IV. General Physics.

294. General Principles. -- The great, striking phenomenon, the study of which enables the "physician" to ascend beyond the details of nature and embrace it all in one synthetic view, is the all-pervading fact of movement or change. On corporeal motion and its divisions, as well as on the theory of substantial change, scholasticism has recourse to Aristotle (50).^{1} But it can lay to its own credit some all-important developments on the rhythmic evolution of forms (50) and on the finality of the cosmos. Matter is a storehouse of potentiality. Still, the pliability in virtue of which it assumes divers forms successively, is not without due limit and direction. Nature never changes a stone into a lion; in its evolution it obeys a law of progress the particular applications of which it is the duty of the special sciences to discover. In scholastic language: the primal matter relinquishes its form, not in order to assume any other form at haphazard, but to assume THE ONE special form that corresponds to the immediately neighbouring type in the natural hierarchy. By a special predetermination, matter thus traverses a series of admirably arranged progressive stages.^{2} Hence it is that the human body, prior to its union with the spiritual soul, assumes successively a number of intermediate forms, until the natural agencies at work have given to the embryo an organic constitution sufficiently advanced and complex to call for the ultimate formative principle, the spiritual soul. Such is the full meaning of the formula: "Corruptio unius est generatio alterius".

This process, productive of forms ("eductio formarum e potentia materiae"), is rightly regarded as one of the most difficult doctrines in scholasticism. The leading doctors all admit the intervention of three distinct factors: the general concurrence of the First Cause; the pre-existing matter ready to unite with the new form for the formation of the new body; and the natural agent or efficient principle which actualizes the receptive subject. But they disagree about the role to be assigned to each of these three factors respectively. St. Thomas lays special stress on the "virtus activa" of the natural agent and the passivity of the primary matter; in opposition to Albert the Great and St. Bonaventure, he vigorously objects to the Augustinian theory of the "rationes seminales" (239), which was then the commonly accepted view; and to account for the evolution of natural substances he assigns as an adequate explanation the simple, natural predestination of matter to receive a successive series of perfections.^{3} By thus reducing the problem of the appearance and disappearance of substantial forms to the simple actualization of a potency in a pre-existing subject, St. Thomas may be said to have once more accentuated the peripatetic character of his system.

Design pervades the entire series of cosmic changes; such is the profound interpretation of Nature bequeathed to us by the Stagirite. But the medieval doctors never regarded Nature as an organism, endowed with a real, physical unity, after the manner of the ancients, including Aristotle himself (50). To all questions about the term of cosmic evolution, the scholastics find an answer in the relation of the world to God. The creature can have no other end but the glory of the Creator. This glory is primarily revealed in the contemplation of the cosmic spectacle by the Infinite Intelligence; a secondary manifestation of it consists in the knowledge which intelligent creatures possess of the wonderful order of the universe. Such is the answer to the question which Aristotle had asked without answering: How is God the final cause of the material universe? {1} The concept of quantity is, however, accentuated. Throughout all the scholastic systems, the primary matter which unites with the form for the constitution of bodies is invariably conceived as having a primordial relation to quantity: quantity, or the pensive diffusion of a body in space, is the fundamental attribute of the corporeal substance, and is a function of its primary matter; as the reduction of its manifold elements to substantial unity is a function of the form.

{2} The scholastics use the term privatio to indicate the absence of the formal principle demanded by the state in which the matter actually is.

{3} Summa Contra Gentes, L. iii., c. 22.