The Astrologer on the Cliff

The house stood on rocks above the sea, on the island of Rhodes, and the path up to it was steep and narrow and had no rail.

The man who lived there was Tiberius. He was not yet emperor. He was in a kind of exile, sulking far from Rome, with a great deal of time on his hands, and he had filled some of it by studying astrology. He wanted to know his future. And he had worked out a way to find out which astrologers were worth listening to.

The Roman historian Tacitus tells how it went. When Tiberius wanted a reading, he went to the top of the house with one servant, a freed slave who could not read and was very strong. The servant led the astrologer up the cliff path. Tiberius heard the reading. Then, if anything in it smelled of fraud,

“he hurled the astrologer, as he returned, into the sea beneath, that no one might live to betray the secret.”

One day the astrologer on the path was a Greek scholar named Thrasyllus. He read the stars for Tiberius and told him grand things: power, empire, the whole future career. Tiberius was impressed. Then he asked one more question. Had Thrasyllus cast his own chart? What did the stars say about him, on this particular year, on this particular day?

Thrasyllus looked at his charts again. Tacitus says

“he first paused, then trembled, and the longer he gazed, the more was he agitated by amazement and terror, till at last he exclaimed that a perilous and well-nigh fatal crisis impended over him.”

Tiberius threw his arms around him. He congratulated him on seeing his danger, told him he was safe, and kept him as a close friend for the rest of his life. Thrasyllus became the most famous astrologer in Rome.

Tacitus, who was not a gullible man, ends the story by admitting he doesn’t know what to make of it:

“When I hear of these and like occurrences, I suspend my judgment on the question whether it is fate and unchangeable necessity or chance which governs the revolutions of human affairs.”

A Test With Real Stakes

Most of us read that story as a story about evidence.

Tiberius, whatever else he was, ran a real test. He didn’t just take the astrologers’ word for it. He made them put something on the line, and he threw out the ones who failed, very literally. We may not like his methods, but we like his instinct. That is how you are supposed to judge a claim to see the future. You check it. Does it work?

And the long story of prophecy in the West, as most of us have absorbed it, is the story of that check being run again and again, and the prophets failing it.

It starts early. Around 550 BC, Croesus, the rich king of Lydia, wanted to know which oracle in the Greek world he could trust. So he ran an experiment. He sent messengers to seven oracles at once. On the hundredth day after they left, each messenger was to ask the same question: what is King Croesus doing right now? Then Croesus did the strangest thing he could think of. He chopped up a tortoise and a lamb and boiled them together in a bronze pot with a bronze lid. According to Herodotus, Delphi got it exactly right. (Croesus thought one other oracle had passed too, though its answer was lost.) It is the first controlled test of prophecy on record, and it has the structure of every one since: fix the question in advance, hide the answer, and see who hits.

Most don’t. Saint Augustine, before he became a Christian, dabbled in astrology, and he gave it up because of a story about two babies. A friend of his named Firminus told him that his own father had been an astrology buff, and that on the night Firminus was born, a slave woman in a neighbor’s house gave birth at the very same moment. The two fathers had messengers waiting and compared notes to the minute. Same moment, same sky, same chart. Firminus grew up rich and honored. The slave’s son grew up a slave. Augustine drew the obvious conclusion:

“It was therefore perfectly clear to me that when predictions based on observations of the stars turn out to be true, it is a matter of luck, not of skill.”

Then he went further, to twins. Twins are born minutes apart under the same stars. If astrology were true, twins should live alike. They don’t. Jacob and Esau were twins, Augustine points out, and their lives could hardly have been more different.

The French essayist Michel de Montaigne, writing in the 1500s, explained why anyone believed the thing anyway. Predictors get to fire a lot of arrows. “For who is there,” he asks, borrowing a line from Cicero, “who, shooting all day, will not sometime hit the mark?” And the misses vanish:

“no one keeps a record of their mistakes, inasmuch as these are ordinary and numberless; and their correct divinations are made much of because they are rare, incredible, and prodigious.”

Montaigne tells a story about Diagoras, a Greek philosopher known as the Atheist. Someone took him into a temple and showed him the walls, covered in paintings put up by sailors who had prayed during storms and been saved. How could he doubt the gods cared, with all this proof? Diagoras answered: “Those who were drowned, in much greater number, are not portrayed here.” It is one of the cleanest statements ever made of what statisticians now call survivorship bias, and it is about two thousand four hundred years old.

