The Peripatetics

Aristoxenus of Tarentum was the most important music theorist of antiquity and one of the most original members of the early Lyceum. Born in Tarentum in southern Italy — a city with strong Pythagorean traditions — he studied under the last Pythagoreans before coming to Athens and joining Aristotle's school. He was reportedly bitter at being passed over for the headship of the Lyceum in favor of Theophrastus, though he continued to write prolifically. The Suda credits him with 453 works, of which only the Elements of Harmonics (Harmonika Stoicheia) survives in substantial form, along with fragments of his Elements of Rhythm (Rhythmika Stoicheia) and biographical works.

His revolution in music theory consisted in rejecting the Pythagorean approach — which defined musical intervals as numerical ratios (the octave as 2:1, the fifth as 3:2) — in favor of an empirical, perceptual method. For Aristoxenus, the ear, not mathematics, is the judge of musical phenomena. Musical intervals are to be understood as continuous magnitudes perceived by trained hearing, not as discrete numerical ratios grasped by reason. The note is a point on a continuum of pitch, and intervals are the distances between such points — measured by perception, not by arithmetic. This represents a fundamental methodological shift: from the Pythagorean conviction that the real structure of music is mathematical and hidden beneath the surface of experience, to the Aristotelian insistence that the phenomena themselves — as perceived — are the proper starting point of scientific inquiry.

The Elements of Harmonics develops this approach systematically. Aristoxenus identifies the fundamental principles of melody — the voice's natural movement through intervals — and classifies the genera of melody (enharmonic, chromatic, diatonic), the species of intervals, and the rules governing melodic succession. He introduces the concept of 'function' (dynamis) — the role a note plays within a musical system, as distinct from its absolute pitch — anticipating by over two millennia the modern concept of functional harmony. His analysis of the tetrachord (the four-note unit that is the building block of Greek scales) remains the foundation of our understanding of ancient Greek music.

Beyond music theory, Aristoxenus wrote influential biographies of Pythagoras, Archytas, and other philosophers. His Life of Pythagoras is one of the earliest biographical works in Greek literature and was used extensively by later writers (Porphyry, Iamblichus). He also wrote on education, ethics, and political philosophy, though these works survive only in fragments. His ethical views apparently emphasized the Pythagorean-Aristotelian theme that the soul is a harmony or attunement (harmonia) of the body — a position Plato's Socrates had argued against in the Phaedo. If the soul is a harmony, it cannot survive the body's dissolution; Aristoxenus may have accepted this consequence and denied personal immortality.

His methodological legacy is significant beyond musicology. By insisting that perception, properly trained, is the criterion of scientific knowledge in its own domain — that the ear is authoritative about music as the eye is authoritative about color — Aristoxenus applied Aristotle's empiricist epistemology in a way that challenged the Pythagorean-Platonic privileging of mathematical reason over sensory experience. The debate between 'Pythagorean' (mathematical) and 'Aristoxenian' (empirical-perceptual) approaches to music theory persisted throughout antiquity and into the Middle Ages, and its echoes are audible in modern disputes between acoustic science and musical phenomenology.

Theophrastus — born Tyrtamus in Eresus on the island of Lesbos — was Aristotle's most important student, his closest collaborator, and his successor as head of the Lyceum, a position he held for thirty-five years (c. 322–287 BCE). The name 'Theophrastus' ('divine speaker') was reportedly given him by Aristotle in recognition of his eloquence. Under his leadership the Lyceum reached its peak of institutional influence: Diogenes Laertius reports that he attracted up to two thousand students (V.37). He was a prolific author on an Aristotelian scale — the catalogue preserved by Diogenes lists 227 works — covering logic, metaphysics, natural philosophy, biology, ethics, politics, rhetoric, and the history of philosophy.

