295. Celestial and Terrestrial Substances. -- Astronomical physics and mathematics, the teaching about sublunary bodies and about the action of the heavens on terrestrial substances, were all deduced from an erroneous postulate borrowed by scholasticism from Aristotle (51): the superior perfection of the astral substance as compared with the terrestrial.

(1) The celestial substance is the nobler both by its constitution and by its movement. It is immutable, because the heavens are not subject to generation and corruption. Albert the Great, St. Thomas and Duns Scotus explain these cosmological properties by saying that the heavenly bodies are in reality made up of matter and form, like terrestrial bodies, but that in the former those two elements are indissolubly united to each other. Hence, as matter only lays aside one form to clothe itself with another ("corruptio unius est generatio alterius"), the celestial body can neither come into being nor pass out of being. Other scholastics, less numerous, however, have recourse to a more radical hypothesis, recognizing in the stars simple substances, exclusive of matter.^{1} From the immutability of the stars, the scholastics do not conclude, like Aristotle, to their eternity, their system on this point being rather in conformity with their teaching on creation (293). But they defend the unicity of the sidereal type, the form either existing alone or determining all the matter it is capable of "informing". Motion being regarded by the physicists of the Middle Ages a necessary manifestation of the essence of bodies, every specific substance ought to possess a specific motion: this is the theory of the "natural motion" and "natural place": one of the antitheses to modern mechanics. It means simply that if a body be displaced by an efficient cause, it determines and directs its local motion, conformably to its nature, in the direction of a place which is natural to it. The heavenly body, superior in constitution to the terrestrial body, is endowed with a nobler mode of motion, viz., circular motion. The movements of the fixed stars are explained by the rotation of concentric spheres, that of the planets by various subsidiary hypotheses: homocentric cycles, and excentric circles or epicycles. Who gives the impetus to the spheres? Not astral souls, intelligent and Divine forms, as Aristotle taught. St. Thomas admits that intelligent motors, extrinsic to the spheres, may communicate to them a gyratory motion.^{2} In connection with this geocentric astronomy, let us emphasize this significant remark of St. Thomas, for it weakens many an accusation brought against scholasticism (v. Fourth Period). Speaking of the motion of the planets, he says: "Licet enim talibus suppositionibus factis apparentia salvarentur, non tamen oportet dicere has suppositiones esse veras, quia forte secundum aliquem alium modum nondum ab hominibus comprehensum apparentia circa stellas salvantur".^{3}

(2) The terrestrial or sublunary body: Reference is everywhere made to Aristotle for the theory of the four elements, their qualities, and their rectilinear motion indicative of their relatively inferior natures.

(3) The action of the heavens in producing generation and corruption in sublunary matter is interpreted as in Aristotle.^{4} This supposed influence explains the exaggerated importance attached to the stars in the Middle Ages -- and also the vogue of the various arts which investigated that influence: magic, which interrogated the secret powers of the heavens; astrology, which explained their influence on man's destiny; alchemy, which sought to substitute for the ordinary course of terrestrial transformation an artificial mode of which man was supposed to be master, and to direct the mysterious power of the heavens so as to make the primary matter pass through all the sublunary forms.

{1} Cf. P. TEDESCHINI, Dissert. historica de corpore simplici quoad essentiam (in the Institut. Philosoph. of Palmieri, iii., pp. 322 sqq.), not free from errors and exaggerations.

{2} Ad hoc autem quod moveat, non oportet quad uniatur sicut farina sed per contactum virtutis sicut motor unitur mobili" (S. Theolog., i., q. 70, a. 3).

{3} In Lib. II. de Coelo et Mundo, lect. xvii. Cf. S. Theolog., I^a P., q. 32, a I ad 2.

{4} S. THOMAS, S. Theol., Ia, q. 115, a. 3.