Thomas Hobbes, in 1651, added the last piece. Oracles hedged. The famous answers at Delphi were “made ambiguous by design, to own the event both ways.” If you can’t lose, you can’t be tested.

And modern science simply finished the job. In 1985 a physicist named Shawn Carlson published a double-blind test of astrology in the journal Nature. Astrologers who had agreed to the rules in advance were given birth charts and asked to match each one to the right personality profile. They did no better than chance.

So here is the view most of us hold, and it deserves to be put as strongly as it can be. Prophecy was rejected because it doesn’t work. The people who rejected it were the clear-eyed ones, the testers and counters: Augustine with his twins, Montaigne with his arrows, Diagoras with his drowned sailors. The believers were the gullible ones. The whole question was always an empirical one, and the answer came back no.

That view has a quiet corollary, and most of us would sign it without thinking. If an astrologer could pass Tiberius’s test every time, if she could stand on that cliff and never once get thrown into the sea, then we would have to take her seriously. We would have no grounds left to object. Accuracy settles it.

What the Critics Actually Said

Here is the problem. Many of the oldest and sharpest critics of prophecy did not sign that corollary. Several of them said the reverse, in so many words.

Start with the law of Moses, which had rules for testing prophets. Deuteronomy 13 lays out a case. A prophet or a dreamer of dreams appears. He gives a sign, a wonder, a prediction. And the prediction comes true. But then he says, let us follow other gods. What should you do? Hobbes quotes the answer:

“thou shalt not hearken to him.”

The sign came true, and it doesn’t matter. Accuracy is not the test.

Augustine built on this passage. Yes, he had his twins. But when he explains why Christians reject the diviners, he says something that should stop the modern reader cold. Scripture, he writes,

“does not warn the soul against following such practices on the ground that those who profess them speak lies, but it says, ‘Even if what they tell you should come to pass, hearken not unto them.’”

Not “because they’re wrong.” Even if they’re right. Augustine’s own reasons were religious: he believed the hits came from “lying spirits” who wanted a hold on people. But he also named what the practice did to the person who took it up. It was, he said, “full of hurtful curiosity, torturing anxiety, and deadly slavery.”

Thomas Aquinas, the most careful mind of the Middle Ages, goes further still. He flatly grants that astrologers are often right. It is one of his objections, and he does not deny it: “astrologers often foretell the truth concerning the outcome of wars, and other human actions.” He concedes the hits and condemns the practice anyway.

Even Herodotus, the man who told the story of Croesus, held back. He had seen prophecies that “speak with clearness” come true, and he said he would neither speak against prophecies himself “nor do I approve of others impugning them.”

So what was the objection, if it wasn’t mainly the evidence?

The Greek Stoic Epictetus put it in one line. He had been a slave in Rome and later taught philosophy in Greece, and he has a short chapter in his Discourses called “How we ought to use divination.” In it he asks why people go to diviners at all. His answer:

“What then leads us to frequent use of divination? Cowardice, the dread of what will happen.”

The case against prophecy was never only a case about the stars. It was a case about the person asking.

It is worth being exact about what that claims and what it doesn’t. The evidence case is real. The critics used it, and they were right to. The greatest ancient attack on divination, Cicero’s On Divination, is mostly a long, sharp argument that it fails, and Carlson’s test is good science. The claim is not that accuracy doesn’t matter. The claim is that accuracy was never the whole trial. The tradition ran a second test alongside the first, one no experiment can reach. It took different forms. For Moses it was loyalty to God. For Augustine it was what the practice bred in the soul. For the prophet Jeremiah it was whose wishes a prophecy served. What these share is that they judge the asking, not the hit rate. And on the hard cases, the prophet who hits, the sign that comes true, the second test is the one that decides. Even Cicero, in the middle of all his counting, stopped to say that knowing the future would not be worth having: “it is wretched to be tormented to no purpose.” The modern retelling kept the first test and quietly dropped the second.

Epictetus says the plainest thing any of them said about it: the second test is about courage. And courage, as the old writers understood it, never works alone. It leans on the virtues next to it.

What the Customer Is Buying

Start with courage itself.

