Aristotle describes two kinds of motion: "violent" or "unnatural motion", such as that of a thrown stone, in the Physics (254b10), and "natural motion", such as of a falling object, in On the Heavens (300a20).
ˈæɹɪstˌɑːɾəl dᵻskɹˈaɪbz tˈuː kˈaɪndz ʌv mˈoʊʃən: "vˈaɪələnt" ɔːɹ "ʌnnˈætʃɚɹəl mˈoʊʃən", sˈʌtʃ ɐz ðæt əvə θɹˈoʊn stˈoʊn, ɪnðə fˈɪzɪks tˈuːhˈʌndɹɪd fˈɪfti fˈoːɹ bˈiː tˈɛn, ænd "nˈætʃɚɹəl mˈoʊʃən", sˈʌtʃ ɐz əvə fˈɔːlɪŋ ˈɑːbdʒɛkt, ɪn ɔnðə hˈɛvənz θɹˈiːhˈʌndɹɪd ˈeɪ twˈɛnti.