Recently I’ve been asked to write a review of a fascinating book by Peter T. Struck: Divination and Human Nature, A Cognitive History of Intuition in Classical Antiquity. While the full book is worth reading for anyone interested in ancient Greco-Roman culture, as well as in the early development of science, there is a whole chapter on Posidonius, one of the major figures of the so-called Middle Stoa, the period during which Stoicism transitioned from its original home in Athens to Rome. Posidonius is a fascinating figure in his own right, and he is often not written about because all we have by him are fragments and indirect sources (as opposed to, say the wealth of stuff by Seneca). Divination is also rarely commented on in Stoic circles, because it is automatically relegated to the category of superstition, something the Stoics were wrong about and that we just don’t need to be concerned with nowadays. But the story is not that simple.

Struck begins the chapter by reminding us of some core concepts of Stoic metaphysics, beginning with the idea that the soul is a unitary thing (not made of parts, like Plato thought), and that the cosmos is a living organism animated by the Logos, its rational principle. Human beings, in turn, are embedded in the fabric of the world, and share with the Logos of the whole.

He then moves on to provide some preliminary information about Posidonius himself. He was a student of Panetius, who was known for a number of unorthodox ideas within the Stoa. Cicero tells us that Panaetius “did not deny the power of divination but had his doubts.” Posidonius, by contrast, wrote a whopping five books on the topic.

A lot of what we know about Posidonius’ take on divination comes from Cicero’s De Divinatione, which is structured as a dialogue with his brother Quintus, who acts as a surrogate for the Stoic position, and whose thoughts are based on Posidonius.

Next Struck summarizes the basic precepts of Stoic physics, focusing in particular on the fact that the Stoics were thoroughgoing materialists who “define god as an extraordinarily fine mist, called pneuma , literally ‘breath,’ that permeates and suffuses inert matter, or hylê, bringing into being everything that exists in the cosmos. The pneuma, which is also co-extensive with logos or reason, steers the universe on its course.”

The pneuma structures everything in the universe, and it comes in four distinct forms, or gradations: “‘tenor’ (hexis), the configuration of pneuma that holds together inanimate objects like stones and logs. … Physis, the pneuma in a slightly higher state of tension, which holds together plants and the human fetus between conception and birth. … Soul (psychê), a higher form of pneuma, has the powers of sensation and movement, and brings perception and locomotion to the hylê of which an animal is made up. … [and] the logical soul (logikê psychê), which is the most energetic form, holds together the inert matter that underlies the human body and uniquely manifests the characteristic of logos, or speech and reason.”

The idea is that this continuum of pneuma pervades the entire cosmos, and it is coextensive with nature and god.

Now, given the above summary, the Stoics — unlike Plato — saw everything as interconnected with everything else, and as Struck points out, “these interconnections are the central innovation of the Stoic school that is relevant to the topic of divination, and they consistently travel under the same technical term. The Stoics label this property of the cosmos ‘sympathy.'”

This notion of sympathy was well known in antiquity, and found application in medicine, for instance, where people had figured out that symptoms in certain parts of the body may indicate a disease in other parts — because the parts are connected within the same body, just like all human beings are connected to the rest of the cosmos. Moreover, “the medical treatises speak of ‘sympathy’ between the body and soul, as when diseases interfere with the mind.”

Struck traces the idea to the early Stoa: “In the surviving Stoic evidence, the idea first appears in an argument assigned to Cleanthes. He speaks of the relation between body and soul, and claims that since it is impossible for an incorporeal to sympathize with [συμπάσχει] a corporeal, the soul must be a body. The soul clearly does show sympathy with the body, he says, as in the case when it is distressed when the body is diseased or cut; and the body with the soul, as in blushing from embarrassment or turning pale from fear.” This, of course, has nothing to do with the “soul” as understood either by Plato or by modern Christianity, but it refers to the materiality of what we would today call the mind.

But it is Chrysippus who greatly expands the concept, essentially applying “sympathy” to the interconnectedness of all the cosmos, by way of the running pneuma, so that Stoic sympathy becomes grounded in the notion of the universe as a single living organism.

And now we arrive back to Posidonius. While earlier Stoics like Cleanthes and Chrysippus got criticized by the Academic Skeptic Carneades for presenting an image of the divine principle as too closely connected to individual divinatory signs, Posidonius is more cautious and suggests that the machinery of the world simply works by cause and effect, which however also means that events have predictable precursors.

Moreover, as Struck reminds us: “the Stoics’ materialist view of the soul allows, and in fact requires, that the soul is fully embedded within the interlocking set of physical causes that produce coming events.” Accordingly, for Posidonius the divine is not a busybody, it simply steers the world in a way that is “good” for the whole organism, but without concern for its individual parts, including, of course, individual human beings.