Epictetus’s chapter on divination is short, and it is almost entirely about fear. What can a diviner see, he asks? Death, danger, disease, “or generally things of that kind.” Fine. Now suppose the question in front of you is whether to stand by a friend who is in trouble.

“If then I must expose myself to danger for a friend, and if it is my duty even to die for him, what need have I then for divination? Have I not within me a diviner who has told me the nature of good and of evil?”

Put plainly: the diviner can tell you whether you’ll get hurt. He can’t tell you whether you should go. And if you already know you should go, then the only thing his answer could change is whether you do. Which means that asking him is a way of looking for permission not to.

Epictetus gives an example from his own time. The emperor Domitian had banished a woman named Gratilla. A friend of hers was about to send her a month’s supply of food. Someone warned the friend that Domitian would just seize the shipment. She answered: “I would rather that Domitian should seize all than that I should not send it.” She didn’t need a diviner. She already knew what was worth doing, and the risk was the cost of doing it.

Most of us are not Gratilla’s friend. Epictetus describes us, and it is not flattering. “But now we trembling take the augur by the hand, and, while we invoke God, we entreat the augur, and say, ‘Master have mercy on me; suffer me to come safe out of this difficulty.’” We are not asking what is right. We are asking to be told we will be safe.

This is the first thing the old critics saw, and it holds whether or not the diviner is any good. The customer at the oracle doesn’t really want knowledge. He wants the risk taken out of the act. He wants to do the brave thing without having to be brave, because someone has promised him it will work out. Divination is a kind of insurance against needing courage.

History shows what happens when the insurance is sold. Montaigne tells of the Marquis of Saluzzo, a French commander in Italy in the 1530s, a man who owed everything to King Francis I. Prophecies were going around Italy that favored the Emperor Charles V, France’s enemy, and so many people believed them that in Rome, Montaigne says, people bet serious money on them. The marquis, “for no reason and even against his own affections,” let himself be “so very terrified” by these predictions that he changed sides. He betrayed his king because a forecast frightened him. He didn’t even betray him well. Montaigne notes that he went about it “like a man torn by conflicting passions.”

The oldest version of this story is the worst one. When the Greek fleet was stuck at Aulis, waiting for a wind to Troy, the prophet Calchas declared that the goddess Artemis wanted a sacrifice: Agamemnon’s daughter, Iphigenia. In Aeschylus’s play, Agamemnon sees the trap clearly. “My fate is angry if I disobey these, but angry if I slaughter this child.” He does it anyway. In Euripides’s version of the same story, Iphigenia’s mother, Clytemnestra, names what is really driving her husband. It isn’t piety. It is “Terror of the army. This base fear is in him.” And Achilles gives the answer the whole tradition would repeat: “Reason can wrestle and overthrow terror.”

The Roman poet Lucretius, looking back on Aulis centuries later, drew his own conclusion: “A mighty counselor, Religion stood, with all that power for wickedness.” Lucretius was attacking religion as such, which the Christian critics would not have accepted. But he saw the same machinery they did. A prophecy gives fear a respectable costume.

So far this sounds like a simple lesson: prophecy makes people cowards. But it’s stranger than that, because it also makes people reckless.

Tacitus tells of Otho, a Roman noble who in AD 69 wanted to seize the empire. He had motives enough: debts, envy, rage. The astrologers gave him nerve. They “urged him to action, predicting from their observation of the heavens revolutions, and a year of glory for Otho.” One of them, Ptolemaeus, had once predicted Otho would outlive Nero and been proved right, and now pressed him further. Otho “received the prediction as the words of wisdom and the intimation of destiny.” Tacitus adds that the astrologer “now even prompted to crime, to which from such wishes it is easy to pass.” Otho did seize the throne. He held it for three months and then killed himself after losing a battle.

And Tacitus’s verdict on the whole profession is one of the best sentences ever written about fortune-tellers:

“This is a class of men, whom the powerful cannot trust, and who deceive the aspiring, a class which will always be proscribed in this country, and yet always retained.”

Put the marquis and Otho side by side. One was frightened into running. The other was flattered into charging. Aristotle’s old picture of courage was a middle point between two failures: cowardice, which fears too much, and rashness, which fears too little. Divination pushes people off that middle point in both directions. It isn’t just a failure of evidence. It is something that knocks courage off its balance, whichever way the prediction happens to point.

