Arcesilaus initiated a sceptical phase in the Academy after taking over in c. 268 BCE. He was motivated in part by an innovative reading of Plato’s dialogues. Where his predecessors found positive doctrines to be systematically developed, he found a dialectical method of arguing and the sceptical view that nothing can be known (akatalêpsia, De Or. 3.67, see DL 4.28, 4.32). He also advanced this conclusion in opposition to the ambitious system of the Stoics, claiming further that the appropriate response to the pervasive uncertainty generated by his method is the suspension of judgement (epochê).

Arcesilaus' dialectical method was practiced without significant modification in the Academy until Carneades, who became head sometime before 155 BCE. Carneades both continued and strengthened Arcesilaus' method (ND 1.11, Acad. 2.16, see also Acad. 1.46, and Eusebius, Praep. evang. 14.7.15). Sextus marks the change by referring to Plato’s Academy as Old, Arcesilaus' as Middle, and Carneades' as New (PH 1.220).

Since the main interpretative issues regarding both Arcesilaus and Carneades depend on the concepts of akatalêpsia and epochê, we must try to determine what they mean, how they are related, and what attitude the Academics take towards them – i.e. in what sense, if any, are these their sceptical doctrines?

What I have described as progress in the last chapter was seen as decline by at least one member of the late Academy. Aenesidemus criticized his fellow Academics for being dogmatic. The long dispute with the Stoics had effectively come to an end, and the Stoics had won. The Academics had completely abandoned their originally strict stance on epochē and now made firm determinations about a whole range of philosophical issues.

Aenesidemus committed his scepticism to writing, probably some time in the early-to-mid first century bce. Unfortunately, only some fragments and testimonia survive. The most extensive reports are about his Pyrrhonist Discourses. One of these comes from the ninth-century Byzantine patriarch Photius, who is remarkable in his own right. In his Bibliothēkē, Photius summarizes 280 books, including the Pyrrhonist Discourses. It is clear from his summary that he thinks very little of Aenesidemus' work because it makes no contribution to Christian dogma and drives from our minds the instinctive tenets of faith (Bib. 170b39–40). Nevertheless, Photius is a generally reliable source (Wilson 1994). So despite his assessment of Aenesidemus' scepticism, he probably provides an accurate summary.

The proper interpretation of that summary, however, along with the general character of Aenesidemus' Pyrrhonism, is disputed. The central issue is whether the revived Pyrrhonism is a type of relativism that affirms the impossibility of knowledge, or whether it is more consistent with Sextus Empiricus' later scepticism, according to which we cannot conclusively rule out the possibility of any kind of knowledge.

In this chapter I hope to show that even if we are uninterested in the prospects of tranquillity, Pyrrhonian Scepticism continues to provide an important, and perhaps insurmountable, challenge. For this challenge to take effect, one must simply accept that an arbitrary preference is not a rational basis for belief, and that we have some sort of obligation as rational agents to justify our beliefs. If the sceptic can systematically block all of our attempts at justification, we will be left in the troubling position of believing that we ought to do what we cannot.

The five modes

As we saw in Chapter 6, the modes are the Sceptic's tools; they are argument forms that may be employed against a wide variety of claims. After presenting Aenesidemus' ten modes, Sextus turns to another set of five that he describes as the property of “more recent Sceptics” (PH 1.164; see 1.36).

What all routes to epochē have in common is that they oppose one thing to another (PH 1.31): for example, the appearance that the tower appears round with the appearance that the tower appears square, or the argument that providence exists with the argument that providence does not exist. The only logical requirement is that the opposed propositions must be contraries: they cannot both be true, but they may both be false. As long as they are contraries, there will be a problem for the proponent of either side: he cannot accept that both are true, so he must explain or justify his preference.

The most persistent objection to Pyrrhonian Scepticism is that life is not possible without beliefs. This may be applied to the practice of scepticism as well as ordinary, day-to-day activities. So while the sceptic goes shopping, makes breakfast or argues against dogmatists, she is deluded if she thinks she has no beliefs. According to the objection, all intentional, purposeful action presupposes some sort of belief.

There are two different types of response the sceptic might make, or that we might make on her behalf. First, we can deny that action presupposes belief. If so, the sceptic is able to act without holding any beliefs, and the scope of epochē may be unrestricted. Secondly, we can agree that action presupposes belief and claim that this is not a problem because the sceptic has the sort of belief necessary for the relevant action. In this case, the scope of epochē is limited: the sceptic does not suspend judgement about everything. I shall follow Gail Fine (1996) in referring to these options as the “no-belief view” and the “some-belief view” respectively.

Whether Sextus offers a no-belief view or a some-belief view remains controversial. The versatility of the concept of belief complicates the issue further by allowing for variations within each camp. One attractive hypothesis is that versions of both views appear in Sextus' work, reflecting competing strands in the history of Pyrrhonism.

When I was a graduate student a slogan was proposed for a department T-shirt: “Philosophy. We've done less in 2,500 years than most people do in a single day.” The slogan was rejected. But it reflects a serious sceptical sentiment. When we consider the remarkable progress made by science in the past 400 years, philosophy appears in an unfavourable light. At a minimum, we would like to know why there is so little agreement among professional academics regarding the solutions to philosophical problems, their proper articulation, and even the methods suitable to resolving them.

My desire to find a satisfactory explanation of pervasive disagreement has sustained my interest in the ancient Sceptics. From their arguments and strategies, we can construct various explanations for why philosophers have not and perhaps will not reach consensus. There are, of course, plenty of non-sceptical solutions as well, and I am not convinced that I have a satisfactory explanation yet. But reflecting on the arguments of the ancient Sceptics has proved to be immensely valuable in getting to grips with the broad issue of the limits of reason.

