Physics of the Stoics

by Samuel Sambursky

Overview

Stoic physics, based entirely on the continuum concept, is one of the great original contributions in the history of physical systems. Building on The Physical World of the Greeks, the author describes the main aspects of the Stoic continuum theory, traces its origins back to pre-Stoic science and philosophy, and shows the attempts of the Stoics to work out a coherent system of thought that would explain the essential phenomena of the physical world by a few basic assumptions.

Originally published in 1987.

The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

Product Details

ISBN-13:	9780691606873
Publisher:	Princeton University Press
Publication date:	07/14/2014
Series:	Princeton Legacy Library , #827
Pages:	166
Product dimensions:	6.80(w) x 9.90(h) x 0.40(d)

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Physics of the Stoics

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By Samuel Sambursky

PRINCETON UNIVERSITY PRESS

Copyright © 1959 S. Sambursky
All rights reserved.
ISBN: 978-0-691-08478-7

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CHAPTER 1

THE DYNAMIC CONTINUUM

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1. Pneuma and Coherence

ACCORDING to the Stoic conception, the cosmic scene of material events, including conglomerate matter as well as space between bodies, is made up of a continuous whole. Like Aristotle, the Stoics exclude emphatically any possible existence of a void within the cosmos. However, their cosmos is, in contradiction to that of Aristotle, an island embedded in an infinite void. The cosmos is filled with an all-pervading substratum called pneuma, a term often used synonymously with air. A basic function of the pneuma is the generation of the cohesion of matter and generally of the contact between all parts of the cosmos. The term coherence was originally used by Aristotle to express continuity in an essentially geometrical and topological sense, but the Stoics gave it the physical and dynamic significance of cohesion within the physical world.

The property attributed to the pneuma of producing coherence can be traced back to pre-Socratic sources. This is of special interest for the study of the development of physical concepts in ancient Greece, as it brings to light the biological origin of some of the basic physical notions. In a well-known fragment of Anaximenes it says: "As our soul, being air, holds us together, so do breath and air surround the whole universe." Sextus Empiricus gives us the reason on which the Pythagoreans and Empedocles based their belief that the fellowship of men is not only with one another and with the gods, but also with animals: "For there is one spirit (pneuma) which pervades, like a soul, the whole Universe, and which also makes us one with them."

Other evidently biological roots of the concept of pneuma can be found in Aristotle's exposition of the doctrine of the soul in the teachings of the Pythagoreans and other early philosophers. Respiration is a characteristic sign of life, a fact which throws into relief the connection between soul (pneuma) and movement. The dynamic properties of the pneuma will be discussed later on; its identification with breath and soul in living organisms led to the Stoic hypothesis of the composition of the pneuma which was supposed to be a mixture of air and fire. This composition was apparently made plausible by analogy of the pneuma with the warm puff of breath. As air represents the principle of Cold, the warmth of the human body makes it likely that the stuff souls are made of is a mixture of cold and hot, of air and fire. Galen tells us that this mixture is so proportioned that it protects the living organism from extremes of temperature. Here we see clear associations with Alcmaeon's equilibrium theory of health and with Hippocratic ways of thinking. The passage in Galen just mentioned shows that the Stoics expanded the idea of mixture by adding the other two qualities of Dry and Moist, thus making possible the differentiation between the pneuma of the soul (psyche) and that of the world of plants (physis). The former is dry and warm whereas the pneuma of physis is moist and cold. We shall see later how important this principle of differentiation was, when it became the basis for the Stoic explanation of physical phenomena.

Another aspect of both components of the pneuma is their activity. Aristotle divided the four elementary qualities of matter into active — warm and cold, and passive — dry and moist and regarded the four elements as a result of the four physically possible combinations of these four qualities. The Stoics who attribute one quality only to each of the four elements, defined Air and Fire as active, and Earth and Water as passive elements. It might seem paradoxical that activity is attributed to the cold element (Air) as Cold was usually regarded as a rather passive quality. The confusion which is found in this respect in Aristotle's writings prevails also in later literature. One of the main reasons for this seems to be the constant confusion of air and water vapour. Whereas Poseidonios makes moisture responsible for the coldness of air over marshy ground, his pupil Cicero stresses the caloric content of air: "Air must be deemed to be a sort of vaporized water, and this vaporization is caused by the motion of the heat contained in the water." On the other hand, Plutarch points to the active participation of air during the freezing of water. Making use of dialectic arguments, Plutarch finally assigns to air an intermediate position between fire and water. Thus, "neutralization" of air emphasizes at the same time its active role in thermic processes. Elasticity, linked with pressure, is another aspect of the active character of air; it was known in Greece already by the fifth century B.C. and is repeatedly mentioned by Aristotle.

