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Aenesidemus (1st Century BCE)

(1st century BCE)

Very little is known about Aenesidemus's life. He was associated with the Athenian Academy around the time of its collapse in 87 BCE; and he was party to the dispute between Philo of Larissa, who advocated a mild form of skepticism in the form of an externalist, coherentist epistemology, and Antiochus of Ascalon, whose epistemology was basically that of Stoic foundationalism. The Academy had been for two centuries the home of epistemological skepticism, directed largely against the optimistic epistemology of the Stoics, who posited "apprehensive impressions" (phantasiai katalêptikai ), which carried their own guarantee of truth. Aenesidemus saw Philo and Antiochus as betraying that heritage, as "Stoics fighting with Stoics" (Photius, Library Catalogue 212), and resolved to "philosophize after the fashion of Pyrrho."

Aenesidemus wrote eight books of Pyrrhonian Discourses, which Photius summarized: "the whole aim of the book is to ground the view that there is no ground for apprehension, whether through perception or thought." The main burden of the Discourses, Photius says, is to establish that nobody really grasps anything. However, only Pyrrhonian skeptics are aware of this ignorance, while everyone else falsely considers themselves to be in possession of secure knowledge. This false conviction, and the inevitable disputes that follow from the evident fact that different people hold different and incompatible beliefs, leads the Dogmatists ("belief-holders," as skeptics styled their opponents) into "ceaseless torments." Skeptics, having no beliefs, avoid these torments; indeed they "are happy in the wisdom of knowing that they have firm apprehension of nothing." "Apprehension" (katalêpsis ) is the Stoic technical term for sure and unshakable knowledge based on apprehensive impressions. When Aenesidemus claims that Pyrrhonists have no apprehension of anything, he is careful not to say that they have apprehension of that second-order fact. Yet they may still be aware of it, since it is evident to them introspectively that they are not certain of anything (thus skeptics seek to avoid the charge that their position is self-refuting).

Moreover, "even in regard to what he knows [this is Photius's language; and he may well be less careful than Aenesidemus in avoiding apparent self-refutation], he takes care to assent no more to its affirmation than to its denial." "Assent" (sunkatathesis ) is another Stoic term, denoting unwavering commitment to the truth of some proposition (positive or negative); and no skeptic will claim that sort of cognitive security, even in regard to his own claims: a skeptic's "positions" (insofar as he really has any) are invariably provisional. In the same vein, "no more" (ou mallon ) is a skeptical slogan: things may appear to be thus and so, but in themselves they are no more one way rather than the other. Diogenes Laertius (DL 9.106) reports Aenesidemus as saying that appearances are the criterion for action; thus he seeks to evade the common charge brought against skeptics (most famously by Hume) that their refusal to hold beliefs renders life impossible (it is a further, difficult question how far this notion of appearance can really be divorced from some concept of belief).

In the first Pyrrhonian Discourse, according to Photius, Aenesidemus distanced himself from the Academics, since they "posit some things with confidence and deny others unambiguously, while Pyrrhonists are aporetic and devoid of dogma; they say neither that all things are inapprehensible, nor that they are apprehensible, but that they are no more so than not so, or sometimes so and sometimes not so, or so for one person but not for another." The Academics are negative dogmatists, positively affirming that nothing can be apprehended according to the Stoic criterion; Pyrrhonists, by contrast, will say that they do not seem to apprehend anything, but will not reject the possibility of there being apprehension. Crucially, "the Pyrrhonist determines absolutely nothing, not even this very proposition, that nothing is determined." That this is the authentic skeptical attitude is confirmed by Sextus Empiricus, Outlines of Pyrrhonism (PH) 1.187209; and Sextus probably relies heavily upon Aenesidemus in that work.

The second Pyrrhonian Discourse casts doubt upon "truth, causes, effects, motion, generation and destruction," while the third "was also about motion and sense perception working carefully through a similar set of contradictions, he puts them too beyond our grasp." These arguments about perception no doubt included the material of the so-called "Ten Modes of Aenesidemus," arguments designed to undermine the Dogmatists' truth-claims, and hence to induce epochê, or suspension of judgment, "which the skeptics say is the goal (telos ), upon which tranquility follows like a shadow, according to Aenesidemus and Timon" (DL 9.107; cf. PH 1.2530). Thus "Pyrrhonian discourse is a kind of recollection of appearances , on the basis of which they are all brought into confrontation with one another, and when compared are found to cause much disparity and confusion; so says Aenesidemus in the summary of his Pyrrhonics " (DL 9.78).

The Ten Modes are attributed to Aenesidemus by Sextus (Against the Professors [M] 7.345); Aristocles ascribes nine Modes to him, and we know the number of the Modes to have been fluid (our earliest source, Philo of Alexandria, records only eight). Neither Sextus in his extant treatment of the Modes (PH 1.31163), nor Diogenes in his shorter summary (DL 9.7988) father them on Aenesidemus; but it is still likely that he was responsible for this organization of earlier skeptical material. The Modes share a common form, involving conflicting appearances: x appears F in conditions C, or to observer O, not-F in conditions C *, or to observer O *; there is no non-question-begging way of privileging either of C or C *, O or O *; so we should suspend judgment as to whether x is F. The Modes are differentiated by different fillers for C or O ; thus the first (in Sextus's ordering) compares the different sensory representations of different animals, the second collects cases of dissonant judgment between different humans, the third conflicts in the deliverances of different sense-modalities, and the fourth includes discrepant reports from the same sense at different times. Other Modes collect cases of ethical or social discrepancy (the tenth), and point to the ways in which differing conditions of the perceiver may affect what they seem to perceive.

