Geomancy

What it is

A system of sixteen figures, each made of four rows containing either one dot or two, generated by making random marks and counting whether the total in each row is odd or even.

Four rows of a binary choice gives exactly sixteen possibilities, and the figures carry names — Via, Populus, Fortuna Major, Puer, Rubeus, Acquisitio, Carcer, Laetitia and the rest — with attributed meanings.

A note on the name first. Feng shui is sometimes called "Chinese geomancy" in older English writing, and the two have nothing to do with each other. Feng shui arranges space; geomancy in this sense is a divinatory technique using figures made of dots. The shared label is a translator's accident.

Where it came from

The system is Arabic in origin — ʿilm al-raml, "the science of the sand", from the practice of making the original marks in sand. It is attested from around the ninth century and spread rapidly across the Islamic world.

It reached Latin Europe through the twelfth-century translation movement, and became one of the most popular divinatory arts of the medieval and Renaissance periods, practised alongside astrology and borrowing astrology's houses wholesale. Hugh of Santalla produced early translations; later Cornelius Agrippa, Christopher Cattan and Robert Fludd wrote on it.

Related systems exist across Africa, and the relationship is genuinely debated. Yoruba Ifá uses sixteen principal figures which pair into 256; Malagasy sikidy is structurally very close to the Arabic system. Whether these descend from Arabic geomancy through trans-Saharan and Indian Ocean contact, whether the influence ran the other way, or whether there is a shared older ancestry, is not settled, and confident claims in either direction should be treated with caution. Ifá in particular is a living religious tradition with its own vast oral corpus, not simply a divination technique, and it belongs in the section on indigenous and folk traditions rather than here.

How it is practised

The diviner makes four lines of random marks — in sand, or as dots on paper — without counting, then counts each line and records one dot if odd, two if even. This produces the first figure. Repeating three more times gives four figures, called the Mothers.

Everything after that is deterministic. The four Daughters are produced by reading the Mothers column-wise rather than row-wise. Four Nieces are produced by combining pairs. Two Witnesses come from the Nieces, and a single Judge from the Witnesses, by a rule that adds rows and takes the parity.

So a full chart of fifteen or sixteen figures contains exactly sixteen bits of actual randomness — the four Mothers — and the rest is computation. The Judge, which carries the verdict, is a function of the Mothers, and can only ever be one of the eight figures with an even number of total dots. Medieval practitioners knew this last constraint and used it as a check on their arithmetic.

The figures are then placed in astrological houses and read for the question.

What we can and cannot say

We can say the structure is mathematically elegant and genuinely interesting. It is a binary system with a deterministic derivation, worked out centuries before binary arithmetic was formalised in Europe, and the parity rule that generates the Judge is a real piece of mathematics. Its structural resemblance to the I Ching — figures built from stacked binary lines — is often remarked on, though there is no evidence of contact between the two traditions and the resemblance is most plausibly what happens when different cultures build systems on the simplest possible unit of information.

We can say the practice was mainstream and respectable for centuries in both the Islamic world and Europe, taught alongside astrology and used by the same clientele.

We can say there is no evidence it yields information, and no test has been attempted.

We can say something structurally revealing. Because the derivation is deterministic, the entire reading is fixed by sixteen random bits — and the elaboration into sixteen figures across twelve houses produces the appearance of a rich, detailed, internally consistent picture from almost no input. That is worth noticing beyond geomancy: a small amount of randomness, run through enough apparatus, generates a great deal of material for a reader to find meaning in. Several systems in this section have that shape.

We cannot say why it declined. It largely vanished from European practice by the eighteenth century, while remaining in use in parts of the Islamic world and West Africa, and the modern Western revival is small and driven by the same traditionalist interest that recovered Hellenistic astrology.

Further reading

• Stephen Skinner, Geomancy in Theory and Practice (2011) — the standard modern historical treatment.
• Emilie Savage-Smith and Marion Smith, Islamic Geomancy and a Thirteenth-Century Divinatory Device (1980) — on the Arabic tradition and a remarkable brass instrument built for it.