PHYSICS

The principles of conservation

Epicurus, Letter to Herodotus 38–9

(1) Having grasped these points, we must now observe, concerning the non-evident, first of all that nothing comes into being out of what is not. For in that case everything would come into being out of everything, with no need for seeds. (2) Also, if that which disappears were destroyed into what is not, all things would have perished, for lack of that into which they dissolved. (3) Moreover, the totality of things was always such as it is now, and always will be, (4) since there is nothing into which it changes, (5) and since beside the totality there is nothing which could pass into it and produce the change.

Lucretius 1.15 9–73

(1) For if things came into being out of nothing, every species would be able to be produced out of everything, nothing would need a seed. (2) Men, to start with, would be able to spring up out of the sea and scaly fish from the land, and birds hatch out of the sky. Cattle and other livestock, and every species of wild animal, would be born at random and occupy farmland and wilderness alike. Nor would the same fruits stay regularly on the same trees, but would change over: all trees would be able to bear everything.

The earliest surviving record of a clash between the two cosmologies is in Plato's Phaedo – perhaps an unexpected place to find it, since the Phaedo is not about the natural world, but about the human soul, its destination after death, and the implications of immortality for the life of man or earth. And yet Plato's discussion of the issue gains an important dimension from its unexpected context, as we shall see, although he himself does not explicitly draw attention to the point.

Socrates is sitting in an Athenian prison waiting for the death sentence to be carried out. He talks with friends and explains the ground for his confidence that a man's soul survives his death. They listen and are convinced – but two of them express lingering doubts. The first doubt is quickly disposed of, but the second, says Socrates, is more troublesome. The first part of his attempt to allay it contains the critique of other philosophers that interests us. We shall return later to the doubt itself and the role of this critique in putting it to rest.

In his youth, Socrates says, he was an avid student of the philosophy of nature. ‘It seemed to me a superlative thing – to know the explanation of everything, why it comes to be, why it perishes, why it is’ (96a).

The kind of question he considered then, he says, was whether the growth of animals came from a fermentation of the hot and the cold, and whether the blood, or air, or fire,

The educational tradition of the Western half of the world has fairly recently adopted a new picture of the physical universe and man's place in it. We are now brought up to believe that we live on one of the planets that move in orbits around a rather minor star, itself a member of one galaxy among a vast number of others. We are taught that our planet is not eternal, but had an origin and will have an end; that it has grown into its present condition through a long evolutionary process, in which the minerals developed first, before any life appeared, and then the complex living species that we see around us, including man, evolved over a very long period of time through the processes of mutation, natural selection, and inheritance.

This picture has largely, though not entirely, replaced an earlier one that held the field for many centuries. According to that, the earth is stationary at the center of the cosmos. The stars, planets, sun, and moon revolve around the earth. The cosmos is finite, being bounded by the outermost sphere of the heavens, and is unique. This cosmic order, according to the prevalent, Judaeo-Christian version of the picture, was determined and brought into being at a certain point of time by God, who adopted it because it is a good order. He also created all the living species that now exist and arranged them in a hierarchical order of decreasing complexity or excellence, with man at the top of the scale.

Aristotle vigorously attacked the Atomists and their predecessors for their theories of the elementary material components of the physical world. He himself held a theory that there are such elements, but it differed from all that had come before: he does not even spare Empedocles, who anticipated him at least in picking out earth, water, air, and fire as the elements. His criticisms are directed at the character given to the elements, and the use made of these elements in explaining the phenomena.

We shall examine these criticisms of early theories of the material elements in the second section. First, we must look at a more fundamental criticism – a criticism of materialism itself. Aristotle argues that an account of the material elements is a necessary part of explanation in the study of the natural world, but it is not sufficient for explanation, except in a minority of less important cases. He accuses his philosophical predecessors of a crucial failure, in that they virtually ignored form and finality in nature. This takes us right to the heart of the differences between Atomism and Aristotelianism.

The failure of Presocratic explanations

The subject under discussion is nature, physis. At the beginning of the second book of his Physics, Aristotle makes some very general remarks, which then form the basis for his criticisms of the Presocratics.

