XENARCHUS

(fl. Seleucia, Cilicia, first century BCE)

celestial physics, Aristotelian tradition.

Xenarchus wrote a book against Aristotle’s thesis that the celestial bodies are made of a special simple body, unique to them: the so-called fifth substance (also known as the fifth body, the fifth element, quinta essentia, or aether). The book is not extant and only a few citations are preserved by Simplicius in his commentary on the De caelo. Xenarchus is often presented as an unorthodox Peripatetic philosopher or even an early commentator. These descriptions are misleading. They obscure the fact that there is no orthodoxy in the Aristotelian tradition at this early stage. The revival of interest in the philosophy of Aristotle that took place in the first century BCE did not involve either the acceptance of the views stated by Aristotle or their codification in the form of commentary.

Strabo asserted that Xenarchus was originally from Seleucia in Cilicia, but that he spent most of his life away from home teaching philosophy first in Alexandria, then in Athens, and finally in Rome (Geo. XIV 5,4,670). Strabo attended his lectures. On the basis of this information, Xenarchus’s activity as a teacher and a philosopher took place in the second half of the first century BCE.

Xenarchus wrote a book against Aristotle’s thesis that the heavens are made of a celestial simple body, unique to them. Simplicius said that this book was titled Against the Fifth Substance. There is no evidence that this title goes back to Xenarchus. Yet, the transmitted title is a significant testimony about the way Aristotle’s thesis was received in antiquity. It was considered a controversial doctrine about the existence of an additional body whose theoretical necessity was dubious at best.

The decidedly polemical nature of the book cannot be disputed. The target was the thesis that the heavens are made of a celestial simple body distinct from and not reducible to earth, water, air, and fire. Xenarchus advanced objections with the intent to refute Aristotle’s thesis. It is also clear that Xenarchus focused on the arguments offered in the De caelo. Since Simplicius is the only source of information about Xenarchus’s book, it must be accepted that a reconstructed text independent of Simplicius’s citations is impossible. In other words, it is impossible for us to evaluate just how many liberties Simplicius took in reporting Xenarchus’s objections.

Xenarchus was not content with demolishing Aristotle’s arguments. He developed a theory of natural motion that was a revision of Aristotle’s doctrine of natural motion. It is possible to reconstruct this theory to some extent.

It was a substantial claim of Aristotle’s that a simple body naturally performs a simple rectilinear motion. If unimpeded, a simple body naturally moves upward or downward until it has reached its natural place. But, at least for Aristotle, the nature of a simple body is such that it stops moving when it has reached its natural place. Xenarchus consciously departed from this crucial tenet by claiming that a simple body in its natural place either is at rest or moves in a circle. This revision can lead to a crucial revision of Aristotle’s physics. If one of the bodies encountered on Earth can move in a circle once it has reached it natural place, there is no need to introduce a special body that naturally performs circular motion in order to account for celestial motion. Consider the case of fire. Nobody in antiquity disputed that mobility is a conspicuous feature of fire. If unimpeded, fire regularly moves upward. But what happens to fire when it has reached its natural place? According to Aristotle, this fire loses its mobility. Xenarchus developed a theory of natural motion that allows one to say that fire does not lose its mobility once it has reached its natural place. Quite the contrary: this mobility manifests itself in a different and more perfect form: circular motion. There is evidence that this theory was successful. Simplicius credits Plotinus and Plolemy with a version of Xenarchus’s doctrine of natural motion (In cael. 20. 10–25). This testimony is confirmed by Proclus in his commentary on Plato’s Timaeus (in Tim. III 11. 24–12; III 11. 104 26–105. 12). In the case of Ptolemy, this doctrine was reconciled with the acceptance of Aristotle’s fifth body.

Xenarchus’s critique was well known and highly regarded in antiquity. At the time when Simplicius wrote his commentary on the De caelo, a more recent criticism was moved against Aristotle’s doctrine by Philoponus in the treatise that is traditionally known by the title De aeternitate mundi: contra Aristotelem. According to Simplicius, the objections of Philoponus depended heavily on those of Xenarchus. Simplicius and Philoponus were only the last link of a long and complicated chain that goes back, ultimately, to Alexander of Aphrodisias and his (lost) commentary on the De caelo. Thanks to Simplicius, Xenarchus’s objections were passed on to the Middle Ages and the Renaissance. Galileo Galilei makes use of these objections in his Discorso sopra i due massimi sistemi del mondo, though he never named Xenarchus and knows of this objection only in a derivative way.

BIBLIOGRAPHY

Falcon, Andrea. Corpi e Movimenti. Il De caelo di Aristotele e la sua fortuna nel mondo antico. Naples, Italy: Bibliopolis Press, 2001, pp. 80–118, 144–174.

———. Aristotle and the Science of Nature: Unity without Uniformity. Cambridge, U.K.: Cambridge University Press, 2005, pp. 62–71.

Hankinson, Robert James. “Xenarchus, Alexander and Simplicus on Simple Motion, Bodies and Magnitudes.” Bulletin of the Institute of Classical Studies 46 (2002–2003): 19–42.

Moraux, Paul. “Xenarchos von Seleukia.” Paulys Realencyclopädie der classischen Altertumswissenschaft. Revised by G. Wissowa. Edited by Konrat Ziegler, Walther Sontheimer, and Hans Gärtner. Stuttgart, Germany: Druckenmüller, 1964. Vol. 9 A2, 1420–1435.

———. Der Aristotelismus bei den Griechen. Vol. I: Die Renaissance des Aristotelismus im I. Jh. v. Chr. I. Berlin: De Gruyter, 1973, pp. 192–214.

Sambursky, Samuel. The Physical World of Late Antiquity. London: Routledge and Kegan Paul, 1962, pp. 125–132.

Simplicius. In De caelo. Edited by Johan Ludvig Heiberg. Berlin: Commentaria in Aristotelem Graeca 7, 1894.

———. On Aristotle’s On the Heavens 1.1.1–4. Edited by Robert James Hankinson. Ancient Commentators on Aristotle series. London: Duckworth, 2002.

Andrea Falcon