The function of fire as an active element is more obvious. Here, too, the starting-point was the observation of biological processes. "Innate heat", an expression coined by Hippocrates, was thought by Galen to be the cause of metabolism. The often-quoted Stoic sentence, "Nature is an artistically working fire, going on its way to create", also points to the significance of heat in organic nature. On the other hand, one can infer from the passage in Cicero's De natura deorum dealing with the nature of heat that the development of the first thermodynamical notions had begun already in the Old Stoa. Cicero quotes Cleanthes' doctrine which describes the function of heat in organic nature as a special case of thermic processes: "It is a law of Nature that all things capable of nurture and growth contain within them a supply of heat, without which their nurture and growth would not be possible; for everything of a hot, fiery nature supplies its own source of motion and activity; but that which is nourished and grows possesses a definite and uniform motion. ... From this it must be inferred that this element of heat possesses in itself a vital force that pervades the whole world." Here the active character of heat as such and its connection with dynamic phenomena is expressed very clearly. Cicero goes on to illustrate thermic effects by various examples, such as generation of heat by friction, and the role played by heat in melting, evaporation, etc.

The greater emphasis placed on the active nature of air and fire during the period of the Old Stoa brought into greater prominence the contrast between the active elements and the passive ones — water and earth. The mixture of air and fire which was identified in Stoic physics with the pneuma thus became the active agent par excellence in their cosmos. Pneuma or one of its components was also defined by the collective "pneuma-like matter", and the Stoics attributed to them the property of coherence in the twofold sense of being cohesive and making cohesive, 25 whereas the passive elements ("hyle-like matter") were denied any such faculty. Without the active interference of air or fire or the two mixed together, the passive elements would disintegrate as they themselves do not possess the "cohesive force". Coherence thus appears here distinctly as a force, and the geometrical continuum of Aristotle is in this way transformed into a dynamic one. It should be mentioned, however, that Aristotle has occasionally used the term "cohesive" in a structural sense; e.g. in his analysis of the concept of unity: continuity is achieved by a binding agent and not by geometrical contact alone, as for instance a faggot which is made one by its string, and pieces of wood by glue. This is a static analogy, but immediately afterwards Aristotle comes very close to a dynamic conception when he calls a thing continuous "whose motion is essentially one and cannot be otherwise". But the Stoics undoubtedly identified continuous with cohesive and thus completed the transformation of the geometrical concept into a physical one.

The most striking proof of this is the specifically physical property ascribed by the Stoics to the pneuma, or the "pneuma-like matter" in general: tension (tonos). On the one hand this latter term was also applied to the living body as shown by a quotation from Cleanthes given by Plutarch, where the vital force of the soul is defined as follows: "Tonas is the heat of fire which, if originating in the soul in sufficient measure to accomplish the task, is called strength and force." Galen uses the expression "vital tension", and according to another source, the Stoics saw the cause of sleep in a relaxation of the sensory tension. On the other hand, several sources such as Plutarch and Alexander quote the Stoic theory of the function of tonos in inorganic matter. These quotations and the comments which accompany them show the gist of this theory: the tension innate in air and fire endows these elements with cohesion which is acquired by water and earth only through their admixture. Pneuma especially, which according to Galen was elevated by the Stoics to a "fifth quality", possesses this tensional power as its most conspicuous property. As a matter of fact, since pneuma pervades the whole universe, the pneuma-like tonos makes the cosmos into a single cohesive unit and thus the pneuma becomes the first version of the aether with all the characteristic functions ascribed to it from the seventeenth century on. We shall return later to this close analogy between pneuma and aether.

The singular significance attached in Stoic physics to air and fire by their being components of the pneuma finds its expression in still another shift in their character, parallel to that which occurred with the "thermal neutralization" of air mentioned above. In pre-Stoic physics air and fire were generally regarded as light and thus rising, water and earth as heavy and thus descending. Aristotle, however, declared that air was both light and heavy at the same time, but he said this in a very specific context. He wanted to describe the role of air in his dynamics of "forced" motion: i.e. in movements which are not "natural" the body is pushed up and down by air.