The Man Who Ignored the Birds

At this point the obvious conclusion is that the brave man is simply the one who ignores signs. Scorn the omens, trust your own right arm, and let the soothsayers talk.

The best-known version of that attitude is in the Iliad. The Trojans are about to storm the Greek ships. An eagle flies over them carrying a huge snake, which twists around and bites it, and the eagle drops the snake and flies off screaming. A Trojan named Poulydamas reads the sign: we will break through, but we will not come back in good order. Let’s not attack the ships.

Hector, the greatest of the Trojans, refuses. His answer has been quoted as a war cry for three thousand years:

“you tell me to put my trust in birds, who spread wide their wings. I care nothing for these, I think nothing of them… One bird sign is best: to fight in defence of our country.”

It is a magnificent line. It is also wrong. Poulydamas was right. The Trojans break through to the ships, just as the omen showed, and then are driven back from them with terrible losses. Later Poulydamas warns Hector a second time to pull back, and Hector refuses again. That refusal is one of the steps that leads to his death and to the fall of Troy.

Homer does this more than once. In the Odyssey, when an old man reads a sign of two eagles and warns that Odysseus is coming home to kill the suitors, the suitor Eurymachos laughs at him. “Many are the birds who under the sun’s rays wander the sky; not all of them mean anything; Odysseus is dead, far away.” He is right as a general rule. Most birds mean nothing. He is one of the first men to die when Odysseus comes home.

And Shakespeare gives the finest speech against astrology in English not to a hero but to the villain. In King Lear, Edmund mocks his father for blaming eclipses:

“This is the excellent foppery of the world, that, when we are sick in fortune—often the surfeit of our own behaviour—we make guilty of our disasters the sun, the moon, and the stars.”

Everything he says is true. He says it in the middle of a scheme to destroy his brother, and a few lines later he gloats about how easy it is to fool “a credulous father” and “a brother noble.” Scorning the stars made Edmund clear-eyed, not good.

So scorn isn’t the answer either. Fearlessness is not courage, and the old writers insisted on this. In Plato’s Laches, a dialogue about courage, the general Nicias puts it plainly: “There is a difference to my way of thinking, between fearlessness and courage.” Animals and small children can be fearless, he says, simply because they don’t see the danger. Courage needs forethought. The brave man is not the one who fears nothing. He is the one who fears the right things, at the right time, in the right amount.

That means courage needs the virtue right next to it, which the tradition called prudence: practical good sense about what is actually coming. And prudence needs a real way to look ahead. The critics of divination never denied that. Francis Bacon, writing in 1605, split foresight into two kinds. One kind reasons from causes: “the astronomer hath his predictions, as of conjunctions, aspects, eclipses,” and “the physician hath his predictions, of death, of recovery.” The other kind reasons only from “a coincidence of the effect,” this bird, then that disaster, and that kind, he said, “for the most part is superstitious.”

Aristotle, writing about whether dreams can predict the future, made the same cut two thousand years earlier. Some dreams, he said, might really be signs: a doctor should pay attention to them, because a small illness starting in the body can show up in sleep before you’d notice it awake. Most so-called prophetic dreams, though, are coincidences, like mentioning a friend just before he walks in the door.

The best illustration of the difference is two eclipses.

Early in the war with Sparta, the Athenian leader Pericles was about to sail with a fleet of 150 ships when the sun went dark. The men panicked. His helmsman froze. Plutarch says Pericles took his cloak and held it up in front of the man’s face so he couldn’t see, and asked if he thought there was anything terrible in this. No, said the helmsman.

“‘Why,’ said he, ‘and what does that differ from this, only that what has caused that darkness there, is something greater than a cloak?’”

Less than twenty years later, in 413 BC, another Athenian general was camped outside Syracuse in Sicily. It was Nicias, the same man who in Plato’s dialogue had drawn the line between fearlessness and courage. His army was losing, and he had finally agreed to sail away while it still could. Then the moon went dark. The historian Thucydides says Nicias was “somewhat over-addicted to divination,” and when his soothsayers told him to wait twenty-seven days, he waited. By the time he moved, it was too late. The army was trapped and destroyed. Almost none of it came home.

Same kind of sign, the sky going dark. Pericles treated it as a cause he understood, and sailed. Nicias treated it as a message, and his men died waiting for its meaning.

