[19] Rene Descartes, _Oeuvres_, Charles Adam and Paul Tannery, Paris, 1897-1910, vol. 2, p. 597 (letter to Mersenne, 16 Oct., 1639), and vol. 11 (Le Monde), p. 39. The original definition can be found in Aristotle, _Physics_, translated by P. H. Wickstead and F. M. Cornford, Loeb Classical Library, London, 1934, 201a10. Aquinas rephrases the definition as "_Motus est actus existentis in potentia secundum quod huius modi._" See St. Thomas Aquinas, _Opera omnia_, Antwerp, 1612, vol. 2, _Physicorum Aristotelis expositio_, lib. 3, lect. 2, cap. a, p. 29. We have seen above that the "motor essentialis" for terrestial change is the "virtus coeli." Thus the enacting source of all motion and change is the heavens and the heavenly powers, while the earth and its inhabitants becomes the focus or passive recipient of these actions. In this manner the scholastic restated in philosophical terms the drama of an earth-centered universe. Although change or motion is normally effected through the above mentioned causal hierarchy, it is not always necessary that actualization pass from the First Cause down through each step of the hierarchy to terminate in the qualities of the individual being. Some of the steps could be by-passed: for instance man's body is under the direct influence of the celestial bodies, his intellect under that of the angels and his will under God.[20] Another example of effects not produced through the tangible prime qualities is that of the tide-producing influence of the moon on the waters of the ocean or the powers of the loadstone over iron. Such causal relations, where some members of the normal causal chain have been circumvented, are called occult.[21] [20] St. Thomas Aquinas, _op. cit._ (footnote 19), vol. 9, _Summa contra gentiles_, lib. 3, cap. 92 (Quo modo dicitur aliquis bene fortunatus et quo modo adjuvatur homo ex superioribus causis), p. 343. [21] St. Thomas Aquinas, op. cit. (footnote 19), vol. 17 _Opuscula, De operationibus occultis naturae ad queindam militem ultramontem_, pp. 213-224. While St. Thomas referred to the loadstone in a number of places as something whose nature and occult properties are well known, it was always as an example or as a tangential reference. One does not find a systematic treatment of the loadstone in St. Thomas, but there are enough references to provide a fairly explicit statement of what he considered to be the nature of the magnet. In one of his earliest writings, St. Thomas argued that the magnet attracts iron because this is a necessary consequence of its nature.[22]