After raising difficulties regarding not-being, the Visitor and Theaetetus become involved with puzzles about being. This is rather surprising because one might have expected the obscurity to be confined to the obviously problematic area of not-being. The puzzles about being come in two families.

The present chapter’s first section addresses the first family of puzzles (243d6–245e5), which concern the number of beings. Both pluralists, who believe there to be two or more entities, and monists, who maintain that only one thing is, are criticized. The second family of puzzles about being (245e6–249d8), tackled in this chapter’s second section, concerns the characteristics shared by all and only beings. A debate is set up between two factions. One party includes thinkers who claim that only bodies are, members of the other hold instead that only changeless forms are. An attempt to reach a compromise acceptable to both parties leads to the result that both change and stability are beings. But then (249d9–250d4) a further difficulty arises which seems to depend on a confusion between sentences used to speak about the kind being itself. This last difficulty is dealt with in the chapter’s third section.

The stated purpose of the Sophist is to define the sophist. The definition is pursued by applying the method of division, to a discussion of which section 1.2 is devoted. Plato offers a salvo of six descriptions of the sophist based on the method of division: each one focuses on certain traits shared by at least some sophists. Section 1.3 is about the connection between the sophist and the concept of appearing. The six descriptions of the sophist show that he appears to have many skills. Precisely the point that the sophist appears to have many skills provides the starting point for a new characterization, which turns upon the concept of appearing: the essence of the sophist is exactly his appearing to have skills which he in fact lacks. But the concept of appearing and the connected concept of falsehood generate puzzles: until these remain unresolved, the sophist can evade ‘capture by definition’.

Characters and task of the dialogue

The task of defining the sophist. Here are the last words of the Theaetetus (210d1–4, Socrates is speaking): ‘Now I must go to the King’s Porch to meet the indictment that Meletus has brought against me; but let us meet here again in the morning, Theodorus’. And here is the beginning of the Sophist (216a1–2, Theodorus is speaking): ‘We have come at the proper time by yesterday’s agreement, Socrates’. Thus, the conversation recorded in the Sophist is a continuation of that of the Theaetetus. It may be inferred that the Sophist’s dramatic date is 399 bc, the year of Socrates’ death. It may also be inferred that the discussants of the Sophist include those of the Theaetetus: Socrates, Theodorus, Theaetetus, Young Socrates, and other unnamed adolescents. But there is also someone else.

Xenarchus and the reception of aristotle’s physics in antiquity

Xenarchus’ criticism of the thesis that the heavens are made of a special simple body, unique to them, was preserved and passed on to late antiquity by Alexander of Aphrodisias. Alexander is ultimately responsible for its integration into the ancient commentary tradition. The role that Xenarchus plays in this tradition can be usefully contrasted with the one played by Eudemus of Rhodes. Eudemus was not only a direct pupil of Aristotle; with the exception of Theophrastus, he was also the most prominent Peripatetic philosopher of his time. What we know about his Physics suggests that Eudemus followed the sequence of topic discussed in Aristotle's Physics very closely – so closely that it is tempting to consider him an interpreter (exegetês) of Aristotle's Physics. Because of what is perceived as loyalty to Aristotle, his views are often cited by Simplicius in his commentary on Aristotle's Physics. There, Eudemus is regarded as an exegetical aid in the exposition of Aristotle's text. By contrast, Xenarchus is treated as an adversary whose objections are to be answered in order to reaffirm the transcendence of the heavens. It is telling that his affiliation to the Peripatos is not recorded by Simplicius. This silence is symptomatic of the role that Xenarchus ended up playing in the commentary tradition. Seen through the lens of a pro-Aristotelian exegesis that ultimately goes back to Alexander of Aphrodisias, Xenarchus played the outsider within the Peripatetic tradition. This role does not do full justice to Xenarchus – or so I have argued.

A study of the reasons which may have led a Peripatetic philosopher like Xenarchus to criticize Aristotle's celestial physics helps us to better appreciate how unusual Aristotle's conception of the natural world was in antiquity. Several aspects of this conception were strongly resisted. But nothing generated more controversy than Aristotle's views on the material composition of the heavens. The mixed reception of these views indicates that his conception of the celestial world was truly exceptional, if not even anomalous. When we say that for Aristotle the natural world consisted of a celestial and a sublunary part, we do not do full justice to his position. In antiquity it was common to think of the celestial world as a somehow special region of the world. Stability and incorruptibility were often given as its differentiating features. In fact, the position recommended by Aristotle was much stronger than a generic commitment to the incorruptibility and stability of the celestial world. Aristotle was committed to the existence of material discontinuity between the celestial and the sublunary regions of this world. The so-called fifth substance or fifth body is not just different from earth, water, air, or fire. At least for Aristotle, this substance cannot be reduced to any of them. The expressions “fifth body” and “fifth substance,” which Aristotle never uses in his writings on natural philosophy, are themselves an indication of how many, if not most, people felt about Aristotle's celestial simple body in antiquity. They felt that this was an additional body, or an additional substance, alongside earth, water, air, and fire whose theoretical necessity was at best very dubious.