So where exactly is the line? It isn’t accuracy. A weather forecast can be wrong and an oracle can be right. The difference is in two other places. First, a real forecast runs on reasons you could check. You can ask the astronomer how he knows, and he can show you. You can’t ask the bird. Second, and this is where courage comes back in, a real forecast leaves the choice with you. The physician says the fever will likely break by Thursday. He does not tell you whether to be kind to your family while you wait. The weatherman says there’s a storm coming. He doesn’t tell you whether to go help your neighbor board up his windows. A good forecast hands you better information and leaves you with the same decision and the same duty. The oracle, like Nicias’s soothsayers, takes the decision out of your hands. That is its appeal, and that is the charge against it.

The Prophet Nobody Wanted

Everything so far could be said by a skeptic. But the fiercest critics of false prophecy in the ancient world were not skeptics at all. They were the prophets of Israel, who believed completely that God did speak through some people. That put them in a hard spot. They couldn’t reject prophecy. They had to tell the true from the false. And the empirical test was often no use to them, because the prophecy was about the future, and the decision had to be made now.

So what test did they use? The Hebrew Bible’s most vivid answer is a scene from about 850 BC.

Ahab, king of Israel, wants to go to war to take back a city called Ramoth-gilead. His ally, the king of Judah, asks him to check with the prophets first. So Ahab calls in four hundred of them, and all four hundred say the same thing: “Go up; for the Lord will deliver it into the hand of the king.” One of them, a man named Zedekiah, even makes himself a pair of iron horns and acts out the victory.

The king of Judah is uneasy. Is there no other prophet? Ahab admits there is one. “There is yet one man… Micaiah the son of Imlah: but I hate him; for he doth not prophesy good concerning me, but evil.”

The messenger sent to fetch Micaiah gives him some friendly advice on the way:

“Behold now, the words of the prophets declare good unto the king with one mouth: let thy word, I pray thee, be like the word of one of them, and speak thou good.”

Micaiah answers: “As Jehovah liveth, what Jehovah saith unto me, that will I speak.” He tells the king he saw all Israel scattered on the mountains “as sheep that have no shepherd.” Zedekiah slaps him in the face. Ahab has him thrown in prison on bread and water until the king comes home safe. Micaiah’s parting words are: “If thou return at all in peace, Jehovah hath not spoken by me.”

Ahab goes to battle in disguise, hoping to dodge the prophecy. A soldier on the other side “drew a bow at a venture,” fired an arrow at no one in particular, and it struck the king of Israel between the plates of his armor. He bled to death in his chariot by sunset.

The shape of this scene is the whole test. Four hundred prophets said what the king wanted. One said what he didn’t, and paid for it. The false prophets weren’t failing a test of evidence. They were passing a test of flattery. They were telling a frightened, ambitious man what he wanted to hear, and they were telling it “with one mouth.”

The book of Isaiah names the demand behind it. It describes a people who “say to the seers, See not; and to the prophets, Prophesy not unto us right things, speak unto us smooth things, prophesy deceits.” Jeremiah is even blunter: “the prophets prophesy falsely… and my people love to have it so.”

Twenty-five centuries later Sigmund Freud noticed the same thing from a completely different direction. He had patients who went to fortune-tellers, and he studied what the fortune-tellers told them. His conclusion was that the fortune-teller “has only given expression to the thoughts, and particularly to the secret wishes, of his clients.” One woman had been promised children by a certain age. The prediction failed completely. Years later she still spoke of it fondly. Of course she did, Freud says. The prophet had told her what she most wanted: “how could she feel otherwise than friendly towards the prophet?”

The false prophet, then, is a mirror. He hands back the customer’s wish in the voice of heaven. Hobbes saw the political danger in that. Whoever claims to know the way to happiness, he wrote, “pretends to govern,” and so ought to be “examined and tried by every man before he yield them obedience.” A mirror that speaks with God’s voice is a very good tool for running a country.

Now the test starts to show its real shape. In the prophet Jeremiah’s day, Jerusalem was under threat from Babylon, and a prophet named Hananiah stood up in the temple and announced that within two years God would break Babylon’s power and bring everything back. Jeremiah was wearing a wooden yoke around his neck as a sign that the people would have to submit. Hananiah pulled it off and broke it in front of the crowd.