Interestingly, Stoic psychology says that we are capable of receiving information from the rest of the world-organism and decide on how to best act: “presentations [i.e., sensorial inputs] arrive from the parts of soul extended toward senses, the commanding part at that point assents to the presentation or not, making a certification of its authenticity. The reasoning function then concludes what kind of action to take and sends out a triggering impulse to make it happen.” This is relevant to the issue of divination because in this way we are connected to the web of causality that permeates the cosmos, which makes it possible for people to “sense” future happenings in an entirely non mystical fashion.

All of the above is coherent with the Stoic acceptance of determinism: “Posidonius links his explanation of divination to a widely-attested Stoic idea of the deterministic, and so predictable, character of the cosmos. What happens in the universe does not just come out of nowhere, it arises from prior causes.”

Here is how Cicero summarizes Posidonius’ view:

“If there were a man whose soul could discern the links that join each cause with every other cause, then surely he would never be mistaken in any prediction he might make. … But since such knowledge is possible only to a god, it is left to humans to presage the future by means of certain signs that indicate what will follow them.”

That is, for the Stoics divination was a science of forecasting, made possible by the very fact that we live in a deterministic universe of causes and effects. They were, obviously, mistaken about which signs to pay attention to and how to interpret them, but the basic idea is the very same one that underlies modern predictions of natural events, especially when we don’t fully understand the underlying mechanisms and rely on heuristics and statistical correlations.

Indeed, again from Cicero’s rendition, it is clear that the Stoics understood full well the limitations of relying on correlation, and the fact that this was something sensible to do in the absence of knowledge of causes:

“I see the efficacy of the scammony root for purging and birthwort for countering snake bites … and this is sufficient; I do not know why they work. In the same way I do not understand adequately the explanation for the signs of wind and rain which I have mentioned; I recognize, I know, and I vouch for the force and the result of them.”

In the same dialogue, Quintus — the Stoic character — defines divination as the study of things that are thought to occur by chance. They are thought to so occur, but in reality they are the result of chains of cause-effect that we do not have sufficient information to understand. This is a remarkably modern view of chance, with which any 21st century physicist would feel comfortable. Struck confirms: “Several texts tell us the Stoics defined chance [tychê, fortuna] as a cause which is unclear [aition adêlon] to human reason.” Divination then is an extension of our cognition about the cosmos, what Struck calls “surplus knowledge.”

Moreover, Posidonius — like many other ancients, including Plato — distinguishes between professional divination and that unconsciously carried out by some people who appear to be especially gifted. The former proceeds by the systematic study and interpretation of outward signs, and it is accounted for by the above discussion; the latter happens under particular conditions, such as in dreams, or characterizes what oracles do (probably under the effect of hallucinatory substances, as at Delphi).

Again, we are prone to reject “natural” divination as nonsense, just like we do with the professional variety. And it is, by modern standards. But the idea isn’t crazy at all. Essentially, what Posidonius was saying was that some people, for whatever reason, have particularly strong pneumatic connections with the rest of the cosmos, so that they can occasionally glimpse what for the rest of us is obscure and inaccessible. Struck interprets this as what modern psychologists call intuition.

Now, the Stoics had a sophisticated theory of causation, articulated chiefly by Chrysippus, who for instance distinguished between external and internal causes. The internal causes are what make us moral agents, and they organically interact with external causes because we stand inside, not outside, the cosmic web of causation (the Stoics didn’t believe in what today is referred to as contra-causal free will).

One of the metaphors they used to explain how cause-effect allows us to make predictions about the future — whether they come from professional divination (i.e., science) or certain individuals’ efforts (i.e., intuition) is by using the analogy with the potential of a seed to become a plant. Here is Cicero:

“As in seeds there is present the power (vis) of those things which are produced from the seeds, so in causes are stored the future events which the soul discerns.”

Struck summarizes the full view in this way: “In addition to the soul’s ability to gain information from the senses about its environment, we now have a soul capable of discerning pneumatic information somehow without the senses. How this works remains opaque, but within strict Stoic understanding, it would not require fairy dust to happen, just contact; and since, via sympathy, all things are in contact with all things, that problem is already solved. … We are the corpuscles receiving the message of the somatic cosmos. The receptors, if you like, taking in the body’s signals. Our position inside it, animated by our logical souls, makes us capable of picking up its information by our senses, and sometimes by surplus means as well.”

The point of this analysis, and of my summary of it, is of course not to say that the Stoics had knowledge or understanding of physics and metaphysics that somehow is still relevant today. Divination, nowadays, is a pseudoscience. But it is important to appreciate the sophistication of their reasoning even in areas where it has been superseded by modern science. Always beware of dismissing the products of other times and cultures as “obviously” primitive, or even downright ridiculous. It is a good lesson to keep in mind, because you can bet that whatever we think today about how the world works will eventually be regarded with equal disdain by future generations.