Aenesidemus of Knossos

Life and Historical Context

Aenesidemus of Knossos was a leading figure of Hellenistic skepticism, generally placed in the 1st century BCE. Almost nothing is known with certainty about his life. Ancient testimonies associate him with Knossos on the island of Crete, while later reports connect him to Alexandria, one of the major intellectual centers of the period. Scholars infer that he was active at a time when the traditional Platonic Academy, which had long been the institutional home of skeptical philosophy, was turning away from skepticism toward a more dogmatic Platonism.

According to later sources, Aenesidemus was initially associated with the Academy, but became dissatisfied as it moved in a more doctrinal direction under figures such as Antiochus of Ascalon. In response, Aenesidemus is said to have revived the Pyrrhonian tradition, tracing his inspiration back to Pyrrho of Elis and his immediate followers. While the historical continuity between Pyrrho and Aenesidemus is debated, the ancient tradition consistently presents Aenesidemus as a restorer and systematizer of Pyrrhonist skepticism rather than an inventor of skepticism from scratch.

The broader Hellenistic context of Aenesidemus’s thought involves debates between Stoics, Epicureans, Peripatetics, and various Academic skeptics about the possibility of knowledge, the criterion of truth, and the good life. Against Stoic claims to secure cognitive impressions (kataleptic phantasiai) and against Epicurean empiricism, Aenesidemus developed a structured set of arguments intended to undermine confidence in any alleged criterion of truth and to support the skeptical attitude of epochē, or suspension of judgment.

Sources and Writings

No work by Aenesidemus survives in full. Knowledge of his philosophy is mediated through later authors, primarily:

• Photius, the 9th‑century Byzantine scholar, who gives a summary of an Aenesidemian work;
• Sextus Empiricus (2nd–3rd century CE), whose surviving skeptical treatises preserve and adapt much Aenesidemian material;
• Diogenes Laertius, who briefly reports on Pyrrhonism and its exponents.

Photius reports that Aenesidemus wrote a work—often referred to by modern scholars as the Pyrrhonian Discourses (Pyrrhôneioi logoi), probably in eight books. This text appears to have set out the principles of Pyrrhonism, methodological arguments against dogmatic schools, and Aenesidemus’s famous list of argument-types known as the Ten Modes.

Beyond this, the existence of other works is uncertain. Some have suggested that Aenesidemus may have also engaged with Heraclitean philosophy, and Photius attests to an Aenesidemus who draws connections between skepticism and the doctrines of Heraclitus. Modern scholars debate whether this represents a genuine synthesis, a strategic reinterpretation, or a misreading by later compilers. In any case, Aenesidemus appears as a figure who both codified existing skeptical tactics and reframed them within a new argumentative architecture.

Because of the fragmentary and indirect nature of the evidence, almost every aspect of Aenesidemus’s thought—from his precise arguments to his historical aims—remains a matter of reconstruction and scholarly dispute. Nonetheless, a consistent image emerges of a philosopher who sought to provide a systematic exposition of Pyrrhonian skepticism as a way of life and a method of inquiry.

Philosophical Views and the Ten Modes

Aenesidemus’s central philosophical commitment is that human cognitive limitations, together with the conflict of appearances and opinions, render dogmatic claims about reality unjustified. He does not assert that knowledge is impossible in a dogmatic fashion; rather, he argues that the skeptic finds no adequate grounds to affirm or deny any non‑evident proposition and therefore adopts suspension of judgment.

Skepticism and Epochē

Following the Pyrrhonian tradition as later reported by Sextus Empiricus, Aenesidemus likely held that the skeptic:

• Investigates philosophical questions with an open mind;
• Encounters equipollence—a balance of opposing considerations—for and against any view;
• As a result, suspends judgment (epochē) about the nature of things;
• And, according to some testimonies, thereby attains a form of tranquility (ataraxia), at least regarding doctrinal disputes.

Aenesidemus’s contribution lies in offering a structured taxonomy of the ways in which appearances and judgments clash, making the skeptical strategy more methodical than in earlier skeptical writings.

The Ten Modes

Aenesidemus is best known for the Ten Modes (or Tropes), a set of recurring skeptical strategies designed to undermine dogmatic confidence. Sextus Empiricus attributes these modes to “the older skeptics,” with Aenesidemus as the chief named source. Though there are variations in formulation, the modes can be summarized as follows:

1. Mode from the variety among animals: Different species perceive the same objects differently, suggesting that no single animal’s perception—human or otherwise—can claim privileged access to how things are in themselves.

