood (cf. Diels, frag. B. 3). Guided by
this principle, Parmenides, through subtle arguments,
came to the conclusion that reality must be one and
immutable, despite the apparent testimony of the
senses to the contrary. At first sight, Parmenides' thesis
does not at all seem favorable for the rise of modern
science. This science is based upon the careful obser-
vation of facts, and the empirical evidence of plurality
and change in nature is indisputable. Nevertheless, a
closer consideration of Parmenides' thesis about the
unity and immutability of reality reveals its profound
meaning for physical science too. Without immutabil-
ity in nature, there would be no room at all for scien-
tific laws. Without unity in nature, there would be no
room for universal laws.

Nevertheless, the approach of Parmenides to under-
standing reality was too one-sided. It did not take the
testimony of the senses seriously enough. This same
testimony led his contemporary Heraclitus to the as-
sertion that everything is mutable (πάνγα ρει) and
nothing remains. Democritus' atomism has to be un-
derstood as an attempt to combine the fundamental
thesis of Parmenides, prerequisite for any rational ex-
planation, with the reality of change and multiplicity.
For this reason the atoms are unchangeable and quali-
tatively the same. The observable multiplicity and
qualitative change can sufficiently be explained by the
quantitative differences between the atoms and by their
motion, which opens up the possibility of their combi-
nation and recombination. Why did Democritus think
his retouching of Parmenides' thesis was justified? Jus-
tification lay in the fact that changes in configuration
and quantitative difference are mathematically intel-
ligible. Thus he did not come into conflict with Par-
menides' first principle: “Only that can exist which can
be understood.”

Parmenides' great impact of Greek thought is shown
by the fact that his considerations form the starting
point of practically all later Greek conceptions of
nature, including those conceptions which greatly
differ from Democritus' atomism.

The main reason why later philosophers, such as
Plato and Aristotle, rejected Democritus' solution of
the problems of change and multiplicity was not on
account of the general idea of atoms, but because of
the one-sidedness of the atomic theory, considered as
a universal theory. For Democritus the theory was not
merely intended as an explanation of natural phenom-
ena, such as evaporation, or the existence of different
states of aggregation. Its aim was to give an ultimate
explanation of change as change, including also those
changes occurring in man's mental activity. For this
reason Democritus assumed atoms of a special form
and shape for the composition of the soul. Because they
are fine and round, the atoms of the soul can penetrate
the whole body, move it, and thus cause its vital func-
tions. Whatever may be the fruitfulness of the atomistic
approach to the explanation of natural phenomena, the
atomic theory of Democritus was not judged on its
merits as a merely physical theory, for such a theory
did not yet exist. The theory was judged on its merits
as a universal explanation of the whole of reality. As
such it was rejected. It was too materialistic, in reduc-
ing all qualities and feelings to quantities of atoms, and
their configuration. Only gradually did man learn to
distinguish methodically the different problems which
are interwoven in Democritus' system. In Greek
thought philosophy and science still formed a unity.
This holds true also of other forms of atomism, which
can be distinguished from atomism in the strict sense.

Plato, for example, in the Timaeus propounded an
atomic theory of his own, but only as a minor aspect
of a much broader idealistic conception of nature. In
order to understand this conception we must return
again to the sharply contrasting views of Parmenides
and Heraclitus. For Parmenides reality was immutable


being; for Heraclitus it was continuous change. Plato
accepts both views, but with respect to different reali-
ties, the unchangeable world of ideas and the changing
world of sense perception, respectively. The latter
world is a shadow of the first, or its imperfect embodi-
ment. Consequently, true knowledge is not directed
towards the imperfect world of change, but towards
the real unchangeable world. On account of the im-
perfection and continuous change of the natural things
natural science could only be an imperfect science. A
further consequence of Plato's conception of the two
worlds is that the principle of order in nature was not
to be found in nature itself, but in the world from
which it is derived. This also explains the great value
Plato assigned to mathematics. For the object of math-
ematics is not the observable realization of a circle
or a triangle, but their ideas or essences in their un-
observable, logical purity.