Jeremiah’s answer is one of the strangest rules in the Bible. The prophets before us, he says, mostly foretold war and disaster. But:

“The prophet that prophesieth of peace, when the word of the prophet shall come to pass, then shall the prophet be known, that Jehovah hath truly sent him.”

In other words, good news carries the burden of proof. The prophecy you want to hear is the one you should trust least until events confirm it, because it’s the one your wishes would have invented anyway. The test is a test of the hearer as much as the speaker.

And then the story takes a turn that goes straight to the heart of the courage question. Jeremiah’s message was not “fight bravely.” It was “surrender.” Babylon will win. “Bring your necks under the yoke of the king of Babylon, and serve him and his people, and live.” He even told people that whoever went out and gave up to the Babylonians would survive.

To the city’s leaders, that sounded like cowardice, or treason. When he tried to leave the city on business, a guard grabbed him: “Thou art falling away to the Chaldeans.” The princes demanded his death because “he weakeneth the hands of the men of war.” They lowered him into a muddy cistern to die, and he sank in the mud. A foreign servant of the king, an Ethiopian named Ebed-melech, pulled him out with ropes and rags.

Meanwhile, the brave-sounding prophets said resist. Hananiah broke the yoke. When Jeremiah’s scroll of warnings was read to King Jehoiakim, the king cut it up column by column with a penknife and fed it to the fire in his brazier. And the text makes a point of what did not happen: “they were not afraid, nor rent their garments, neither the king, nor any of his servants that heard all these words.”

Not afraid. It’s said as an accusation.

The last king, Zedekiah, secretly called Jeremiah in and asked him what to do. Jeremiah told him again: surrender and live. And Zedekiah’s reply is the most human line in the whole story: “I am afraid of the Jews that are fallen away to the Chaldeans, lest they deliver me into their hand, and they mock me.” He wasn’t afraid of Babylon. He was afraid of being laughed at. He held out. The Babylonians took Jerusalem in 587 or 586 BC, burned the temple, killed Zedekiah’s sons in front of him, and blinded him.

This is the tradition’s own telling, of course, written by the side that turned out to be right. But what it says about courage is hard to miss. The true prophecy looked like cowardice. The false prophecies looked like courage. The king who was “not afraid” was foolhardy, and the king who was afraid was afraid of the wrong thing. The one person in the story who needed real courage, and had it, was the prophet in the mud, saying what nobody wanted to hear.

So the test for a true prophet turns out to be a courage test with two ends. The prophet has to be brave enough to say the unwelcome thing. The hearer has to be brave enough to take it. And the law of Moses, when it gets to the false prophet whose words don’t come true, ends with a command that is easy to skip past: “thou shalt not be afraid of him.” The false prophet’s power was always fear. Take away the fear, and he has nothing.

Why the Astrologers Hit

That leaves the strangest part of the old argument, and it goes back to Thomas Aquinas’s odd concession. He admitted that astrologers are often right. How could a Christian theologian, who believed the stars could not control a human will, admit that?

He gave two answers. One, borrowed from Augustine, was that demons whisper the truth to astrologers to lure people in. The other is the one that has lasted, and it turns the whole question inside out:

“The majority of men follow their passions, which are movements of the sensitive appetite, in which movements heavenly bodies can co-operate; but few are wise enough to resist these passions. Consequently astrologers are able to foretell the truth in the majority of cases, especially in a general way. But not in particular cases, for nothing prevents man resisting his passions by his free-will. And so the astrologers themselves say that the wise man is stronger than the stars, in so far as, that is, he conquers his passions.”

In another place he gives it more briefly, in a saying he credits to the ancient astronomer Ptolemy: “the wise man governs the stars.”

Aquinas believed, as most people did in his day, that the heavens influence the body: they stir moods, appetites, tempers. His astronomy is wrong. The shape of his argument survives it anyway. His claim is that the forces that push people around, whatever they are, work on appetite and fear. A person who is run by appetite and fear can be predicted, because you know which way he will be pushed. A person who has mastered them can’t be, not in the same way, because he decides what to do with the push.