His most enduring contribution is to botany: the two surviving works, the Enquiry into Plants (Historia Plantarum) and the Causes of Plants (De Causis Plantarum), together constitute the founding texts of systematic botany. They classify, describe, and explain the structure, reproduction, and ecology of plants with an empirical precision that had no predecessor and no rival until the Renaissance. Theophrastus distinguished trees, shrubs, undershrubs, and herbs; identified the difference between monocotyledons and dicotyledons (without using those terms); described plant geography and the effects of climate, soil, and cultivation on growth; and recorded practical knowledge from farmers, root-cutters, and drug-sellers alongside his own observations. Linnaeus called him 'the father of botany.'

Philosophically, Theophrastus was no mere disciple. His short but penetrating Metaphysics — a collection of aporiai (puzzles) rather than a systematic treatise — raises fundamental challenges to Aristotle's system. He questions the scope of teleology: 'with respect to how far the limit extends, for not all things are for the sake of an end or in the best state' (11a27–b2). He challenges the relationship between the unmoved mover and nature: if the heavenly bodies are moved by desire for the divine, what moves the sublunary world? He probes the awkward boundary between the teleological and the mechanical. These are not student exercises but genuine philosophical difficulties that Aristotle had not resolved, and Theophrastus's willingness to articulate them openly set a precedent for internal critique within the Peripatetic tradition.

His lost work On the Opinions of the Natural Philosophers (Physikōn Doxai), compiled with the assistance of his students, was the foundation of the entire ancient doxographic tradition. Though the original is lost, its structure — organizing philosophical opinions systematically by topic rather than by thinker — was transmitted through Aetius (1st–2nd century CE) to pseudo-Plutarch and Stobaeus, and through them to all subsequent histories of philosophy. When we read that 'Thales said the principle is water' or 'Heraclitus said it is fire,' we are ultimately depending on the framework Theophrastus established. His Opinions of the Natural Philosophers is arguably the single most influential lost work in the Western philosophical tradition.

The Characters, a collection of thirty brief sketches of moral types — the Flatterer, the Boaster, the Penurious Man, the Surly Man — is the only other surviving work. It became enormously influential as a literary form: La Bruyère's Caractères (1688) are direct imitations, and the tradition extends through the English 'character' writers to the modern novel's interest in psychological types. Whether Theophrastus intended them as an appendix to his ethical lectures, a rhetorical exercise, or a free-standing work is debated. What they demonstrate is an eye for the concrete detail of human behaviour that complements the abstract analysis of Aristotle's Nicomachean Ethics.

Theophrastus also made important contributions to mineralogy (On Stones, partly surviving), the study of fire (On Fire, surviving), sensory psychology (On the Senses, substantially preserved by later sources), and the history of logic. His modifications of Aristotle's syllogistic — particularly the addition of prosleptic premises and hypothetical syllogisms — were transmitted through the Stoic logical tradition and influenced the development of formal logic. Aristotle bequeathed him his library and his manuscripts, and Theophrastus in turn left them to Neleus of Scepsis, whose family's neglect of the collection contributed to the notorious loss and later recovery of Aristotle's esoteric works — a story told by Strabo (XIII.1.54) that has shaped the entire transmission history of the Aristotelian corpus.

Eudemus of Rhodes was, alongside Theophrastus, one of Aristotle's two most distinguished students — and, according to ancient tradition, the runner-up to succeed him as head of the Lyceum. Aulus Gellius reports (Noctes Atticae XIII.5) that Aristotle chose Theophrastus over Eudemus by tasting wines from the two men's home regions: he preferred the Lesbian wine (Theophrastus's homeland) to the Rhodian (Eudemus's), a jest concealing a serious institutional decision. Eudemus returned to Rhodes after Aristotle's death and may have established a Peripatetic circle there, though the evidence is uncertain.

His most important works were histories of the mathematical sciences: the History of Geometry, the History of Arithmetic, and the History of Astronomy. None survives intact, but substantial fragments are preserved by later commentators — above all Simplicius, Proclus, and Eudemus's fellow Peripatetic Porphyry. These works established the genre of the history of science: they traced the development of mathematical knowledge from its origins in Egypt and Babylon through the Greek geometers and astronomers, assigning credit to individual discoverers and establishing a narrative of progressive intellectual achievement. When Proclus credits Thales with specific geometric theorems, or when we know the sequence of discoveries leading to Euclid's Elements, we are usually relying on Eudemus's research. His History of Geometry is the single most important source for pre-Euclidean mathematics.