The four roots

It is customary to regard the Sicilian Empedocles as a particularly archaic and reactionary thinker, but I believe that is a mistake. Like Parmenides, he wrote in hexameter verse, and he chose the language of myth. He adopted the Pythagorean idea of metempsychosis, and wrote of himself as a fallen daimon, one who had been ‘a bird, a bush, and a dumb sea-fish.’ Historians of science find it hard to take someone like that seriously. Nevertheless, he was responsible for three innovations of great importance to the history of science: the invention of elements, the postulation of two physical forces, qualitatively different from each other, and the first statement, so far as we know, of the concept of natural selection by survival of the fittest.

His theory of elements claimed that there are just four substances in the physical world: earth, water, air, and fire. There is a finite and unvarying quantity of each of them, and between them they make up all the material objects that there are, by mixing in different proportions. Empedocles gives an idea of how the theory works by offering a comparison with fourcolor painting:

In chapter 2 we have already taken a preliminary look at Plato's first observations on the cosmology of his predecessors. He gave us an unforgettable picture of Socrates reviewing his life of philosophizing, as he sat in prison with friends on the last day of his life. The main target of his criticism of earlier philosophy was its neglect of the question ‘to what end?’ or ‘what is the good of this?’ The physicists gave him nothing but a story of matter in motion, of how, not why, things come to be as they are. This Socrates found so unsatisfying that he turned to a quite different kind of philosophy.

Plato returned to criticism of materialism in works written later than Phaedo, and that is the subject of this chapter. First we must observe that Plato seldom names the targets of his criticism. Of the ‘physicists’ we have reviewed in earlier chapters, Plato mentions Thales, but not Anaximander or Anaximenes; Anaxagoras several times, but Empedocles only twice; and Leucippus and Democritus not at all. The omission of the last two has surprised the commentators from ancient times to the present day. There is a story in the biography of Democritus in Diogenes Laertius (ix.40) that Plato wanted to burn all of Democritus' books that he could collect, but was prevented by two Pythagoreans. It was no use, they said, the books were too widely published.

The crucial importance of Parmenides of Elea in the history of cosmology has been explained. We must now turn to his followers, who reinforced his arguments with contributions of their own. Some of these played an extraordinary and fascinating part in the development of Atomism, and their authors have an assured place in history. It happens that these connections have been fairly thoroughly explored in recent literature, especially in the English language, and this chapter will accordingly be relatively brief.

We hear of two followers of Parmenides, each about one generation younger than he was. Zeno came from Parmenides' home town of Elea, Melissus from the Aegean island of Samos, the birthplace of Pythagoras. Their work can be dated about the middle of the fifth century. There is no doubt about their allegiance. For Zeno, we have Plato's authority: his dialogue called after Parmenides presents Parmenides, as a man of sixty-five, with Zeno as a companion then aged about forty, in conversation with Socrates, at the time still very young. The occasion itself is probably fictitious, but the relative ages are likely to be about right, and when Plato has Zeno say that his book was in support of Parmenides' argument, he may be believed. Against those who ridiculed Parmenides' thesis ‘One thing is,’ Zeno is made to say, he wrote to show that still more ridiculous things follow from the contrary thesis, ‘Many things are’ (Parmenides 128b–c).

This volume tells one side of the story of ancient Greek cosmology, and takes it as far as the criticisms directed by Aristotle at his materialistic predecessors. The book is to be completed in a second volume, The Teleological World Picture and its Opponents. In that I shall give an account of the cosmology of Plato's Timaeus and Aristotle's works on the philosophy of nature, followed by the attempted rehabilitation of the atomic theory of the universe by Epicurus and his followers. Then I shall try to describe the arguments of philosophers of the Hellenistic period and later, including the Stoics and the later Peripatetics. I hope to take the story as far as the time of Simplicius and Philoponus, in the sixth century A.D.

I have tried to make the book readable by those with no specialized knowledge of Greek philosophy, and no Greek. Although it is not designed to be a course textbook, in writing it I have kept in mind the students to whom I lecture every year in Princeton University in a course called ‘Introduction to Ancient Philosophy’. They include some who are taking a major in Classics or Philosophy, but also many who have never before studied the ancient world or read anything of the Greek philosophers. I have tried to keep the philological technicalities to a minimum.

