Occult virtues: why “occult” meant hidden

The word “occult” today evokes something murky or forbidden. For a sixteenth-century reader, by contrast, it had a technical and almost neutral meaning: it indicated what is hidden from the senses, something whose effects are visible but whose cause is not. The very title of Cornelius Agrippa's work, De occulta philosophia, should be understood in this sense. And at the centre of his construction stands the doctrine of the occult virtues of things, one of the most debated problems of natural philosophy before the scientific revolution.

Manifest qualities and hidden qualities

Aristotelian physics explained the properties of bodies through four fundamental qualities: hot, cold, dry and moist. From their combination came the four elements and, by mixture, all substances. These qualities were called manifest, because they could be perceived by touch. There remained, however, a set of phenomena that could not be reduced to them. Why does the magnet attract iron? Why does a poison kill in tiny amounts? Why do some substances purge and others not?

Scholastic philosophers called these properties occult qualities. Thomas Aquinas and his successors admitted that they derived from the specific form of a substance, or from the influence of the heavenly bodies, and not from the simple mixture of elements. Their existence was therefore recognised by everyone; what was debated was their origin, and on this point the answers diverged profoundly.

Agrippa's explanation

In the tenth chapter of the first book Agrippa addresses the question directly. Occult virtues, he writes, do not arise from the elements but are infused from above: from the ideas of the divine mind, through the world soul, with the concurrence of the celestial rays. That is why they are called occult, because their causes are hidden and the human intellect does not easily reach them. Only experience and long observation allow us to recognise them.

The examples he reports come largely from ancient and medieval authors, such as Pliny the Elder, Albertus Magnus and the lapidaries: stones that tradition believed capable of protecting, animals endowed with singular properties, plants linked to a planet. Agrippa gathers them as an encyclopaedist rather than as a direct observer.

From the sixteenth century to Newton

The question did not concern magicians alone. Jean Fernel, physician to the King of France, published in 1548 the dialogue De abditis rerum causis, devoted precisely to the hidden causes of diseases and remedies. Girolamo Fracastoro, in 1546, investigated sympathies and antipathies between bodies. The theme ran through the whole of university medicine.

In the seventeenth century the mechanical philosophers, from Descartes onwards, tried to eliminate occult qualities by reducing every phenomenon to matter, shape and motion. Paradoxically, when Newton introduced in the Principia of 1687 a gravitation acting at a distance without any contact, some critics, Leibniz among them, accused him of having reintroduced into physics precisely an occult quality.

The story shows how closely “occult philosophy” was intertwined with the great questions of modern science. Agrippa did not invent the problem of hidden virtues, which physicians and theologians had been debating for centuries: he gave it a coherent metaphysical answer, founded on the world soul and the celestial rays, which for more than a century remained one of the possible alternatives in the explanation of nature.