Philosophically, Eudemus was the most faithful of Aristotle's students — his works were so close to Aristotle's own that the Eudemian Ethics, one of the three ethical treatises in the Aristotelian corpus, is named after him (either because he edited it, because it was based on his notes, or because it was dedicated to him). He wrote his own Physics and Analytics, closely following Aristotle's but with modifications and clarifications. On the question of the eternity of the cosmos, he appears to have been a strict Aristotelian: the universe had no beginning and will have no end. He reportedly also investigated the concept of the infinite, arguing that time alone is actually infinite — spatial magnitude and number are only potentially infinite, a refinement of Aristotle's own analysis.

His contribution to the history of astronomy was particularly significant. He recorded early observations and theories — attributing the discovery of the obliquity of the ecliptic to Oenopides of Chios, the theory of concentric spheres to Eudoxus of Cnidus, and various refinements to Callippus. Through Eudemus, the entire pre-Ptolemaic history of Greek astronomy was preserved for later scholars. Simplicius's commentary on Aristotle's De Caelo — one of our primary sources for early Greek astronomy — draws heavily on Eudemus's History of Astronomy.

Dicaearchus of Messana (modern Messina, Sicily) was one of Aristotle's most versatile students — a polymath whose interests ranged from philosophy and political theory to geography, history, and literary criticism. He was a contemporary and rival of Theophrastus, and Cicero considered him one of the greatest Peripatetic thinkers, praising his learning and eloquence repeatedly (Ad Atticum II.2, VI.2, XIII.32). None of his works survives intact, but the fragments and testimonia preserved by later authors reveal an ambitious and original mind.

His most philosophically significant contribution was the denial of the soul's separate existence. Dicaearchus argued that the soul is not a distinct entity — neither an immaterial substance (as Plato held) nor a form of the body (as Aristotle argued) — but simply the body itself in a certain condition. The term he used was apparently harmonia, but not in the Pythagorean sense of a mathematical ratio: rather, the soul is an emergent property of the body's physical organization, inseparable from it and perishing with it. This is the most thoroughgoing materialism in the early Peripatetic school, going beyond Aristoxenus's similar but more cautiously formulated position. Cicero discusses his view extensively in the Tusculan Disputations (I.10.21, I.24.77), noting that Dicaearchus denied immortality entirely.

In political philosophy, Dicaearchus advocated for the mixed constitution — a blend of monarchy, aristocracy, and democracy — as the best form of government. His Tripolitikos (The Three Constitutions) argued that each pure constitution is unstable: monarchy degenerates into tyranny, aristocracy into oligarchy, democracy into mob rule. Only a mixture that balances the virtues of all three can achieve stability. This theory was enormously influential: Polybius later developed it systematically in his analysis of the Roman constitution (Histories VI), and through Polybius it influenced Cicero, the American founders, and modern constitutional theory. Whether Dicaearchus was the originator of the theory or merely its most influential Peripatetic formulation is debated, but his Tripolitikos was clearly a key text.

His geographical work was pioneering. He reportedly produced one of the earliest attempts to measure the heights of Greek mountains, and his Circuit of the Earth (Gēs Periodos) combined geographical description with cultural and historical observation. He established a baseline for geographical measurement running from the Pillars of Heracles (Strait of Gibraltar) through the Mediterranean to the Taurus mountains — an early attempt at what would later become the concept of a prime parallel. Eratosthenes, the greatest geographer of antiquity, built directly on Dicaearchus's foundations.

In cultural history, his Life of Greece (Bios Hellados) traced the development of Greek civilization from a primitive golden age through pastoral, agricultural, and urban phases — an early stadial theory of cultural development that anticipates Enlightenment conceptions of historical progress. He also wrote on Homer, on Alcaeus and Sappho, and on the dramatic festivals at Athens, making him one of the earliest literary historians.

