Contested concept in the history of physics. Aristotelian physics holds that every motion requires a conjoined mover. Action can therefore never occur at a distance, but needs a medium enveloping the body, and which parts before its motion and pushes it from behind (antiperistasis). Although natural motions like free fall and magnetic attraction (quaintly called ‘coition’) were recognized in the post-Aristotelian period, the rise of the corpuscularian philosophy again banned ‘attractions’, or unmediated actions at a distance: the classic argument is that ‘matter cannot act where it is not’. Cartesian physical theory also postulated ‘subtle matter’ to fill space and provide the medium for force and motion. Its successor, the aether, was postulated in order to provide a medium for transmitting forces and causal influences between objects that are not in direct contact. Even Newton, whose treatment of gravity might seem to leave it conceived of as action at a distance, supposed that an intermediary must be postulated, although he could make no hypothesis as to its nature. Locke, having originally said that bodies act on each other ‘manifestly by impulse and nothing else’ (Essay, 1st edn., ii. viii. 11), changes his mind by the 4th edition, and strikes out the words ‘and nothing else’, although impulse remains ‘the only way which we can conceive bodies operate in.’ In the Metaphysical Foundations of Natural Science Kant clearly sets out the view that the way in which bodies repulse each other is no more natural, or intelligible, than the way in which they act at a distance; in particular he repeats the point half-understood by Locke, that any conception of solid, massy atoms requires understanding the force that makes them cohere as a single unity, which cannot itself be understood in terms of elastic collisions. In many cases contemporary field theories admit of alternative equivalent formulations, one with action at a distance, one with local action only.