The Sceptics also have some fascinating things to say about the proper response to our rational limitations. These responses are meant to show how Scepticism is a viable position and not merely a set of arguments confined to the study or the classroom.

Whatever became of Pyrrhonian Scepticism in the wake of Aenesidemus, it did not make much of an impression on Seneca, who remarks in the early 60s ce that there is no one to hand on the teachings of Pyrrho (Natural Questions 7.32). On the other hand, looking back from the third century ce, Diogenes draws a lineage of Pyrrhonian teachers and students from Pyrrho all the way up to Sextus Empiricus and his student Saturninus (DL 9.115–16). The list probably exhibits Diogenes' passion for genealogy more than historical truth. But it is likely that there was some continuity of sceptical practice from the time of Aenesidemus since, more than 200 years later, Sextus draws on a rich tradition of sceptical argument that clearly was not the work of just one Sceptic.

Indeed, Sextus himself sometimes seems not to be just one Sceptic either. There are a number of strands in his works, not all of which fit comfortably together. The most likely explanation is that there were earlier, incompatible versions of Pyrrhonism recorded in his sources, and that Sextus drew from them with little concern for consistency. Whether he should have been concerned with consistency is a topic to be explored later. Despite these apparently inconsistent sceptical strands in Sextus' texts, we can discern a distinctive voice at work, and we can extract a coherent philosophical position, or rather practice.

The history of ancient Scepticism officially begins with the enigmatic character, Pyrrho of Elis (c.365–275 bce). Pyrrho wrote nothing himself, so what little evidence we have regarding him comes mostly from the writings of his pupil Timon and a book by a nearly contemporary biographer, Antigonus of Carystus. Only fragments of these works survive in the accounts of other, later writers. Diogenes Laertius, for example, draws from both in his biography of Pyrrho (DL 9.61–108). These later accounts look at Pyrrho through the lens of what Pyrrhonism had become hundreds of years later, so they must be used with caution.

Nevertheless, in seeking to understand the history of Greek Scepticism it is necessary to reconstruct Pyrrho's position. This is so for two reasons. First, the founder of Academic Scepticism, Arcesilaus, was influenced by Pyrrho. And secondly, the founder of neo- Pyrrhonian Scepticism, Aenesidemus (Chapter 6) looked to Pyrrho as an inspirational model of the sceptical life. Clearly we need some account of Pyrrho's views and his character to make sense of this historical influence.

Two recurrent themes in the evidence are Pyrrho's remarkably tranquil life, and his refusal to make any firm determinations about things. There is nothing unusual about a Greek philosopher promoting tranquillity as the proper goal. And there is nothing unusual about the generally sceptical thrust of his view.

There are two narratives we can tell regarding the end of the sceptical Academy. According to the first, the fallibilist interpretation of Carneades is historically wrong, and philosophically ill advised. It was a mistake to positively endorse the claims that earlier Academics advanced dialectically. Compared to the originally radical and pure scepticism of Arcesilaus, the later product is an exhausted and degenerate compromise. Arcesilaus was right to insist on the universal suspension of judgement; to compromise on this point is to abandon what is best about Academic Scepticism. This is the line pursued by the renegade Academic Aenesidemus, whom we shall meet in Chapter 6.

According to the second narrative, one version of which we shall develop in this chapter, Arcesilaus' successors were right to modify and mitigate his originally radical scepticism. In particular they were right to limit the scope of what we must suspend judgement about, and to enlarge the scope of sceptically appropriate assent. Viewed in this way, their concessions constitute progress: the culmination of many years of philosophical debate with the Stoics.

At the heart of this dispute is the issue of whether it is ever wise to accept a proposition that is not known to be true. In other words, what will the sage do in the absence of certainty? Arcesilaus and Carneades agree that he will suspend judgement. But by modify-ing what it means to suspend judgement, Carneades also allows for practical deliberation leading to fallible beliefs in accordance with convincing impressions.

Some time around 387 bce, Plato began meeting with his students in a grove, or public park, just outside the city wall of Athens, where he had studied philosophy as a young man before becoming a pupil of Socrates (DL 3.5). This particular grove was named aft er a Greek hero, Hekademos. Thus was born Plato's Academy.

Plato also bought (or was given) a private residence nearby where he and his students ate their meals and had the occasional drinking party (symposium). The main philosophical business was conducted both within the private house and in the open space of the park. With the exception of a few anecdotes recorded long aft er the fact, we have no explicit account of the methods and curricula of the original Academy.

However, given the prominent role played by the theory (or theories) of the Forms in Plato's dialogues, and Aristotle's extensive critique in On Ideas (and Met. 1.9), it is likely that they were much discussed in the Academy as well. Plato's first two successors, Speusippus, followed by Xenocrates, both developed elaborate and distinct metaphysical theories apparently designed, like the Forms, to explain the intelligible order in the world (among other things). Since Speusippus and Xenocrates were both members of the Academy prior to Plato's death, the origin of their metaphysical theories is probably to be found in their training with Plato.

None of Arcesilaus' first three successors (Lacydes, Evandrus and Hegesinus) are credited with any noteworthy achievements: they merely continue Arcesilaus' practice. Carneades, by contrast, surpasses even Arcesilaus in his rhetorical and philosophical brilliance. Accordingly, he invites an even greater measure of criticism:

Carneades' persuasiveness only serves to accomplish greater evil if we assume that he leads people away from true, or at least beneficial, beliefs. The pressing question is: what did he hope to achieve? It is uncharitable to assume that he was malicious. But it is easy to see how he might have appeared so.

Both Socrates (Ap. 23c) and Arcesilaus (DL 4.37) had attracted a large following by publicly deflating the intellectual pretensions of prominent people.