Concept

Theory of impetus

Overview

The theory of impetus, developed in the Middle Ages, attempts to explain the forced motion of a body, what it is, and how it comes about or ceases. In ancient and medieval times, motion was always considered absolute, relative to the Earth as the center of the universe. The theory was introduced by John Philoponus in the 6th century, expanded upon by Islamic scholars in the 11th and 12th centuries, and established in Western scientific thought in the 14th century, before ultimately being disproven in the 17th century. It is the intellectual precursor to the concepts of inertia, momentum and acceleration in classical mechanics.

The theory of impetus is an auxiliary or secondary theory of Aristotelian dynamics, put forth initially to explain projectile motion against gravity. Aristotelian dynamics of forced (in antiquity called "unnatural") motion states that a body only moves when an external force is constantly driving it. The greater the force acting, the proportionally greater the speed of the body. If the force stops acting, the body immediately returns to the natural state of rest. It also states – as clearly formulated by John of Jadun in his work Quaestiones super 8 libros Physicorum Aristotelis – that both motion and force are transmitted to the medium, such that this force propagates continuously from layer to layer of air, becoming weaker and weaker until it finally dies out. This is how the body finally comes to rest.

Although the medieval philosophers held to the intuitive idea that only a direct application of force could cause and maintain motion, they recognized that Aristotle's explanation of unnatural motion could not be correct. They therefore developed the concept of impetus. Impetus was understood to be a force inherent in a moving body that had previously been transferred to it by an external force during a previous direct contact.

Impetus as a force has no equivalence in modern mechanics, which provides a different explanation of objects in motion: Motion is not absolute but relative to a reference frame (observer), and the permanent state of an object is not rest but uniform motion. The continuity of motion does not require an external or internal force, being based on the inertia of the object. At most, impetus approaches the meaning of "linear momentum" of a mass, because it is linear momentum as the product of mass and velocity that maintains motion due to the inertia of the mass (conservation of linear momentum). But momentum is not a force; rather, a force is the cause of a change in the momentum of a body, and vice versa.

Overview adapted from Wikipedia (CC BY-SA 4.0; see the article history for authors).

Possible passages

Found by search and not yet checked by a person; they may only mention the idea in passing.

_I answer that,_ As in corporeal movement that is called the mover which gives the form that is the principle of movement, so that is said to move the intellect, which is the cause of the form that is the principle of the intellectual operation, called the movement of the intellect. Now there is a twofold principle of intellectual operation in the intelligent being; one which is the intellectual power itself, which…
Aquinas Thomas, Summa Theologica, Part I
States what Theory of impetus is; the author is tied to Theory of impetus in the knowledge graph.
Read the chapter · Open in the reader
Gravitation is nothing more than a mode of moving--a tendency towards a centre: to speak strictly, all motion is relative gravitation; since that which falls relatively to us, rises, with relation to other bodies. From this it follows, that every motion in our microcosm is the effect of gravitation; seeing that there is not in the universe either top or bottom, nor any absolute centre. It should appear, that the…
Paul Henri Thiry Holbach, The System of Nature, or, the Laws of the Moral and Physical World. Volume 1
States what Theory of impetus is.
Read the chapter · Open in the reader
In the physics of Aristotle, it is by the concepts "high" and "low," spontaneous displacement and forced displacement, own place and strange place, that the movement of a body shot into space or falling freely is defined. But Galileo thought there was no essential moment, no privileged instant. To study the falling body is to consider it at it matters not what moment in its course. The true science of gravity is…
Henri Bergson, Creative Evolution
States what Theory of impetus is.
Read the chapter · Open in the reader

Theory of impetus

Overview

Possible passages

Philosophers who discussed Theory of impetus

Related ideas

Place in the tradition