Strato of Lampsacus — known in antiquity as 'the Physicist' (ho Physikos) — succeeded Theophrastus as the third head of the Lyceum around 287 BCE and led the school for eighteen years until his death around 269 BCE. His nickname reflects his distinctive philosophical orientation: where Aristotle had balanced physics, metaphysics, ethics, and logic, and Theophrastus had maintained that breadth while excelling in natural history, Strato concentrated his energy on natural philosophy and pushed it in a radically naturalistic direction that departed significantly from Aristotle's own views.

His most consequential move was the elimination of the unmoved mover and teleology from natural explanation. Where Aristotle had posited an unmoved mover as the ultimate cause of cosmic motion and had explained natural processes teleologically (rain falls 'for the sake of' the crops; teeth grow sharp in front 'for the sake of' cutting), Strato argued that nature operates through its own inherent powers without external direction or purpose. All natural phenomena can be explained by the properties of matter itself — weight, motion, heat, cold — without recourse to divine movers or final causes. Cicero reports (Academica II.38.121, De Natura Deorum I.13.35) that Strato 'freed god from the great task' of governing the universe and attributed all natural operations to 'natural weights and motions.' This represents the most thorough-going naturalism in the ancient Peripatetic tradition — closer in spirit to the Atomists than to Aristotle.

Strato also challenged Aristotle's physics on specific points. He denied the existence of a genuine void (in agreement with Aristotle) but argued that matter contains micro-voids — tiny interspersed vacua that explain compression, the penetration of light through water, and the mixing of substances. This 'micro-void' theory attempted to account for the phenomena that the Atomists explained by void without accepting their ontology of atoms and empty space. He also argued against Aristotle's theory of natural places (the doctrine that each element has a natural place to which it tends): for Strato, all bodies simply tend downward by their own weight, and what appears to be an upward tendency (as with fire) is actually the effect of being squeezed out by heavier bodies pressing down — an explanation by relative weight rather than by natural place.

His experiments with falling bodies are particularly noteworthy. Simplicius preserves his argument that falling bodies accelerate: 'if one drops a stone from a height of one finger, the impact on the ground is negligible; but if one drops it from a height of a hundred feet or more, the impact is powerful' (In Phys. 916.12–17). Strato concluded that the body gains speed as it falls — that is, it accelerates — and offered the observation that a stream of water breaks into droplets as it descends (because the leading drops accelerate away from those behind) as additional evidence. These observations anticipate Galileo's work on falling bodies by nearly two millennia, though Strato lacked the mathematical framework to express acceleration quantitatively.

On the soul, Strato maintained that all mental activities — sensation, thought, emotion — are functions of a single cognitive faculty located between the eyebrows (in the region of the brain). He rejected Aristotle's distinction between the nutritive, sensitive, and rational parts of the soul, arguing instead for a unified psychic principle. Sensation requires attention: we can look without seeing or hear without listening, because the sensory organs transmit impressions to the cognitive center only when the mind is actively engaged. This 'attention theory' of perception is strikingly modern and anticipates debates about the role of attention in consciousness.

Under Strato's leadership the Lyceum remained scientifically productive but began to lose the philosophical breadth that had characterized it under Aristotle and Theophrastus. His naturalistic programme stripped away metaphysics, theology, and much of ethics, concentrating on physics and psychology. Later ancient writers sometimes blamed him for the decline of the Peripatetic school, seeing his narrow focus as an abandonment of Aristotle's comprehensive philosophical vision. But his innovations were genuine and influential: his micro-void theory influenced Hellenistic physics; his arguments about acceleration entered the tradition of commentary on Aristotle's Physics; and his naturalism represented a permanent option within Peripatetic thought.

Lyco of Troas succeeded Strato as the fourth head of the Lyceum around 269 BCE and held the position for approximately forty-four years until his death around 226 BCE — the longest tenure of any scholarch in the school's history. Despite this extraordinarily long leadership, remarkably little of his philosophical output survives. The ancient sources suggest that Lyco was more celebrated as a teacher and orator than as an original thinker: Diogenes Laertius (V.65) reports that he was called 'Glyco' (the Sweet One) because of the charm and elegance of his speaking style.