2. Mode from the differences among humans: Even within our own species, individuals differ in temperament, bodily constitution, and sense organs, producing divergent impressions of the same objects.

3. Mode from the different senses: The same object appears differently to sight, touch, taste, etc. Skeptics infer that we lack a neutral standpoint from which to decide which sense, if any, correctly reveals the object’s true nature.

4. Mode from circumstances (conditions): Perceptions change with conditions such as health, sleep, age, motion, or rest. What appears hot or sweet or beautiful under one set of circumstances appears otherwise under another.

5. Mode from positions, distances, and locations: Objects appear altered when near or far, above or below, inside or outside a medium. The variability of appearance across spatial relations undermines claims to a stable underlying reality evident through perception.

6. Mode from mixtures and compositions: Nothing is perceived in isolation; every perception involves media (air, light, moisture) and the perceiver’s bodily state. Since we cannot separate the object from these conditions, we cannot determine what the object is independently of them.

7. Mode from quantities and constitutions: Differences in amount, number, or structure alter how things appear. For example, a small amount of wine may be pleasant, a large amount harmful; what counts as sweet or bitter, large or small, often depends on relative measures.

8. Mode from relativity: Perhaps the most general and influential mode, it holds that all appearances are relative—to the observer, the circumstances, the comparison class, or conceptual framework. From relativity, the skeptic infers that we can only report how things appear to us, not how they are in themselves.

9. Mode from frequent or rare occurrences: The familiar often ceases to astonish, while the rare fascinates or disturbs. The same event—such as an eclipse—provokes different beliefs and reactions depending on its frequency, suggesting that habituation shapes judgment.

10. Mode from customs, laws, and beliefs: Different cultures and communities uphold divergent norms, moral codes, and religious views. Since people are typically persuaded by the conventions of their own society, the skeptic concludes that no one set of customs can claim rational priority over others.

Collectively, these modes aim to show that for any dogmatic claim, one can produce counter‑considerations grounded in experience, perception, or social practice. The skeptic then withholds assent, adopting the formula of no-more (ou mallon): things are no more this way than that way.

Relation to Other Schools

Ancient reports suggest that Aenesidemus criticized the Academic skeptics for failing to maintain strict neutrality, accusing them of sliding into dogmatic claims (for example, that knowledge is impossible). At the same time, he argued against Stoic and Epicurean epistemologies that sought firm criteria of truth.

The alleged connection between Aenesidemus and Heraclitus is particularly controversial. Photius attributes to Aenesidemus the claim that a genuine skeptic can, in some sense, accept Heraclitean doctrines about flux and the unity of opposites. Some interpreters view this as an attempt to portray skepticism as compatible with a deeper metaphysical insight; others think it reflects a rhetorical or exegetical strategy rather than an actual dogmatic commitment. The tension here—between radical suspension and any positive doctrine—remains a central point of discussion in scholarship on Aenesidemus.

Influence and Reception

Despite the loss of his works, Aenesidemus exercised a long‑term influence through the Pyrrhonian tradition preserved by Sextus Empiricus. The Ten Modes and the general method of arguing from the conflict of appearances became standard tools in ancient skepticism and shaped later reflections on fallibilism and epistemic relativization.

In late antiquity, Christian authors encountered skeptical arguments indirectly, often via doxographical collections, and sometimes adapted them in theological debates—for instance, to criticize pagan philosophy or to highlight the limits of unaided human reason.

During the Renaissance and early modern periods, when Sextus Empiricus’s works were rediscovered and translated, Aenesidemus’s modes re‑entered philosophical discussion. Thinkers such as Montaigne and later Descartes, Bayle, and Hume engaged with Pyrrhonian strategies, whether to deploy them or to craft responses. While Aenesidemus is rarely named in these debates, many of the arguments trace back, via Sextus, to his formulation.

Modern scholarship treats Aenesidemus as a key figure in the institutional reconfiguration of skepticism: he marks the shift from Academic skepticism, centered on the Platonic Academy, to a self‑conscious Pyrrhonian movement defined more by method and practice than by allegiance to an institution. His Ten Modes remain a canonical set of considerations in the history of skepticism, frequently cited in contemporary discussions of perceptual relativity, cultural diversity, and the limits of justification.

Because the direct evidence is sparse, assessments of Aenesidemus’s originality and coherence differ. Some view him primarily as a compiler and systematizer of earlier skeptical arguments; others emphasize his role in rearticulating Pyrrhonism as a way of life responsive to the intellectual conflicts of the late Hellenistic world. Whatever the verdict, Aenesidemus of Knossos occupies a central position in the genealogy of philosophical skepticism from antiquity to the present.