1 The Birth of Science - MSRI

11.7 Newton’s Natural Philosophy 323
Thus Newtonian force (often claimed to be a cardinal novelty of modern
science) is the efficient cause of absolute motion, that is, of displacement
relative to “immovable space” — itself something whose only established
property is that of not being connected to “sensible measures”. And here page 396
moreover Newton seems to be saying that “true motion” is not possible
in the absence of a causing force, in contradiction with the principle of
inertia. 119
In spite of the approach just exemplified, Newton eclectically borrows
from Euclid a hypothetico-deductive expositive framework. His Principia,
like the Elements, contain definitions and axioms. But consider the first few
definitions:
Definition I. The quantity of matter is the measure of the same, arising
from its density and bulk conjointly.
Definition II. The quantity of motion is the measure of the same, arising
from the velocity and quantity of matter conjointly.
Definition III. The innate force [vis insita] of matter is a power of resisting,
by which every body, as much as in it lies, continues in its
present state, whether it be of rest, or of moving uniformly forwards
in a right line.
Definition IV. An impressed force is an action exerted upon a body,
in order to change its state, either of rest, or of uniform motion in a
right line.
Definition V. A centripetal force is that by which bodies are drawn or
impelled, or any way tend, towards a point as to a centre. 120
In later developments force came to be a physical magnitude, but here
it is defined as a disposition or action directed to a particular end. It seems
that Newton does not mean to build scientific theories in the sense we
have given the expression — in the sense of Euclid and Archimedes. His
intention, to judge from the passage just quoted, would seem rather to develop
a natural philosophy based on Aristotelian concepts such as efficient
cause and final cause. page 397
Here one must face what appears at first to be a very difficult problem.
How did Newtonian mechanics, leaning on such foundations, develop
into a true scientific theory?
119 It may be objected that force is said to be necessary to cause notion and not to maintain it. But,
supposing that Newton does admit true motion in the absence of force, how can force as the cause
of motion provide the desired distinction between true and relative motion? For this reason I think
that in this passage the Aristotelian view (upon which is based the characterization of true motion
as the effect of a force) coexists with its opposite (according to which a force is needed only to alter
motion). The origins of this second view will be discussed shortly.
120 Newton, Principia mathematica, Definitions, Motte/Cajori translation.
Revision: 1.11 Date: 2003/01/06 07:48:20