On some subjects I have adopted positions that need more detailed defense than I have been able to give in this context. I have sometimes argued the case in articles that have been published separately in the professional journals, or in papers that are not yet published.

In response to Parmenides, Anaxagoras worked out the astonishing theory that all physical change is nothing more than an emerging into view of something previously latent. Thus, we have no need to speak about what is not, but only about what we do not yet see.

The interpretation and assessment of Anaxagoras put forward here differ substantially from what is found in most contemporary histories of philosophy, in at least three respects. I believe Anaxagoras' theory is quite independent of Zeno, and that Zeno, in fact, criticized him. I also think that Anaxagoras did not criticize and modify Empedocles' theory of elements, but represents a more primitive stage of response to Parmenides than Empedocles. Moreover, my interpretation of his theory of matter is not quite the same as any other that I have read. These are all controversial positions, which cannot be defended in detail here.

Anaxagoras

Anaxagoras began his book with a striking affirmation that at once made clear his opposition to Parmenides:

Parmenides had agreed that what is is invariant in time, so that only ‘it is,’ and not ‘it was’ or ‘it will be,’ is correct, and that it is uninterrupted in space by any interval or change of density.

The men and their books

The writers of the ancient world were confident that Leucippus and Democritus were the founders of the atomic theory, but they were not at all careful in allocating the credit between the two of them. Attempts have been made by modern scholars to isolate the contribution of Leucippus – the earlier and less well-known of the two – but their arguments, if not demonstrably wrong, are too tenuous to rely on. Little attempt will be made in this book to distinguish their doctrines, at least with regard to the physical world; on the subject of human society and ethics, it appears that Democritus was very much the major contributor.

The early Atomists are associated with the remote country town of Abdera, on the northern Greek mainland close to the island of Thasos. Democritus lived and worked there; probably Leucippus did so too, although ancient biographers were uncertain whether to connect him with Abdera, Elea, or Miletus. Democritus had a long life – some say he lived to be a hundred. His precise dates are not known. He himself wrote in The Small World Order, according to Diogenes Laertius (IX.41), that he was forty years younger than Anaxagoras. Probably he was born about 460 b.c., and Leucippus was somewhat older. The atomic theory, then, must date from about the time of the Peloponnesian War, in the last decades of the fifth century; it is approximately contemporary with Socrates' philosophical activity, and with the birth of Plato.

One of the finest products of the Greco-Roman imagination is the fifth book of Lucretius' poem De rerum natura. In magnificent language the Latin poet pictures the successive stages of the evolution of the cosmos from clouds of atomic dust, and with great intensity displays the explanatory advantages of his materialism over the cosmic gods of ancient mythology and rival philosophies. First comes the framework of the cosmos: earth and sun, the stars, planets, sun and moon, and the seasons of the earth. Then the poem goes on to tell of the first growth of vegetation, the emergence of animal life, the earliest human communities, the growth of civilization and technology, the development of political structures, and the genealogy of morals.

There can be little doubt that the Atomists of the fifth century b.c. wrote about these things too, and that many of their ideas were adopted by Epicurus and his Greek followers, who were themselves the sources of Lucretius' inspiration. It happens that Lucretius' poem has survived intact, while the work of Leucippus and Democritus has vanished. The fifth book of De rerum natura, then, is our best source for this aspect of ancient Atomism. Nevertheless, it seems appropriate to study Lucretius in his Epicurean context, rather than as an appendix to Democritus. Although there is in all probability much in Lucretius book v that comes from Democritus, there is certainly much that does not. So at the cost of leaving our history of Preplatonic Atomism embarrassingly threadbare in this area, we shall not draw on Lucretius' anthropology for evidence here, and postpone the attempt to give a more satisfactory account of the atomic theory to volume 2.

