"Not only Locke's valid opinions, but even his errors, were useful in practice. Take, for example, his doctrine as to the primary and secondary qualities. The primary qualities are defined as those that are inseparable from the body, and are enumerated as solidity, extension, figure, motion or rest, and number. The secondary qualities are all the rest: color, sounds, smells, etc. The primary qualities, he maintains, are actually in bodies; the secondary qualities, on the contrary, are only in the percipient. ...But Berkeley pointed out that the same argument apply to primary qualities. Ever since Berkeley, Locke's dualism on this point [primary and secondary qualities] has been philosophically out of date. Nevertheless, it dominated practical physics until the rise of quantum theory in our own day. Not only was it assumed, explicitly or tacitly, by physicists, but it proved fruitful as the source of many very important discoveries. The theory that the physical world consists only of matter in motion was the basis of the accepted theories of sound, heat, light, and electricity. Pragmatically, the theory was useful, however mistaken it may have been theoretically. This is typical of Locke's doctrines."