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Thomas Aquinas
Similar arguments to those of Roger Bacon and Albertus Magnus are
expressed by Thomas Aquinas. In the Summa contra gentiles he adopts
the theory of eduction rather than bestowal of separate forms by an
intellectual moving substance.
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Rejecting the Platonic theory that the
actualization of a generated natural thing is achieved by Ideas as sepa-
rate determiners, Aquinas asserts that what causes natural things to be
is not form alone but the composite of form and matter.
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However,
he accepts that these substantial forms are derived from ‘first princi-
ples’ through the power and movement of the heavenly bodies that
are moved by an intellectual substance.
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Through the mediation of
the heavenly bodies, the forms of inferior things arise in the material
substance.
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The intellectual substance moving the planets and stars is
not the active intellect acting by emanation but the principle of celestial
motion which leads to the actualization of substantial forms.
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Due to
the motion of celestial bodies and their link with the generated world,
and due to the fact that the celestial bodies are noble and closer to intel-
lectual substance than elementary bodies, lower things are ordered and
‘governed’ by the celestial bodies.
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In the lower bodies alteration is a
type of motion and the heavens are declared by Aquinas to be the cause
of all their motions.
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This is compatible with Abu Ma‘shar’s astrological
theory in the Great Introduction which Aquinas mentions in the Summa
contra gentiles.
The issue that most concerned Aquinas with regards to astral causa-
tion is determinism. In Bacon and Albertus too discussion of causation is
followed by establishing free will. Bacon points out that false astrologers
are those who believe in determinism and the necessity of astral judge-
ments and agrees that astrology involves a lot of speculation because
the interpretation of diverse signs (signis diversis) is never certain and
Magic in the Thirteenth Century
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because God can change the whole order at will. True astrologers know
that they are dealing with the possible and their judgements are general
and not final.
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He writes:
Ptolemy also and Aristotle, Avicenna, Messehalac, Hali, and
Albumazar, who in comparison with others have spoken with greater
authority in regard to these matters, do not maintain that there is an
absolute necessity in things below due to the influence of the heav-
ens, because free will is not subject to the things of nature. [
. . . ] they
do not place any necessity on free will.
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It is probable that al-Kindi is excluded from this list of authorities as
in De radiis he insists that contingency is only perceived by human
beings on account of their inability to comprehend causal totality; astral
influences are in fact necessary.
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Bacon, elsewhere, explains that astral
influences are necessary as universal causes but are restricted by the par-
ticularization of nature; meaning that the effects of these causes are not
necessary as their manifestation in nature might be altered by other ter-
restrial causes.
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So, how can he reconcile this notion with his belief in
true astrology? He adopts Abu Ma‘shar’s concept of inclination and adds
that astral influences remain stronger than terrestrial ones. Astrologers,
he explains:
Do not presume certain knowledge, but they consider how the body
is altered by the heavens, and when the body is changed the mind
is aroused now to private actions and not to public ones, yet in all
matters is the freedom of the will preserved. For although the ratio-
nal soul is not compelled to its acts, yet it can be strongly influenced
and aroused, so that it gratuitously desires those things to which the
celestial force inclines it [
. . . ] the forces of the heavens are stronger
than those of the visible and audible things below and of many
other objects of sense perception, and are able to alter substance, not
merely accidents, and to corrupt and destroy all things here below.
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Similarly, Albertus Magnus points out that when it is said that the stars
exert power on inferior things, it is to be understood that they have
such power as universal first causes moving immediate and ‘propter
quid’ causes; for that reason a constellation is not always followed by
its effects in a necessary way.
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In his commentary on Aristotle’s De fato
he explains in a very al-Kindian terms that terrestrial effects are derived
from the intersection of astral rays, the quality of the places in which
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