Tetrabiblos

What it is

Ptolemy's four-book treatise on astrology, written in Alexandria in the second century CE. The Greek title is Apotelesmatika, "Effects"; Tetrabiblos — "four books" — is the name it acquired and kept.

Its structure:

• Book I — the defence of astrology as a legitimate natural science, followed by first principles: the natures of the planets, the signs, aspects and the definition of the zodiac
• Book II — mundane astrology: the character of nations, weather, earthquakes, plagues, eclipses and their effects on countries
• Book III — the nativity from conception and birth through to temperament, parents, siblings and the shape of a life
• Book IV — material fortune, occupation, marriage, children, travel, friends and enemies, and the manner of death

Where it came from

Ptolemy wrote it as the companion to the Almagest: the first establishes where the celestial bodies are, the second what their positions signify. He treated the pair as one project, and the Tetrabiblos opens by acknowledging that the second is the less certain of the two.

That admission is characteristic. Ptolemy's defence is not that astrology is infallible but that it is a difficult natural science, like medicine or navigation — subject to error, dependent on the practitioner's skill, and worth pursuing anyway.

How it is read

It has been read for eighteen centuries as the authority. It passed into Arabic as the Kitāb al-Arbaʿ Maqālāt, into Latin as the Quadripartitum, was printed in 1484, and remained the theoretical foundation of Western astrology throughout.

Its naturalistic framing is a large part of why. By grounding astrology in physical influence rather than in divination or spirits, Ptolemy gave it a defence that could be made to philosophers and, later, to theologians — celestial bodies act on terrestrial ones as the sun and moon manifestly do, which is physics rather than fortune-telling. That argument protected the subject for a very long time.

A companion text, the Centiloquy or Karpos — a hundred aphorisms — circulated under Ptolemy's name for centuries and is now regarded as pseudepigraphic.

What we can and cannot say

We can say it is the most influential astrological text ever written, and that its account of the planets, the signs and the aspects is the ancestor of what Western astrologers still use.

We can say it settled the tropical zodiac for the West. Ptolemy defined the signs from the equinoctial point, knowing perfectly well about precession, and gave his reasons — which is why the divergence from the Indian tradition described in tropical and sidereal traces to a deliberate decision in Book I.

And we can say the thing most often got wrong about it: it is not a good guide to Hellenistic astrology. Ptolemy was a reformer with strong methodological opinions, and he pruned. He largely omits the lots, central to practice as Vettius Valens shows it. He gives little attention to the elaborate time-lord systems. He minimises sect, one of the tradition's organising doctrines. He is uninterested in much of the technical apparatus that working astrologers of his period plainly used.

The reason is consistent with everything else about him: he kept what could be justified as physical influence and set aside what looked like arbitrary symbolism. That made his book defensible and made it survive, and it also means that for fifteen hundred years the West inherited a filtered version of the tradition.

The consequence is real. Much of what the modern revival described in Hellenistic astrology has recovered was not lost in some general catastrophe. It was left out of the one book everybody read.

We cannot say what Ptolemy would make of that. He does not claim to be reporting standard practice; he says he is setting out what can be defended, and he was clear about the difference.

Further reading

• Tetrabiblos, in F.E. Robbins's Loeb translation (1940) — the standard English text, freely available.
• Vettius Valens, Anthology — read alongside it, to see what an ordinary practitioner did that Ptolemy left out.