e incorporation, into this
system of efficient agents, of other qualities, such as the qualities
of gravity and levity that are responsible for upward and downward
motion.

Besides the causal hierarchy of forms, the natural world of St. Thomas
existed in a substantial and spatial hierarchy. All substances whether
an element or a mixture of elements have a place in this hierarchy by
virtue of their nature. If the material were removed from its proper
place, it would tend to return. In this manner is obtained the natural
downward motion of earth and the natural upward motion of fire.

Local motion can also be caused by the "virtus coeli" generating a new
form, or through the qualitative change of alteration. Since each
element and mixture has its own natural place in the hierarchy of
material substances, and this place is determined by its nature,
changes of nature due to a change of the form can produce local
motion. If before change the substance is in its natural place, it
need not be afterwards, and if not, would then tend to move to its
new natural place.

It will be noted that the scholastic explanation of inanimate motion
involved the action and passion of an active external mover and a
passive capacity to be moved. Whence the definition of motion that
Descartes[19] was later to deride, "motus est actus entis in potentia
prout quod in potentia."

[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