De Motu: Sive, de Motus Principio & Natura, et de Causa Communicationis Motuum (On Motion: or The Principle and Nature of Motion and the Cause of the Communication of Motions), or simply De Motu, is an essay written by George Berkeley and published as a tract in London in 1721. The essay was unsuccessfully submitted for a prize that had been offered by the Royal Academy of Sciences at Paris. Berkeley rejected Sir Isaac Newton's absolute space, time and motion. With this essay, Berkeley is considered to be the "precursor of Mach and Einstein" (Karl Popper). Concrete and abstract motion We must pay attention to facts regarding things and their nature, not to words or to someone's authority. The mind should be concerned with particular and concrete things themselves, not with abstract terms. The effects that are felt on our senses should be noticed. The causes of these effects are rationally inferred and are occult qualities. Gravity and force are examples of occult qualities. But they can be useful terms. Gravity can designate concrete bodies in motion and force can...