A few words remain to be said about Plato's atomic
theory. He accepts the elements which Empedocles
has proposed on empirical grounds: fire, air, water, and
earth. In accordance with his views of the importance
of mathematics, Plato tries to characterize the atoms
of these elements by means of their mathematical form.
Fire has the form of a tetrahedron, air of an octahe-
dron, water of an icosahedron, and earth of a cube.
The concrete characterization is, of course, arbitrary,
but the idea of connecting physical properties with
mathematical structures is in itself extremely fruitful.
Yet no physical science could arise from this general
idea. The distance between the observable phenomena
and the hypothetical structure was too great.

Aristotle approached the problems of natural science
much more empirically than either Democritus or
Plato. He could not accept the viewpoint of Democ-
ritus that all observable change is only accidental, and
differed with Plato mainly on the theory of knowledge.
For Aristotle experience was the sole fount of human
knowledge, including the knowledge of abstract pure
concepts. As a consequence, he rejected the distinction
between the unchangeable world of ideas and the
observable world of change. Having safeguarded the
reality of the sensible world, Aristotle faced the diffi-
culties of Parmenides. How is real change possible?
Atomism certainly opened up the possibility of ex-
plaining accidental changes, in which things did not
change their nature, but it could not explain changes,
in which things lost their nature. Nor did it account
for the existing variety of natural things. It could ac-
count for the accidental variety, but not for the fact
that nature showed a fixed order in the variety of
things, an order which remained constant through the
transitory existence of phenomena. Thus the first task
imposed upon Aristotle was a critical reexamination
of Parmenides' thesis.

In order to be intelligible, the possibility of change
presupposes a certain fundamental complexity rather
than simplicity in material things. Otherwise it is not
possible to account for both aspects that are present
in change: the aspect of certain permanence, and the
aspect of something that is really new. To a certain
extent Democritus followed the same line of thought.
Democritus, however, “substantialized” the permanent
aspect, for the atoms were things, which existed as
such. Thus the possibility of change was narrowed
down to accidental changes of configuration. In the
eyes of Aristotle, the permanent aspect could not be
a thing, but only a principle, i.e., something that does
not exist in itself, but only as a component of a whole.
Aristotle calls it matter (hylē). Matter is not a substance,
but only the capacity to receive “forms.” Since matter
is not itself a substance, even the most radical changes
are possible. For existing things are composed of matter
and form, i.e., matter that has received a form. Things
can change, because one form can supersede another.
The most radical changes are those in which things
receive a new substantial form. In this case their nature

The way in which Aristotle discusses the possibility
of change makes it clear that he has quite another
conception of nature than that of Democritus' atomism.
With Aristotle nature refers to something fundamental
in material things, which is both the source and the
end of their growth, their movements, and their
changes. “The primary and proper meaning of 'na-
ture' is the essence of things which have in themselves
(qua themselves) a principle of motion” (Metaphysics,
Book IV). Because each thing acts according to its
nature, i.e., according to its natural form, each thing
strives for its proper end, i.e., its place in the whole
of the cosmos. Thus there is a kind of “built-in” natural
order, based upon a natural series of final causes.

In these conceptions of Aristotle there is, of course,
no place for unchangeable atoms. This does not mean,
however, that Aristotle had no thought of minute par-
ticles. Although he did not propound a corpuscular
theory of his own, we find a few remarks that could
have been the starting point for such a theory. The
remarks form part of Aristotle's criticism of Anax-
agoras' theory about the infinite divisibility of material
things. Somewhere there must be a limit to divisibility,
which limit is determined by the specific nature of the
things in question. It was left to Aristotle's Hellenistic,
Arabian, and medieval commentators to develop the
casual remarks of their master into the minima natu-
theory, stating that each kind of substance has


its specific minima naturalia. These commentators
combined the empirical approach of Aristotle with the
more rational approach of Democritus. Thus they pre-
pared the way for the scientific atomic theory of mod-
ern times.

Before we discuss, however, the gradual shift of
emphasis from a philosophic to a scientific atomic
theory, we must first turn our attention to an aspect
of the Greek ideas of “nature,” which played an im-
portant role namely, the ethical dimension of their
concept of nature. It is interesting to note that the use
of the term “natural law” arose first in the ethical
realm. The reason may have been that the phenomena
of nature at first impressed man as much more unpre-
dictable than human behavior, which was subject to
fixed laws. The Greeks themselves did not even use
the term “natural law” in the sense it has nowadays
in physical science: the latter sense dates from modern
times. This does not mean, as we have seen, that the
Greeks were not familiar with the idea of a rational
order in nature. On the contrary, this idea was central
to their thought, but they did not express it in terms
of laws. Even with respect to the ethical sense, the
coupling of nature and law into a single expression does
not seem to have occurred before the last phase of
Greek philosophy as represented by Stoicism.

