The Foundations of Stoic Physics

  Hensley, Ian. "The Foundations of Stoic Physics." CHS Research Bulletin 9 (2021).


This project examines the nature and function of several important bodies in Hellenistic Stoic physics: the active principle, the passive principle, primary matter, the elements, pneuma, and the cosmos. The terms that refer to these bodies are used in many ways by the Stoics and their sources. By clarifying the different senses of these terms and reinterpreting several pieces of evidence in light of this analysis, we can determine the nature of the bodies, their relations to each other, and how the Stoics use them to explain the natural world.


Physics was one part of Stoic philosophy as it developed during the third century BCE. According to the Stoics, knowledge of physics is not impractical or disconnected from living a good life. Rather, a wise and happy person is also a physicist, and physics is a virtue.[1] So, to understand how a Stoic sage achieves eudaimonia, one must also understand what their knowledge of nature consists of.

Stoic physics is noteworthy in at least two respects. First, bodies (rather than immaterial forms or hylomorphic entities) play a starring role in the Stoics’ explanations of natural phenomena. Second, Stoic physics is systematic: a recurring cast of bodies at different levels of complexity feature prominently in these explanations. Furthermore, these bodies are related to each other in important ways; so, understanding how one body explains an event requires understanding its relation to other bodies at other levels of complexity. Thus, to grasp how Stoic physics seeks to produce knowledge of the world, it is necessary to grasp the nature of these interrelated bodies, their relations to each other, and their functions. This is the focus of my research.

The bodies fundamental to Stoic physics are the active principle, the passive principle, primary matter, the four elements, pneuma, and the cosmos. These bodies play key roles in most of the Stoics’ explanations of natural phenomena, and they are related to each other in the way described above. One of the challenges for constructing a coherent understanding of these bodies is the state of the evidence for early Stoicism. No complete original treatises written by the founders of the school survive. Our evidence is constituted by second-hand reports spanning several centuries. Thus, we rely on condensed summaries, hostile criticisms, or brief quotations to gain an understanding of the Stoics’ views. Furthermore, in examining the reports on the fundamental bodies, we see that the terms used to refer to them are not used consistently or univocally. This is unsurprising, given both the fragmentary nature of the evidence and the Stoics’ tendency to use these terms in many ways. As a result, unclarities and apparent conflicts in our evidence arise which must be untangled.

Given these challenges, I use the following methodology. By surveying the evidence for Stoic physics, I identify the various senses of the terms used for the fundamental bodies—including “god”, “matter”, “substance”, “element”, “cosmos”, “soul”, and “state”. Then, I determine which sense is used in each key report on these bodies. Consequently, many of the interpretive puzzles arising from the state of the evidence can be resolved. Additionally, the nature and functions of the fundamental bodies and the relations between them can be clarified. As a result, we gain a better picture of the wise Stoic physicist’s knowledge.

According to the Stoics, there are two “principles” (archai): the active principle and the passive principle. The active principle acts on the passive principle to construct the world and everything in it. As a result, the passive principle constitutes the world. The Stoics call the passive principle by two names: “matter” and “substance”. They use these terms in two other ways: to refer, in general, to what something is made of (e.g., a piece of bronze is the matter of a statue; pneuma is the substance of the soul) and to refer to primary matter—a formless, eternal body with a constant mass and variable density. While Zeno, the founder of the Stoic school, seems to identify primary matter and the passive principle, there are philosophical problems with this position. On the other hand, our evidence suggests that Chrysippus, the third scholarch of the school, did not identify primary matter and the passive principle. In my work, I explain the philosophical problems facing Zeno’s position and describe how Chrysippus’s modification of the orthodox Stoic view is an improvement. Thus, during the first century of the school, the Stoic theory of the principles and primary matter seems to develop at least partially in response to philosophical challenges.

According to the Stoics, the world is an animal. Animals are composed of bodies and souls, and the Stoics often treat the active principle as the world’s soul. Furthermore, they call the active principle “god” or “Zeus”. However, in Stoic physics, “god” is not reserved for the active principle alone. The Stoics also call the fiery peripheral region of the world, or “the heavens”, by this name. Finally, they call the entire world “god”, as well. The active principle, the heavens, and the world are three distinct bodies. Why did the Stoics label each of them “god”? By comparing the world to other animals, such as human beings, I show that the Stoics had good reasons for using “god” in many ways.

