Mass is the quantity of matter composing a body. Isaac Newton (1642-1727), in Principia Mathematica (1687), gave the first definition of mass. He determined that universal (gravitational) attraction is proportional to mass. Unlike weight, which varies with the distance of bodies from the centers of attraction, mass remains constant.

For Aristotle (384-322 B.C.E.), the weight or "gravity" of bodies is an intrinsic property that impels them toward the center of the Earth. René Descartes (1596-1650) was the first to asset that weight is not an intrinsic property of bodies, but the product of the mechanical action exerted by particles of subtle matter from the periphery of vortexes toward their centers. Isaac Newton, in the Principia, interpreted weight as the effect of universal attraction on bodies. Given that universal attraction is inversely proportional to the square of distance, it follows that the weight of bodies is not constant, but varies with the distance between their centers.

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