By speaking of “natural law” the Stoics revived to
a certain extent the pre-rationalistic conception of
mythological thought, in which hardly any distinction
was made between the orders of nature, of culture,
and of ethics. The difference was, however, that the
Stoics were convinced of the rationality of the all-
embracing order of nature. Because nature is perme-
ated with rationality, and rationality constitutes the
nature of man, the Stoic philosophers could return to
the identification of the natural and the ethical order,
without losing sight of the special position which man
occupies in the natural order. For precisely the reflec-
tion on this position has marked the transition from
the mythological to the rational phase in Greek
thought. By his reason (ratio) man could know his own
place in the natural order, and by his free will he could
live in accordance with the ethical obligations, which
followed from his own place in this order. It is, there-
fore, no coincidence that long ago the term “nature”
also acquired the meaning of “essence.” The study of
the order of natural things and the place of man in
it revealed both the essence of natural things and that
of man.

It is not easy to summarize in a few words the
general characteristics of the Greek conception of
nature. The great diversity of opinion among Greek
philosophers makes it even impossible to speak of the
Greek conception. Yet the ethical connotation, which
the term “nature” could acquire, indicates a certain
common conviction with almost all Greek philoso-
phers, namely, that the order of nature is an order
which man cannot change and which he has to accept
as it is. Within the order there was a certain room
for human intervention, but the possibilities of this
intervention did not apply to nature as a whole, but
only to those aspects of it which invited man, as it
were, to utilize them. The room in nature for human
culture confirmed rather than weakened the idea of
the order of nature.

Closely connected with the conception of nature as
an unchangeable order, which man has to respect both
in his technology and in his ethics, was the conviction
that the order of nature was something “supernatural.”
It should be noted, however, that this last conviction
was less general than the first.

In the Christian era the Greek conceptions of nature
were taken over, albeit with an important correction.
The order of nature became a created order. Christian
thought broke with the identification of the natural
and the supernatural, and with the identification of the
natural and the ethical order. The transcendent Creator
as the origin of nature and of man made it possible
to place man apart from nature, even though their
common origin was maintained. The ethical concept,
“natural law,” could be preserved, but it now had a
different sense, because man's relationship to the Crea-
tor differs fundamentally from his relation to nature.
Both nature and man are subjected to the divine law,
but not on the same level. Man's freedom with respect
to nature was, in principle, safeguarded; nature was
at his disposal. To what extent man could “cultivate”
nature was, however, not clear. Man had still to find

As long as natural science seemed to be no more
than a rational affair, the conception of nature as an
order which man could understand but not change
remained predominant. Even after the Middle Ages
when mathematical rationality superseded philo-
sophical rationality, this predominance was not seri-
ously affected. The new rationality of science changed
the idea of the order of nature insofar as this order
became the order of a mechanism. It became the con-
struction of a chief engineer, but it still remained an
order, which could not be altered by man. He simply
seemed to lack the necessary power. Only after it
became clear that in order to reach its rational end
science had to use experimental means, could a new
conception of nature appear. The laws of nature (in
the scientific sense) were not only the signs of the
rationality of nature, but also the means for the manip-


ulation of nature. The actual order of nature lost its
supernatural and normative character. Through an
apparently fixed order of nature man saw an unlimited
field of possibilities, based upon the fundamental forces
of nature.

In the process that led to this view of nature, the
development of the atomistic conceptions into an
experimental scientific theory played an important
role. How did this development take place? For the
answer to this question let us return to the Greek
atomic conceptions and follow their development
through the centuries until the seventeenth century. As
we have pointed out, there could not have been a
straightforward development from the Greek atomic
theories to the scientific atomic theories of modern
times. The main reason why this was impossible was
not that the Greek theories were, first of all, philo-
theories, propounded to explain the possibility
of change in general. This was, of course, a reason,
but there were nevertheless enough physical aspects
in the atomistic doctrines which could function as the
beginning of a fruitful physical theory. The main diffi-
culty was that there was too great a gap between the
observable phenomena and the theories in question,
even with respect to their physical aspects. Accord-
ingly, it was not the atomic theory which was destined
to be the first fruitful physical theory. This had to be
a theory in which the observable phenomena were
much more directly suited to be fitted in a mathe-
matical model. It is, therefore, not accidental that the
first fruitful general physical theory, namely mechanics,
was of astronomical origin. Only after a general theory
of mechanics was developed, which could be applied
to the hypothetical atoms, was the time ripe for a
physical atomic theory.