Fire, air, water, and earth—the four elements—are important theoretical entities. The Stoics analyze many events and processes in the world in terms of the interactions between these bodies. In contrast to the “formless” principles, the four elements have defined “forms”. For example, fire is hot, and air is cold. Thus, Diogenes Laertius tells us that the Stoics “say that the principles and elements differ” (D.L. 7.134). Yet, throughout the source texts (including Diogenes Laertius a few lines later at 7.137), we find apparent identifications of the principles with subsets of the four elements. For example, in descriptions of the active principle’s various activities on the world, it is often described as fiery. Matter is variously described as being made up of different subsets of fire, air, water, and earth or the aggregate of these elements. Thus, our evidence suggests that the principles are both distinct from the four elements and the same as these bodies. Despite this apparent contradiction, I argue that the Stoics can consistently endorse these two views. While not strictly being identical to the four elements, the principles bear a close relationship to them that justifies treating the two groups as the same in some contexts.

According to the Stoics, one of the important features of the four elements is that they change into each other by means of changing density. The causes of condensation and rarefaction are cold air and hot fire, respectively. These two elements combine to form a body called “pneuma”, which the Stoics employ in many different kinds of explanations. As pneuma moves throughout the world, it modulates the densities of other elements. As a result, complex objects such as animals, plants, and inanimate wholes are produced. Thus, pneuma seems to be the vehicle of the active principle’s creative functions. Some have thought that pneuma is something over and above the fire and air that compose it. In opposition to this, I show that the functions of pneuma are reducible to the functions of fire and air, at least according to the early Stoics. This thesis allows us to resolve several puzzles in our evidence about the makeup of pneuma and its relation to fire, air, and the active principle.

During my time as a fellow, I have conducted research that has led me to these conclusions, both in residence in Washington, D.C. and remotely. I was grateful to present this research to the CHS in April 2021. Additionally, during my fellowship, I have drafted five chapters of a monograph dedicated to these topics. I am currently revising these drafts while writing the sixth and final chapter. I am very grateful to the CHS and its staff for providing me with time and resources to use in service of this project.

Select Bibliography

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Bénatouïl, T. 2009. “How Industrious can Zeus be? The Extent and Objects of Divine Activity in Stoicism.” In God and Cosmos in Stoicism, ed. R. Salles, 23–45. Oxford.

Cooper, J. M. 2009. “Chrysippus on Physical Elements.” In God and Cosmos in Stoicism, ed. R. Salles, 93–117. Oxford.

Frede, M. Forthcoming. “Diogenes Laertius 7.134.” Phronesis.

Gourinat, J.-B. 2009. “Matter and Prime Matter: ‘Corporealism’ and the Imprint of Plato’s Timaeus.” In God and Cosmos in Stoicism, ed. R. Salles, 46–70. Oxford.

Hahm, D. E. 1977. The Origins of Stoic Cosmology. Columbus, OH.

Hahm, D. E. 1985. “The Stoic Theory of Change.” The Southern Journal of Philosophy 23:39–56.

Hensley, I. 2018. “On the Separability and Inseparability of the Stoic Principles.” The Journal of the History of Philosophy 56(2):187–214.

Hensley, I. 2020. “The Physics of Pneuma in Early Stoicism.” In The Concept of Pneuma After Aristotle, ed. O. Lewis, D. Leith, and S. Coughlin, 171–201. Berlin.

Helle, R. 2021. “Self-Causation and Unity in Stoicism.” Phronesis 66(2):178–213.

Lapidge, M. 1973. “ἀρχαί and στοιχεῖα: A Problem in Stoic Cosmology.” Phronesis 18(3):240–278.

Long, A. A., and D. N. Sedley. 1987. The Hellenistic philosophers, 2 volumes. Cambridge.

Menn, S. 1997. “Physics as a Virtue.” In Proceedings of the Boston Area Colloquium in Ancient Philosophy, vol. XI, ed. J. J. Cleary and W. C. Wians, 1–34. Lanham, MD.

Reydams-Schils, G. 1997. “Posidonius and the Timaeus: Off to Rhodes and Back to Plato?” Classical Quarterly 47(2):455–476.

Sedley, D. 2011. “Matter in Hellenistic Philosophy.” In Materia, ed. D. Giovannozzi and M. Veneziani, 53–66. Florence.


[1] This view is attributed to certain Stoics by Diogenes Laertius at 7.92. He says that while Panaetius said that there were two kinds of virtues, theoretical and practical, “others” divide them into “logical, physical, and ethical” kinds. Cicero attributes this view to the Stoics generally at De Fin. 3.72. His character Cato then explains why knowledge of physics is necessary for a good life at 3.73. For discussion of physics as a virtue, see Menn 1997.