The upshot is that we cannot in any case say how things really are, but only how they seem in particular circumstances. Things are judged relatively to the perceiver and their circumstances. Sextus is careful not to draw relativistic conclusions (although the facts of relativity figure both as a particular Mode, the eighth, and in general in the articulation of all the Modes): He does not positively assert that things are for the observer as they appear. By contrast, Aenesidemus, judging from Photius's summary, is quite happy to accept the relative judgments as such, since they do not (cannot) count as Dogmatic.

In the fourth Discourse, Aenesidemus discussed signs. Sign-theory and its associated epistemology was of overwhelming importance in post-Aristotelian philosophy. The Stoics (along with various Dogmatic medical schools) held that it was possible to infer directly from the phenomena to the underlying structural conditions responsible for them. Skeptics (and Empiricist doctors) denied the validity of such inferences, allowing only that memories of past conjunctions of phenomena might allow us to expect (although fallibly) similar conjunctions in the future. Aenesidemus advanced the following paradigmatically skeptical argument: If apparent things appear alike to all in a similar condition, then signs should appear alike to all in a similar condition; but they do not; hence signs are not apparent (M 8.215). That is, it is not unequivocal what they are signs ofdifferent doctors, for example, draw radically different conclusions from the same symptoms (M 8.219220).

In the fifth Discourse Aenesidemus turned to causes; again Sextus retails some of his arguments (M 8.218226)yes; crucial to them is the idea that a cause should operate from its own resources; but if it does, then, since it requires nothing else in order to exercise its causal power, it should do so invariably and continuously. More impressive are the Eight Modes against the Aetiologists, mentioned in Photius and ascribed to Aenesidemus by Sextus at PH 1.180185. These are eight general arguments against the possibility of inferring from evident phenomena to the hidden structures of things that are supposedly causally responsible for those phenomena, in the manner of Dogmatist philosophers and scientists (notably Epicureans, but also Peripatetics and Stoics). Aenesidemus's basic claim foreshadows the modern maxim that theories are invariably underdetermined by the available data. No amount of evidence can ever entail that any particular theory must be true: There are always many ways in principle of accounting for the same set of phenomena (1.181182). Moreover (and here Aenesidemus turns from general methodological issues to castigating particular recurrent theoretical foibles), theorists sometimes offer piecemeal, unrelated explanations for what are evidently related sets of phenomena; and they tend to suppose, without justification, that the structure of the hidden, subperceptual realm will mirror in all important respects that of the phenomenal world (1.182; this point is particularly well-taken against Epicurean physics).

Furthermore, Aenesidemus notes (and this too is a staple of contemporary philosophy of science) that researchers are inclined to favor explanations that concur with their own prejudices (1.183), and indeed on occasion to prefer explanations that not only conflict with the facts, but also with their own theories (1.184). Finally, he notes that Dogmatists "frequently seek to explain doubtful things on the basis of things equally doubtful" (1.184). Taken together, the eight Modes are an impressive attack on the possibility of arriving at any soundly based understanding of the hidden natures of things. As such, they are obviously of a piece with, and complement, the rest of Aenesidemus's skeptical argumentation. The last three Pyrrhonian Discourses dealt with ethical issues, with Aenesidemus arguing that the lack of philosophical agreement regarding good and bad, choice and avoidance, virtues, and finally the end, preclude the possibility of arriving at any secure judgments about them.

All of the evidence so far reviewed makes Aenesidemus a consistent and powerful skeptic. However, a number of passages in Sextus portray him in a much more Dogmatic light, as holding various views about the intellect (M 7.350), and endorsing the view that there are two types of change (M 10.38). Elsewhere he is said to be in agreement with Heraclitus, whom Sextus explicitly describes as a Dogmatist. These discrepancies are too widespread simply to be brushed aside. But there is as yet no scholarly agreement as to what to do about them.

See also Ancient Skepticism; Antiochus of Ascalon; Philo of Larissa; Pyrrho; Sextus Empiricus.


Annas, J., and J. Barnes. The Modes of Scepticism. Cambridge, U.K.: Cambridge University Press, 1985.

Hankinson, R. J. The Skeptics. London: Routledge, 1995.

Long, A. A, and D. N. Sedley. The Hellenistic Philosophers. Cambridge, U.K.: Cambridge University Press, 1987.

Rist, J. "The Heracliteanism of Aenesidemus." Phoenix 24 1970: 309319.

Tarrant, H. Skepticism or Platonism? Cambridge, U.K.: Cambridge University Press, 1985.

R. J. Hankinson (2005)

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