With respect to a chemical atomic theory, the situa-
tion was different. Of course, this theory had to wait
also until more general theories were developed, but
typical corpuscular considerations could play a part
in the coming into existence of these theories. It was
easier to characterize the hypothetical atoms by their
chemical than by their physical properties. It is obvious
that of the two important Greek corpuscular schools
of thought, the atomism of Democritus and the minima
doctrine of the commentators of Aristotle, the latter,
with its specific minima for each kind of substance,
offered more possibilities for chemical considerations.
The interesting point is that one of the main aspects
of the development of the minima doctrine lies pre-
cisely in a gradual development from a more philo-
sophical to a more chemical theory. Atomism does not
show such a development. This is mainly due to the
fact that its spiritual father Democritus had already
elaborated a more or less complete doctrine, and that
it had only a few adherents between the fifth century
B.C. and the seventeenth century; the most important
are Epicurus and the Latin poet Lucretius Carus.

Epicurus introduced the idea of a chance swerve
(clinamen) of the atoms, which he and Lucretius
thought would make intelligible new qualities and free
will. What atomism needed for its development was,
above all, the connection with observable chemical
phenomena. And for such a development it was less
suited than the minima theory. When atomism came
to occupy again a central position in the seventeenth
century, it could profit from the development the
minima theory meanwhile had made. What were the
main stages in this development?

To Aristotle himself the minima did not mean much
more than a theoretical limit to divisibility; they were
potentialities rather than actualities. Nothing indicates
that he attributed to the minima a certain measure of
independent existence. As early as the Greek commen-
tators of Aristotle—Alexander of Aphrodisias (second
century A.D.), Themistius (fourth century), and John
Philoponus (sixth century)—we find an important de-
velopment. In their thought the smallest particles are
more than just potentialities, the commentators speak
of the elachista, the Greek equivalent of minima, as
if they actually existed. Furthermore, in reading these
commentators we get the impression that they used
the term elachista in a special, technical sense. Just
as for Democritus the term atomos does not mean
exclusively indivisible, but is also a technical term for
smallest particles, so also something similar seems to
be the case with the word elachista. Each specific
substance has its own elachista.

Averroës (1126-98), the most important Arabian
commentator on Aristotle, followed up this line of
thought. With him we find the suggestion that the
elachista or minima play an important role during
chemical reactions. His Latin followers elaborated this
point. They do not leave us in doubt that they attrib-
uted to the minima not only an independent existence,
but also a certain function in several physical and
chemical reactions. In their opinion the minima were
the actual building stones of reality. Consequently, the
increase or decrease in quantity of a substance amounts
to the addition or subtraction of a certain number of
minima. Agostino Nifo (1473-1538) assures us that a
chemical reaction takes place among the minima.
When elements react upon each other they are divided
into minima.

The next step was now to examine how the proper-
ties of the minima, responsible for chemical and physi-
cal reactions, could be connected with the specific
observable properties of different substances. The first
attempt to do so was, of course, not very spectacular,


but the attempt in itself was already important for the
coming into existence of chemical and physical the-
ories. According to J. C. Scaliger (1484-1558), some
properties of matter, such as fineness and coarseness,
depend on the properties of the minima themselves,
while others depend on the manner in which they are
joined. Rain, snow, and hail are all of the same coarse-
because they are composed of the same minima,
but their density is different, because the minima of
these three substances are at a smaller or a greater
distance from one another. Scaliger knew not only
minima of elements, but also of compounds, although
he does not mention the latter as systematically as the
former. As to the chemical reaction, Scaliger is not
satisfied with the Aristotelian definition: the union of
the reagents, because the role of the minima is not
mentioned. He is not satisfied with Democritus either,
because Democritus' atoms could not really form a new
unity. Hence Scaliger's definition: chemical composi-
tion is the motion of the minima towards mutual con-
tact so that union is effected. As a follower of Aristotle,
Scaliger is convinced that for a true chemical compo-
sition a new substantial form is required.