The beginning of the fifth century b.c. – the time when Greek civilization was menaced by the expanding might of the Persian Empire, and managed to save itself from being overwhelmed – is a period of some confusion for the literary historian. The materials he has to work with consist of the Victory Odes of Pindar, some other specimens of lyric poetry, and a few of Aeschylus' plays. In philosophy, we have a chancy collection of the fragments of Heraclitus, Parmenides, Anaxagoras, and Empedocles. Hardly anything else is preserved in sufficient quantity to allow a critical view to be taken of it, or even to provide background against which we can locate the works that have survived. It is possible that our histories do some injustice, by concentrating on the known names and works, to others whose efforts were at the time of equal importance. But we have no choice: we can only endeavor to make the most of the available evidence. Some have tried to supply the background that is missing from the Greek inheritance by looking at other cultures, especially those of the Near East. It is, of course, right to make the effort, and for the period when the transmission was not by written texts but oral, and when myth dominated the poetic scene, the comparisons have proved invaluable. In my opinion they have been much less successful and illuminating for the period when, with the growth of literacy, philosophy and the exact sciences were emerging from the mythical background.

The infinite universe

It is particularly important, when writing about the Atomists, to be consistent in making the distinction between the universe or ‘the all’ (to pan, in Greek) and the world (kosmos) – that is, a particular part of the universe consisting of our earth, sea, air, and visible stars, planets, moon, and sun. Leucippus and Democritus held that ‘the all’ consists of an unlimited expanse of void space and an unlimited number of atoms. Of these, some are – temporarily – arranged so as to form worlds, including the world in which we live. We must study, first, the arguments used to defend the notion of the unlimitedness of the universe, and then the mechanisms that were suggested to account for the formation of worlds.

If we are to understand the controversy of classical antiquity, we must keep clearly in mind one important difference between ancient and modern cosmology. In the twentieth century we have grown accustomed to the notion that what we see in the sky on a starry night is the beginning of a universe that continues far beyond the limits of our vision. We can see some of the stars, because they are relatively large, bright, and near; with telescopes we can see further into the distance and see more stars, and with better telescopes we could see further still. In the cosmology of classical antiquity, on the other hand, it was a matter of common agreement that the stars we see are part of our world: they are the boundary beyond which the infinite universe (if it is infinite) begins.

There are many good histories of early Greek philosophy, and it is not necessary to the purpose of this book to attempt to rival them. We must try to pick out the themes outlined in the earlier chapters, without encumbering the narrative with too much detail. The reader must be warned, however, that this selectivity is certain to be controversial. The evidence for the theories of the sixth and fifth centuries b.c. is scrappy and ambiguous: we lack the context of the short quotations that survive – the longest consecutive fragment is sixty-six lines of verse. It is just possible that we have the whole intent and direction of some fragments wrong. That is not likely, because the tradition has been subjected to the most careful criticism by many generations of scholars and philosophers; but there is no general consensus on some important questions, and the possibility of misinterpretation is open. This is as a rule not true in the case of philosophers like Plato and Aristotle, whose work survives either completely or in bulk.

The main theme to be picked out is the one emphasized by Plato, in the passage of the Phaedo that we discussed in chapter 2: the explanation of the world in terms of matter in motion. We must begin, however, briefly, at the beginning: that means, with the three philosophers who lived in the sixth century b.c. in the prosperous Greek colony at Miletus, on the coast of Asia Minor – Thales, Anaximander, and Anaximenes.

The value of logos

The course of the history of philosophy and science was utterly changed by these two men. Many would claim, in fact, that philosophy in the modern sense began with Parmenides – or at least that a historian of philosophy today must begin with Parmenides. Earlier writers are now represented by a few isolated phrases or sentences of their own words and some garbled second-hand reports; but by great good fortune – and the far-sighted wisdom of Simplicius, who understood the historical importance of being able to quote the original text, a thousand years after it was written – a substantial portion of Parmenides' argument survives, in his own poetic words. It is undeniably an argument and undeniably philosophical; and its importance can hardly be exaggerated. Often in history, and especially in the history of science, although each progressive step is credited to the account of a single person or group, it can be seen that if the advance had not been made then, it would inevitably have been made soon by someone else. In the case of Parmenides, this is not true. There is nothing quite like his argument: it seems startlingly original.

Nevertheless, there is a point in including Heraclitus with Parmenides in the same chapter, although they were separated physically from each other by the whole of the Greek world – Heraclitus lived in Ephesus, Parmenides in the South Italian town of Elea – and they wrote in different genres.