IMPETUS

An anglicized Latin term from the Latin roots in and petere, meaning a thrust toward some goal, it was commonly used in classical, ecclesiastical, and scholastic Latin without the technical connotation it was to acquire in the 14th century. Hence, in Roman and Biblical literature it was used for military attacks, the force of a river's current, and human drives; it was almost synonymous with nisus. In the first quarter of the 14th century, however, the term acquired a technical, philosophical meaning that eventually was rendered as impeto and momento by galileo in the field of mechanics. The term lost much of its original scholastic significance in discussions of the "quantity of motion" by Galileo, R. descartes, and G.W. leibniz. The last replaced it with the concept of force, lebendige Kraft, which is measured by mass times velocity squared (mv ²). Here the term is considered in its technical scholastic context.

The origin of the notion was the Aristotelian problem of explaining violent, or compulsory, motion after the body was separated from the impelling agent. Natural motion springs spontaneously from nature (φύσις), an internal, innate principle of motion and rest motion of heavy bodies, but it cannot explain the unnatural motion of a heavy body upward. All unnatural, or violent, motions must be explained in terms of an extrinsic, alien mover; insofar as a thing is moved at all, it must be moved by something distinct from its own nature. plato suggested that the original mover also moves the medium, which continues to move the projectile by rushing behind and pushing it forward ἀντιπερίστασις (Tim. 80C). aristotle rejected this as insufficient and held that the original mover gives not only movement, but also the power of moving (δύναμις το[symbol omitted] κινε[symbol omitted]ν) to the medium (Phys. 226b 27–267a 22; Cael. 301b 17–33). In this way the reality of nature was preserved and violent motion could continue as long as there was sufficient force in the medium to move the body. Aristotle argued further that if there were no medium, there could be no violent motion (Phys. 215a 1–18).

john philoponus presented many commonsense objections to Aristotle's theory and concluded: "It is necessary that a certain incorporeal motive power (κινητικήν τινὰ δύναμιν ἀσώμκτον) be given to the projectile in the act of throwing" [Commentaria in Aristotelem Graeca 17 (Berlin 1888) 636–642]. He pointed out that the motive "energy" (ἐνέργεια) is only borrowed and is decreased by the natural tendency of the body and the resistance of the medium. This new theory of an alien energy given to the body by the original mover was attacked by Simplicius (d. 549), who did not present the new view clearly and adequately. The scholastics did not have a Latin translation of Philoponus; thus it is doubtful that he played any role in the scholastic concept of impetus.

The scholastic theory seems to have been first suggested by Francis of Marchia. While discussing sacramental causality in his commentary on the Sentences, bk. 4, he used a concept of impetus to explain how both Sacraments and projectiles have a certain force resident within, by which something is produced. This virtus derelicta in lapide a motore is a "certain extrinsic form" that will in time diminish. john buridan, perhaps independently, reached the same conclusions in his Quaestiones super lib. Physicorum, 8.12, and Quaestiones de caelo et mundo, 2.12–13, 3.2. For him, the mover impresses a certain impetus on the body itself by which the body continues to move until overcome by air resistance and natural gravity. He insisted that impetus is "violent and unnatural, since it is violently impressed by an extrinsic principle and foreign to the natural form." He even suggested impetus as a possible explanation for the perpetual rotation of celestial bodies, in which there is no resistance or fixed natural inclination. albert of saxony and marsil ius of inghen promulgated the theory and continued to speak of impetus as an "accidental and extrinsic force," thus preserving the Aristotelian notion of nature and violence. Later scholastics—such as Laurence Londorius, first rector of St. Andrew's; Agostino nifo; Tommaso de Vio cajetan; Alessandro piccolomini; and J. C. Scaliger—interpreted Aristotle's words in a wide sense consistent with the theory of impetus. Thomists such as John capreolus and Domingo de soto claimed it as the "opinion of St. Thomas."

Some scholastic authors considered impetus to be a "mover" accompanying the body. However, if impetus were a true efficient cause of motion, then it would be philosophically impossible to distinguish projectiles from living things, which move themselves. Domingo de Soto argued against this misconception and explained that impetus is only an instrument of the agent who is the true efficient cause. He pointed out the analogy between impetus and nature. Both are formal principles of motion, not efficient causes; just as the cause of natural activity is the progenitor, not nature, so too the cause of violent motion is the agent, not impetus.

See Also: science (in the middle ages).

Bibliography: a. maier, Enciclopedia filosofica (Venice-Rome 1957) 2:1299–1301. j. a. weisheipl, "Natural and Compulsory Motion," The New Scholasticism 29 (1955) 50–81; "The Principle Omne quod movetur ab alio movetur in Medieval Physics," Isis 56 (1965) 26–45. m. clagett, The Science of Mechanics in the Middle Ages (Madison, Wis. 1959).

[j. a. weisheipl]