Arcesilaus of Pitane

Arcesilaus of Pitane

Arcesilaus did something no one saw coming: he took over Plato's Academy, the most prestigious school in Athens, and turned it into a weapon against certainty. The reigning philosophy of his day was Stoicism, brash and confident, armed with a theory of knowledge that promised the wise man could be sure of the truth. Arcesilaus set out to prove he could not, and he did it with a move of beautiful cunning. Rather than build a rival system, he attacked the Stoics with their own principles, leading them step by step, by their own logic, to conclusions they could not stomach. He wrote nothing, taught entirely by argument, and asked his students not to memorize his opinions but to argue both sides of every question until the certainties came apart in their hands. He is one of the most important philosophers almost no one has heard of: the man who turned the school of Plato into the headquarters of doubt.

The indistinguishable twin. The Stoics' whole claim to knowledge rested on a single idea: that some perceptions are self-certifying. A truly clear and distinct impression, they said, arrives stamped with its own guarantee, like a seal pressed into wax, so vivid and so exact that it could only have come from the real thing and could not possibly be false. Assent to those alone, and you have certainty. Arcesilaus destroyed the idea with one relentless strategy: for any impression the Stoic swore was self-certifying, he produced an indistinguishable twin that was false. Identical twin brothers, whom even their own mother mixes up. A dream as vivid as waking life that refers to nothing at all. The vision of a madman, every bit as forceful as the sight of a real friend. A wax apple the eye cannot tell from fruit. If for every "guaranteed" impression there is a perfect counterfeit you could not tell apart, then no impression carries a guarantee, and the Stoic's certainty collapses. Twenty-three centuries later this is still the sharpest blade in the theory of knowledge: it is the dream argument Descartes would run, the brain in a vat, the simulation, the question that refuses to go away, how do you know your most vivid experience is not a flawless fake?

Fighting with the enemy's sword. The genius of Arcesilaus was not only the argument but the way he wielded it. He never stood up and declared a doctrine of his own, that nothing can be known; that would have been just one more dogma to defend. Instead he fought entirely on the enemy's ground. He granted the Stoics their premises and then showed that those very premises, followed honestly to the end, led straight to the suspension of judgment the Stoics were so desperate to avoid. It was the method of Socrates, the cross-examiner who refutes you out of your own mouth without ever asserting a counter-thesis, and Arcesilaus claimed that this, and not the grand metaphysics of Plato's late dialogues, was the real legacy of the Academy. He read Plato himself as a kind of skeptic. Whether or not the reading was fair, it let him do something rare and devastating: tear down a confident philosophy without ever exposing a position of his own for it to tear down in return.

How to act when you cannot be sure. The Stoics had an obvious counterstrike. If you can be certain of nothing, they said, then you cannot act at all, because every action requires assent to some impression, the belief that this is bread, that the bridge will hold. A consistent skeptic, they jeered, would simply sit still and starve. It is the strongest single objection to skepticism, and the Academy spent generations answering it, in what may be its most lasting gift. You do not, Arcesilaus and above all his great successor Carneades replied, need certainty in order to act; you need only the persuasive. An impression can be more or less convincing, can survive more or fewer checks, and a reasonable person acts on the impression that has been tested and held up, while never pretending it is guaranteed. Carneades even graded it: the merely plausible, the plausible-and-unshaken, and the plausible-and-thoroughly-examined, and you stake your action on the third kind while keeping your certainty in your pocket. This is, two thousand years early, the logic of acting under uncertainty, of probability and reasonable confidence, the way a careful scientist or a sensible person actually thinks. You do not have to be sure. You have to be scrupulous.

Two ways to doubt. The Academy was not the only school of skeptics. Across town ran the rival tradition of Pyrrho, and the two quarreled for four centuries over the right way to doubt. The difference is subtle and genuine. The Pyrrhonist suspends judgment as a settled disposition, meeting every claim with the same calm refusal from the very start, and finding tranquility as the reward. The Academic suspends as the conclusion of an argument, investigating each claim, running the demolition, and withholding assent because the investigation came out balanced. The Pyrrhonists accused the Academy of cheating: by announcing that nothing can be grasped, Arcesilaus had let a doctrine slip back in, a confident claim about the limits of knowledge, which is not skepticism but dogma in a mask. The Academics answered that they asserted nothing of the sort, that "nothing can be grasped" was only where the argument had landed so far, ready to be revised the instant a Stoic produced a counter-example that actually worked. The dispute has never been fully settled, and it is the same fault line that still runs beneath every modern theory of knowledge. The room next door, under Pyrrho, argues the other side.

The long argument. The room reads Arcesilaus the only way it can, since he wrote nothing: through his ancient biography and, above all, through Cicero, the Roman who studied in the Academy's skeptical tradition and carried its method into Latin; the citations point there. The line of descent is long and unexpected. Cicero translated the persuasive into Latin as the probabile and made the both-sides method his own. Augustine, freshly converted, wrote a whole book trying to refute the Academics. Descartes, facing the Academy's challenge that no impression comes guaranteed, drove the doubt all the way to the bottom precisely in order to find the one thing, the thinking self, that survives even the indistinguishable twin. Hume aligned his own "mitigated skepticism" not with the radical Pyrrhonists but explicitly with the moderate Academics, who doubt without despairing and act on the probable. And the entire modern apparatus of reasoning under uncertainty, of confidence short of proof, is the great-grandchild of a Greek who taught his students to argue both sides until the false certainties fell away.

Arcesilaus has no monument and almost no fame, because he built nothing you could put a name to, only a way of taking confident claims apart. But that turns out to be one of the most valuable things a mind can learn to do. We live, even more than the Greeks did, surrounded by people utterly sure of things, and the rarest and most useful skill is not to have a better certainty but to know how to test the ones on offer: to ask of any confident impression, however vivid, whether it might be a perfect counterfeit, and then to act anyway, on the most persuasive case, without ever mistaking persuasion for proof. Arcesilaus taught that you can live, and act, and think well, in the absence of certainty, simply by staying scrupulous about the difference between what is sure and what is merely convincing. It may be the most grown-up lesson philosophy has to teach.