What fragmentary evidence we have suggests that Lyco focused primarily on ethics and education. He apparently continued the Peripatetic tradition of character analysis that Theophrastus had made famous in his Characters, and he may have written on psychology and the emotions. His ethical interests seem to have been practical rather than theoretical: he was concerned with the formation of character through habit and education, following Aristotle's emphasis in the Nicomachean Ethics on the role of habituation (ethismos) in moral development.

Lyco's significance in the history of the Peripatetic school is largely institutional rather than philosophical. Under his leadership the Lyceum continued to function as an educational institution, but its philosophical vitality declined. The school produced no major systematic works during this period, and its influence waned relative to the rising Stoic and Epicurean schools. His will, preserved by Diogenes Laertius (V.69–74), provides valuable information about the organization and property of the Lyceum in the mid-Hellenistic period. The long but philosophically quiet period of Lyco's leadership is often cited as evidence of the general decline of the Peripatetic school between the creative early generation and the scholarly revival associated with Andronicus of Rhodes in the first century BCE.

Critolaus of Phaselis (in Lycia, modern Turkey) was the most philosophically significant figure in the Middle Peripatos — the long period between the creative early generation of Aristotle's direct students and the scholarly revival of the first century BCE. He succeeded Aristo of Ceos as head of the Lyceum sometime around 155 BCE, and his most famous public appearance came in 155 BCE when he was one of three philosophers sent by Athens on an embassy to Rome (along with the Academic Carneades and the Stoic Diogenes of Babylon). This embassy — the first major encounter between Greek philosophy and Roman culture — made a powerful impression on Rome. Cicero reports (De Oratore II.155) that the three philosophers lectured publicly in Rome, and their visit is often credited with sparking Roman interest in systematic philosophy.

Philosophically, Critolaus is notable for his defense and reformulation of core Aristotelian positions in the context of Hellenistic debates. On the question of the highest good, he argued against both the Stoics (who identified it with virtue alone) and the Epicureans (who identified it with pleasure), maintaining the Aristotelian position that happiness requires a combination of virtue, bodily goods (health, strength), and external goods (wealth, friends, good fortune). But he gave this position a more precise formulation: he compared life to a balance in which virtue outweighs all bodily and external goods combined. Virtue is the dominant component of happiness, but the other goods are genuinely good and their absence genuinely diminishes well-being. This 'weighing' metaphor became an influential way of expressing the Peripatetic ethical position.

Critolaus also defended the eternity of the world against Stoic cosmology, which held that the universe is periodically destroyed and reconstituted by fire (ekpyrosis). He argued that if the world had a beginning, there must have been a cause of its coming into being; but any such cause would itself require explanation, leading to an infinite regress. The world has therefore always existed and will always exist — a position that Aristotle had also maintained (De Caelo I.10–12) and that Critolaus sharpened against Stoic objections. His arguments on this topic were evidently powerful enough that they continued to be discussed and responded to by Stoic philosophers.

On rhetoric, Critolaus took a controversial position, arguing that rhetoric is not a genuine art (technē) but merely a knack or empirical practice (tribē) — an opinion that aligned with Plato's critique in the Gorgias rather than with Aristotle's more balanced treatment in the Rhetoric. This stance generated debate within the Peripatetic school itself and reflects the diversity of views on rhetoric that existed among Aristotle's followers.

Diodorus of Tyre is one of the more obscure figures in the Peripatetic succession, but his philosophical position on the highest good was distinctive enough to attract the attention of Cicero, who discusses it in De Finibus V.5.14. Diodorus modified the standard Peripatetic doctrine that happiness consists of virtue combined with external and bodily goods. Where Critolaus had argued that virtue outweighs all other goods on the balance, Diodorus went further: he defined the highest good as 'virtue combined with the absence of pain' (honeste vivere cum vacuitate doloris). This formulation represents a striking convergence between Peripatetic ethics and Epicureanism — the absence of pain (aponia) being a characteristically Epicurean value. Whether Diodorus was genuinely influenced by Epicurean ethics or simply arrived at a similar position from different premises is unclear from the fragmentary evidence.

