That complex sequence of events which we have come to refer to as the scientific revolution brought about radical changes not merely in the content of the explanations accepted but also in the forms of explanation considered appropriate. The idea that one of the main aims – perhaps the main aim – of a natural philosopher should be the discovery of the laws governing the natural world emerged clearly for the first time during the seventeenth century. The Greeks had made little use of any concept of a law of nature. The phrase itself occurs infrequently indeed in the original texts (though more often in some translations), and when it can be found, its manner of use often seems to suggest that the whole idea was recognised as being odd and somewhat paradoxical. Given the extremely wide acceptance in the post-sophistic period of a fundamental antithesis between nomos (law, or convention) and phusis (nature), the marginal character of any idea of a law of nature is easy to understand. Even in later centuries, however, when the force of the nomos–phusis antithesis had greatly weakened and the idea of a moral law of nature had become familiar and widely accepted, there was still no parallel acceptance of any idea of nature as a system governed by, and explicable in terms of, physical laws. The theory of scientific explanation set out by Aristotle and accepted by such scientists as Ptolemy and Galen had no room for any such concept. Aristotelian explanations – or, rather, explanatory ideals – were essentialist in that they took as their fundamental premises definitions setting out the essences of things. There was no way in which anything analogous to Newtonian laws of motion could be inserted into such explanations, and neither Aristotle nor any of his successors made the slightest attempt to do so.