A survey of the sources on the Stoic view of the structure of air and fire shows at first sight a certain semantic ambiguity. Sometimes these elements are denoted as "light" and sometimes as "not heavy". However, two of these sources which are fairly reliable are at the same time quite definite on this point. One of them, Plutarch, gives two quotations from the second book of Chrysippos' work On Motion and from his Physical Arts, both of them referring to air and fire as "nonheavy": "Fire is non-heavy and rises, and the same applies to air, so that water has to be assigned rather to earth and air to fire." "Air in itself possesses neither gravity nor lightness." The other source is Arios Didymos, who refers to the Stoic and especially to Zeno's cosmological conceptions. The stability of the cosmos is secured by the tendency of all its parts towards the centre, and particularly of those which possess gravity. "But not all bodies have gravity, for air and fire are non-heavy. These spread over the centre of the whole sphere of the cosmos and bring about the union with its periphery. By their nature they frequent the upper parts because they have no share of gravity. He (Zeno) says likewise that the cosmos has no gravity because it is all composed of heavy elements and non-heavy elements." It is significant that the term "levity" or the attribute "light" has been avoided throughout the passage. It is also not said of air and fire that they are "rising" but that they "frequent the upper parts". The whole emphasis in this text is on the gravitationally neutral character of air and fire who both participate in the cosmic tendency towards the centre, and at the same time stretch from there to the extreme regions, and thus contribute to the communication between all the parts of the universe. Obviously some of the essential characteristics of the pneuma are attributed here to its two components air and fire, and it is in this sense that we have also to interpret the last sentence of the text quoted. If the cosmos had gravity, the pronounced pull of the heavy elements towards its centre would make it collapse. But it is kept extended because of the admixture of the non-heavy elements which spread everywhere from the centre to the outer regions of the cosmic sphere.

2. The Physical State of a Body

Cohesion of matter is not the only effect produced by the tension of the pneuma. The latter has a twofold function: besides being a binding force, it is an agent which generates all the physical qualities of matter. From the point of view of the history of ideas we have to see in Anaximenes the father of this notion, in view of the function he assigned to air, out of which all matter originated by condensation and rarefaction. The next step in the direction of the Stoic conception was made by Diogenes of Apollonia who took up Anaximenes' basic idea and combined it with elements from Empodocles' and Anaxagoras' teachings. We shall be returning to Diogenes very soon. By their conception of the pneuma as the generator of physical qualities the Stoics generalized their continuum theory into a field theory; the pneuma is the physical field which is the carrier of all specific properties of material bodies, and cohesion as such thus gets a more specific meaning by becoming hexis, the physical state of the body. The following quotation from a book by Chrysippos On Physical States is very instructive: "The physical states are nothing else but spirits, because the bodies are made cohesive by them. And the binding air is the cause for those bound into such a state being imbued with a certain property which is called hardness in iron, solidity in stone, brightness in silver." And a little later he continues: "Matter, being inert by itself and sluggish, is the substratum of the properties, which are pneumata and air-like tensions giving definite form to those parts of matter in which they reside." This gives some idea of the central position in the Stoic theory of matter of hexis, which denotes the structure of inorganic matter in a similar way to which physis expresses organic structure and psyche the structure of the living being. Galen and Philo describe the analogy between these three notions and their delimitations. Philo calls hexis "a very strong bond", and in another context he characterizes it as "a bond not unbreakable but hard to dissolve".

The structural concept of hexis, also defined as the "binding spirit" of a body, represents the highest entity in the hierarchy of inorganic structures as conceived by the Stoics. These entities are divided into discrete, contiguous and unified. At the lowest level we have an assembly of bodies in a disordered state, such as a crowd which does not lend itself to numerical determination. The following level is also a discrete state, but here the elements are arranged in an order which allows for numerical determination, such as a choir or an army in formation. This is a "denumerable" entity. Contiguous structures are composed of conjoined elements, like the links of a chain or the planks of a ship or the stones of a house. What the discrete and contiguous structures have in common is that each of their elements can continue to exist even if the rest are destroyed, which is characterized by a simple additive relationship between the elements. True, the whole structure is more than the sum of its units (except in the case of disordered discrete structures which are very much the same as Lucretius' concilia of atoms), and each of these structures is held together by forces which have to be overcome before the whole is reduced to the sum of its elements. But each of n given elements would not be affected by anything that happens to the (n–1) others, in fact there is no "communication" between them which can lead to such a result.

(Continues...)

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Excerpted from Physics of the Stoics by Samuel Sambursky. Copyright © 1959 S. Sambursky. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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Table of Contents

• FrontMatter, pg. i
• PREFACE, pg. v
• INTRODUCTION, pg. vii
• CONTENTS, pg. xi
• I. THE DYNAMIC CONTINUUM, pg. 1
• II. PNEUMA AND FORCE, pg. 21
• III. THE SEQUENCE OF PHYSICAL EVENTS, pg. 49
• IV. THE WHOLE AND ITS PARTS, pg. 81
• APPENDIX: TRANSLATIONS OF TEXTS, pg. 116
• SELECTED BIBLIOGRAPHY, pg. 146
• INDEX TO PASSAGES QUOTED, pg. 148
• GENERAL INDEX, pg. 151