Diodorus's modification reflects a broader trend in Hellenistic philosophy toward eclecticism — the willingness to combine doctrines from different schools when doing so seemed philosophically productive. Antiochus of Ascalon, the Academic philosopher who tried to reconcile Platonism, Aristotelianism, and Stoicism, was a near-contemporary, and the philosophical atmosphere of the late second and early first centuries BCE was increasingly characterized by cross-school borrowing. Diodorus's ethics can be seen as a Peripatetic contribution to this eclectic movement.

His position on the scholarch list is uncertain. Some sources place him as head of the Lyceum, but the succession in this period is poorly attested. What is clear is that he belonged to the middle period of the school's history — after the creative early generation and before the editorial revival associated with Andronicus of Rhodes.

Andronicus of Rhodes is arguably the most consequential figure in the entire history of the Peripatetic school after Aristotle himself — not because of his original philosophical contributions, but because his editorial work made the Aristotle we know possible. Around 60 BCE, Andronicus produced the first systematic edition of Aristotle's esoteric works (the treatises intended for the school, as opposed to the published dialogues), organizing them into the form in which they have been transmitted to the present day. Without his editorial labors, the Aristotelian corpus as we know it would almost certainly not exist.

The background to his achievement involves one of the most consequential episodes in intellectual history. According to Strabo (Geography XIII.1.54) and Plutarch (Life of Sulla 26), Aristotle bequeathed his library — including his unpublished lecture notes and treatises — to Theophrastus, who in turn left it to his student Neleus of Scepsis. Neleus's heirs, uninterested in philosophy, stored the books in a cellar in Scepsis (in the Troad), where they remained for nearly two centuries, damaged by damp and insects. Around 100 BCE, the bibliophile Apellicon of Teos acquired the manuscripts and brought them to Athens; when Sulla conquered Athens in 86 BCE, he seized Apellicon's library and transported it to Rome. There the grammarian Tyrannion gained access to the manuscripts and began preparing copies, and Andronicus subsequently produced his systematic edition.

While the details of Strabo's story are debated by modern scholars — some argue that copies of Aristotle's treatises were available in the Hellenistic period and that the 'rediscovery' narrative is exaggerated — it is clear that Andronicus's edition was a watershed event. He organized the treatises thematically, wrote introductions and explanations, and established the canonical ordering that later scholars followed. The very title 'Metaphysics' (ta meta ta physika, 'the [books] after the Physics') is traditionally attributed to Andronicus's editorial arrangement, though this attribution is also debated. His edition also included a catalogue of Aristotle's works (pinax) and a biographical study.

Andronicus's editorial principles were themselves philosophically significant. He organized Aristotle's works according to the structure of Aristotle's own philosophical system: logic (the Organon) first, then physics, then metaphysics, then ethics and politics. This ordering implies an interpretation of Aristotle's philosophy — that logic is a tool (organon) rather than a substantive part of philosophy, that physics precedes metaphysics, that practical philosophy depends on theoretical foundations. Later Neoplatonic commentators debated Andronicus's arrangement and proposed alternatives, but his basic framework proved remarkably durable.

The consequences of Andronicus's edition for the history of philosophy were immense. It inaugurated the great tradition of Aristotelian commentary that dominated later ancient philosophy: Alexander of Aphrodisias, Themistius, Simplicius, and Philoponus all wrote commentaries on the works in Andronicus's edition. Through Arabic translations of these commentaries and of Aristotle's own treatises, the Andronican corpus shaped medieval Islamic and Christian philosophy. The Aristotle of Aquinas, Averroes, and the European universities is Andronicus's Aristotle — organized, edited, and presented according to his editorial vision.

Alexander of Aphrodisias — known in the later tradition simply as 'the Commentator' (ho Exēgētēs) — is the most important Aristotelian philosopher between antiquity and the medieval period. He held an imperially endowed chair of Aristotelian philosophy in Athens around 200 CE, during the reigns of Septimius Severus and Caracalla (to whom he dedicated one of his works), and produced the most philosophically rigorous and influential commentaries on Aristotle ever written. Where earlier commentators had often read Aristotle through Stoic or Platonic lenses, Alexander insisted on understanding Aristotle on his own terms, defending genuinely Aristotelian positions against rival schools — particularly against the Stoics and the emerging Neoplatonists.