So predictions work best on crowds and worst on individuals. They work “in a general way” and fail “in particular cases.” That is a surprisingly good description of what modern prediction actually looks like. Insurance companies can tell you, very accurately, how many drivers in a group will crash next year. They cannot tell you which ones. In 2020 a team of social scientists led by Matthew Salganik at Princeton published the results of a large contest in the journal PNAS. They gave 160 research teams a huge dataset on thousands of American families, collected over fifteen years, and asked them to predict how the children’s lives would turn out: grades, hardship, eviction, and so on. The best predictions, from the best machine-learning methods, were not much better than a simple model using a handful of basic facts, and none of them predicted individual children well. This does not prove anything about free will. Plenty of other things could explain it. But it fits the pattern Aquinas described: general trends are easy, particular people are hard.

Now take Aquinas’s point one step further than he stated it. If he’s right, then when a forecast works on you, it is not only a fact about the forecaster. It is a fact about you. It means you went the way the push went. The astrologer’s hit is a report on how much of your choice was made by your fear and your appetite instead of by you.

That is why Augustine could say something that sounds, at first, like pure rhetoric. When a free man goes to an astrologer, he wrote, “he gives money that he may come away the slave either of Mars or of Venus.” It sounds like an insult. It’s closer to a description. The customer pays to be told which of his passions will run him, and then, having been told, he lets them.

This is where the virtues next to courage all come together. Aquinas taught that courage and temperance feed each other: a man who can hold his appetites in check finds it easier to hold his fear in check, and the other way around. Temperance handles the pull of what we want. Courage handles the push of what we fear. And those are exactly the two forces a forecast rides on. The virtues, it turns out, are the things that make a person a bad subject for prophecy. Not because they give him better information about the future, but because they leave less of him to be predicted.

So the modern corollary, the one that seemed so obvious at the start, fails. Suppose an astrologer could pass Tiberius’s test every time. The old critics would not be forced to take her seriously. They’d say she had found a very good way of measuring something. And what she had measured would be her customers.

There is a way to hear a prophecy that the old writers admired, and Plutarch records it. Brutus, one of the killers of Julius Caesar, was sitting alone in his tent late one night before the war that would decide his fate. He looked up and saw a strange and frightful figure standing by him, saying nothing. Brutus asked it who it was. It answered: “I am your evil genius, Brutus; you shall see me at Philippi.” And Brutus, “not at all disturbed,” replied:

“Then I shall see you.”

He did not ask how the battle would go. He did not change his plans. He did not run. He heard a prophecy of his own death and treated it as one more fact about the ground he’d have to fight on. He lost at Philippi and died there. But the prophecy got nothing from him. It could tell him what was coming. It could not decide what he would be when it came.

Back to the Cliff

What really happened on the path at Rhodes?

One easy reading is that Thrasyllus read nothing in the stars at all. He read the man in front of him, and the cliff, and the silent strong servant, and the question that had been saved for last. Anyone that clever, asked about his own fate by Tiberius, at the top of that path, on that day, would have seen that something “perilous and well-nigh fatal” was very close. Tacitus doesn’t say this. He says he doesn’t know. But if it’s true, then Thrasyllus passed the test by doing exactly what a real forecaster does: he read causes he could see.

The more interesting question is what the test measured about Tiberius.

He was, by every account, a hard, clever, suspicious man. The biographer Suetonius says he cared little for the gods or religion, “being addicted to astrology.” He also says that Tiberius was terrified of thunder, and whenever the sky looked threatening he wore a wreath of laurel on his head, because laurel was believed to be safe from lightning. When he became emperor, astrologers were expelled from Italy under his rule, in AD 16, and one of them was thrown from the Tarpeian Rock in Rome. Thrasyllus stayed by his side until his own death, a year before the emperor’s. Tiberius spent his last years on the island of Capri, behind cliffs, afraid of plots, reading the stars.

He ran one of the harshest tests ever run on prophets, and he ran it out of fear. The astrologers on that path were being tested for accuracy. The man at the top of it was being tested too, by the older standard, the one that asks not whether the forecast is true but why you needed it. Tacitus, in the same passage where he suspends his judgment about fate, remembers what the old philosophers said about good and bad fortune: that “many who seem to be struggling with adversity are happy; many, amid great affluence, are utterly miserable, if only the first bear their hard lot with patience, and the latter make a foolish use of their prosperity.”

Thrasyllus got off the cliff alive. The question the tradition kept asking was who, on that path, was really being tried.