The interesting point with such men as Scaliger and
Nifo is that on the one hand they develop their ideas
within the conceptual framework of Aristotle's philos-
ophy, whereas on the other hand they transcend it.
Unlike certain Aristotelian circles, as those of the
Thomists and Scotists, who limited themselves to the
discussion of the minima and of chemical composition
merely with philosophical concepts such as actual,
potential, form, etc., the Averroists make efforts to
express in a more scientific way the relation between
the forms of the elements and the form of the com-
pound. Without abandoning the Aristotelian unity of
form in the compound, they transform this concept
in the direction of what we may call the concept of
“structure.” Thus they paved the way for a new
framework of concepts, in which scientific theories
could be formulated.

In the seventeenth century the creation of this new
conceptual framework made great progress. This can
be shown from the fact that the philosophical differ-
ences between the corpuscular systems were soon
pushed into the background, while the more scientific
aspects that were held in common came to the fore-
ground. Daniel Sennert (1572-1657) offers a clear ex-
ample of this tendency. Basically, his corpuscular the-
ory was derived from the minima doctrine, but it also
contained typical ideas of Democritus' atomism, that
in Sennert's days came to occupy a central position.
Sennert was, however, interested mainly in a chemical
theory, not in philosophical disputes. He was of the
opinion that, from a chemical point of view, atomism
and minima theory amount to the same thing. In order
to maintain this opinion, Sennert had to revise the
atomistic doctrine. As a chemist, Sennert was con-
vinced that elementary atoms differ qualitatively and
that a clear distinction should be made between ele-
mentary atoms and atoms of compounds (prima mista).
Modern chemistry followed Sennert in this respect. It
shows how important the minima doctrine has been.

Does this mean that the revival of atomism in the
strict sense did not have any real effect? The contrary
is true. The real significance of this revival must, how-
ever, not be sought in the revival of the general idea
of reducing the observable phenomena to changes in
the configuration of atoms, but in the inspiring convic-
tion that nature formed a unity, which quantitatively
could be analyzed. This fundamental idea remained
alive in modern science, even when at first the number
of qualitatively different elementary atoms had to be
increased from four to more than ninety. Whereas the
minima doctrine (and later on the chemical atomic
theory) accepted the existence of qualitatively different
elementary atoms as an irreducible fact, atomism did
not accept such an irreducibility. Whatever may be
the differences of the elementary atoms, these have
to be reduced to the fundamental properties of all
matter. This idea has been proved successful when the
structure of the atoms themselves became an object
of scientific research. And it is still vivid in our times
when the number of sub-atomic elementary particles
has increased tremendously. Science is again looking
for unity in this diversity.


R. G. Collingwood, The Idea of Nature, 2nd ed. (New
York, 1960). E. J. Dijksterhuis, The Mechanization of the
World Picture
(Oxford, 1960). R. W. Hepburn, “Philosophical
Ideas of Nature,” in The Encyclopedia of Philosophy (New
York, 1967), V, 454-58. F. A. Lange, Geschichte des Materi-
alismus und Kritik seiner Bedeutung in der Gegenwart

(Iserlohn and Leipzig, 1866); trans. E. C. Thomas as History
of Materialism
(reprint, New York, 1950). K. Lasswitz,
Geschichte der Atomistik vom Mittelalter bis Newton, 2 vols.,
2nd ed. (Leipzig, 1926). A. G. M. van Melsen, From Atomos
to Atom, the History of the Concept Atom,
2nd. ed. (New
York, 1960); idem, Physical Science and Ethics, a Reflection
on the Relationship between Nature and Morality
burgh, 1967). C. J. de Vogel, Theoria, studies over de griekse
(“Studies Concerning Greek Philosophy”)
(Assen, 1967). L. L. Whyte, Essay on Atomism: From Democ-
ritus to 1960
(London, 1961).


[See also Atomism in the Seventeenth Century; Causation;
Epicureanism; Law, Natural; Matter; Nature; Platonism;
Rationality; Stoicism.]