Mesmer’s dissertation of 1766 described the effect of the Moon and planets on the human body. This was not medical astrology. An astrological influence would be determined as a function of planetary positions qua astrological signifiers on the parts of the human body according to some traditional system of correspondences. Mesmer was talking about the purely mechanical effects of planetary and lunar gravity on a subtle fluid in the body that can feel its force.

The Moon and planets influence human biomechanics through a force called animal gravitation. Given Newton’s stunning identification of Earth gravity with the force of planetary motion, the existence of such an all-pervading and invisible field that communicated action at a distance did not seem far-fetched. The oceans respond to planetary gravity. What about the fluids in the human body? Earthly tides are indeed caused by the gravitational attraction of astronomical masses, so it stands to reason that there are similar tidal effects on the human body (there are)—and that these might affect our health (they do not).

In his dissertation, Mesmer postulated the existence of an undetectable fluid filling all of space and continuously streaming through and around objects. The idea that the universe was permeated by an aether was popular at the time. Even Newton considered it as a possible mechanism to explain the transmission of light, gravity, and magnetism.

The aether that binds and fills the universe also circulates through tiny channels in the human body, where it functions as a life-supporting vital fluid. In a healthy person, the fluid circulates evenly and unblocked. In sick and emotionally disturbed people, the flow is blocked and the distribution is lumpy.