Faraday introduced the concept of field and of field lines of force that exist outside material bodies.
Descartes believed that all material bodies, including the human body, are machines that operate by mechanical principles.
Some theologians, such as Augustine in the 4th and 5th centuries, stated that angels, who have ethereal bodies, may be able to assume material bodies.
As archons, they rule over the material elements (fire, wind, earth, and water) and therefore also over the material bodies of future human beings.
It is this equivalence that entails that any given gravitational field will accelerate any two material bodies, whatever their weights or their constitutions, to precisely the same degree.
Ibn al-Haytham also objected to Ptolemy's habit of defining motions with respect to immaterial points and lines as if they were real material bodies.
But he was driven to the apparent absurdity of denying causality by his desperation to solve the problem that Descartes couldn't: How can immaterial mind affect material bodies, and vice versa?
Rich ontologists include Plato, who recognized immaterial Forms as well as material bodies, and the Austrian philosopher Alexius Meinong (1853 1920), who embraced merely possible and even impossible objects alongside actual objects.
The law would state that the trajectories of material bodies are geodesics rather than Euclidean straight lines and that gravitation is not a force but rather a departure from the laws of Euclidean geometry.
In general relativity, which, though less firmly established than the special theory, is intended to explain gravitational phenomena, a more complicated metric of variable curvature is employed, which approximates to the Minkowski metric in empty space far from material bodies.
This equivalence also entails that any two material bodies, whatever their weights or their constitutions, will share precisely the same set of physically possible trajectories in the presence of a gravitational field, just as they would in empty space.