Xenarchus of Seleucia is best known for his vigorous criticism of Aristotle's thesis that the celestial bodies are made of a special simple body, unique to them: the fifth substance, also known as the fifth body, fifth element, quinta essentia, or aither. However, his activity was not confined to the study of physics, let alone celestial physics. Although the surviving evidence is slim, and at times frustratingly so, there is no doubt that Xenarchus was concerned with issues of ethics and psychology as well as of physics. In this book I consider all areas of his activity in order to offer as complete a picture of Xenarchus as our sources permit.

For two reasons, particular emphasis is placed on Xenarchus’ criticism of Aristotle's doctrine of the fifth substance. The first reason is that the sophistication and ingenuity involved in this criticism presuppose a close textual study of Aristotle's works. This study led Xenarchus to a brilliant revision of the conceptual apparatus developed in Aristotle's writings on natural philosophy. Xenarchus elaborated a creative interpretation of Aristotle's theory of natural motion which made the celestial simple body expendable. There is conceptual discontinuity between this creative interpretation and what we know about the Hellenistic theories of motion. Xenarchus developed his theory of natural motion as a direct response to Aristotle's theory of motion. His critical engagement with Aristotle strongly suggests that his activity is best understood in the context of the return to Aristotle which took place in the first century bce. While some of his views are rooted in the philosophical debates of the late Hellenistic period, his activity as a whole presupposes the distance from Aristotle that confronted post-Hellenistic philosophers. In Xenarchus’ case, this distance prompted direct attention to Aristotle's text.

Life

From Strabo we learn that Xenarchus was originally from Seleucia on the Calycadnus in Cilicia Tracheia, but that he spent most of his life away from home, teaching philosophy first in Alexandria, then in Athens, and finally in Rome [T1]. Strabo adds that he himself attended Xenarchus’ lectures. Finally, he names two people that must have been very important in Xenarchus’ life: Arius and the emperor Augustus. Arius is Arius of Alexandria, a philosopher who may have been the same person as the doxographer Arius Didymus and the Stoic philosopher listed on the index locupletior to Diogenes Laertius’ Lives. Arius was not only a friend, a teacher, and a confidant of Augustus; he was also a senior political advisor and a very influential member of Augustus’ most inner circle. Strabo seems to suggest that the friendship with Arius played a significant role in Xenarchus’ career. More directly, it is possible that Xenarchus owed his friendship with Augustus to Arius. On the basis of this biographical information, we can date Xenarchus’ activity to the second half of the first century bce.

Elsewhere Strabo tells us that he (Strabo) studied Aristotelian philosophy with Boethus, the Peripatetic philosopher native to Sidon. This piece of information has been combined with what Strabo says about Xenarchus to suggest that Boethus and Strabo were fellow students in Rome, and Xenarchus was their teacher. But the information in our possession does not justify this conclusion. In all probability, Strabo is saying that he was a student of Boethus, not that they were fellow students. Moreover, there is no evidence linking Boethus to Xenarchus, except what Alexander of Aphrodisias says in his Mantissa [T16]. There, Alexander names Boethus and Xenarchus as champions of the attempt to elaborate an Aristotelian doctrine of the first appropriate thing (prôton oikeion) out of what Aristotle says on love (philia). But this doctrinal agreement does not suffice to establish that Boethus studied with Xenarchus, or that Boethus and Xenarchus were personally close in any other way.

Aristotle's writings had a limited circulation in the Hellenistic period. Strange as it may seem to us today, the most common attitude toward Aristotle at the time was not opposition or resistance to his ideas, but rather indifference. The situation changed in the first century bce. The so-called return to Aristotle is one of the most conspicuous features of the century. This return marked a new beginning in the Peripatetic tradition, which is clearly divided into a Hellenistic and a post-Hellenistic period. The post-Hellenistic period begins with the return to Aristotle in the first century bce and ends with Alexander of Aphrodisias in the late second and early third centuries ce.

The division of the Peripatetic tradition into two phases is not a modern invention but was already recognized in antiquity. The ancient explanation for this curious phenomenon was that the loss of Aristotle's books deprived the Hellenistic Peripatos of its most important theoretical tools. Without these books, the Peripatos could not keep up with the other schools. When they became available in the first century bce, the school of Aristotle could flourish again. It has been pointed out that this explanation is based on the anachronistic assumption that serious philosophizing requires direct attention to a written text. While this is a common assumption in the post-Hellenistic return to Aristotle, it has no parallel in the Hellenistic era. Although anachronistic, it points to a central feature of the post-Hellenistic return to Aristotle: it took the form of a vigorous engagement with Aristotle's writings. At least in the Peripatetic tradition, this was a strikingly new development.