His commentaries on the Metaphysics, Prior Analytics, Topics, Meteorology, and De Sensu survive in Greek; his commentary on the De Anima is lost but partially recoverable from later references. In addition to the commentaries, Alexander wrote independent treatises (the De Anima, De Fato, De Mixtione, and the Quaestiones) that constitute original philosophical works of the first order. These treatises demonstrate that Alexander was not merely an exegete but a creative thinker who developed Aristotle's positions in new directions.

Alexander's most consequential philosophical position concerns the nature of the intellect — the question that would dominate Aristotelian psychology for the next millennium. Aristotle had distinguished in De Anima III.5 between a passive intellect (which receives intelligible forms) and an active or 'agent' intellect (nous poiētikos) that 'makes all things' and is described as 'separable, impassible, and unmixed.' The meaning of this notoriously obscure passage was debated from the earliest period: is the agent intellect part of the individual human soul, or is it a separate divine substance? Alexander argued decisively for the latter interpretation: the agent intellect is God — the unmoved mover of the Metaphysics — which illuminates the human material intellect from outside, enabling it to think. The individual human intellect is entirely material and mortal: it comes into being with the body and perishes with it. Only the divine agent intellect is eternal.

This interpretation had enormous consequences. It meant that, for Alexander, individual immortality is impossible: the human soul is the form of the body, inseparable from it, and does not survive death. Personal identity, memory, character — all perish with the body. This is the most consistently naturalistic reading of Aristotle's psychology, and it provoked intense opposition from thinkers committed to the soul's immortality. The Aristotelian commentator Themistius argued that the agent intellect is an individual possession of each human soul; Averroes (Ibn Rushd) later argued that both the agent and material intellects are single and shared by all humans; Thomas Aquinas argued that the agent intellect is individual and immortal. The debate — which raged from the second century to the sixteenth — is fundamentally a debate about the meaning of Alexander's interpretation and its alternatives.

On fate and determinism, Alexander wrote the treatise De Fato, one of the most important ancient discussions of free will. Against the Stoics, who held that every event is determined by an unbreakable chain of antecedent causes (heimarmenē), Alexander defended a libertarian conception of human freedom. He argued that human beings possess a genuine capacity for choice (prohairesis) that is not determined by prior causes: when we deliberate and choose, the outcome is genuinely 'up to us' (eph' hēmin). This does not mean that human action is random or uncaused, but that the human rational soul is a special kind of cause — one that can originate new causal sequences rather than merely transmitting impulses from prior causes. Alexander's De Fato became the standard Aristotelian text on free will and was translated into Arabic (as part of a broader translation movement), influencing Islamic philosophical discussions of qadar (divine decree) and human agency.

His treatise De Mixtione (On Mixture) addresses the problem of chemical combination — how two or more substances can combine to form a genuinely new substance while their constituents remain potentially recoverable. Against the Stoic theory of total blending (krasis di' holōn), which held that two bodies can fully interpenetrate and occupy the same space, Alexander defended the Aristotelian position that mixture involves a genuine transformation of the ingredients into a new substance. His arguments against the possibility of two bodies occupying the same place simultaneously were technically sophisticated and influenced both Arabic and Latin discussions of mixture and chemical change.

Alexander's influence on subsequent philosophy is difficult to overstate. His commentaries were translated into Arabic in the ninth century and became foundational texts for the falāsifa (Islamic Aristotelian philosophers), particularly al-Fārābī and Averroes. Through the Arabic tradition, his interpretations shaped the reception of Aristotle in medieval Latin philosophy. His reading of the agent intellect as divine and external provoked the great medieval debate about the unity of the intellect — a debate that was still generating papal condemnations (the condemnation of 1270) and university controversies in the thirteenth century. In a real sense, the history of Aristotelian psychology from the second century to the sixteenth century is a history of responses to Alexander of Aphrodisias.