Newtonianism

The "philosophy of body" was deemed a crucial branch of philosophy in the eighteenth century dealing with the nature of matter. According to the traditional Cartesian view, the only essential property of a material body was extension. Figure, position, and motion were only "modes of existence" of an extended being. As a result, all natural effects should be processed on the basis of changes that occur in the shape, the relative positioning, and the motion of the bodies or of their parts. A significant advantage of this approach, according to the proponents of Cartesian philosophy, was that it made clear the distinction between the material agent of natural phenomena and the external cum immaterial causes of motion. This way it was made possible to disengage material bodies from the notorious "occult qualities" they inherited from Renaissance and some aspects of ancient philosophy.

Newtonianism brought about two important transformations to this view: Firstly, it maintained the implicitly theological idea that it is in principle impossible for people to grasp all the qualities of natural bodies. Thus, not only is extension not a unique essential quality of material bodies, but also the few other qualities we are able to know are but a subtotal of the qualities God may have provided the bodies with. Almost all the followers of Newtonian philosophy subscribed to this voluntaristic view of the divine design. Voltaire, Willem Jacob 'sGravesande (1688–1742), and Petrus van Musschenbroek (1692–1761)—to mention only the most active of them—insisted on the constitutional inability of human beings to penetrate God's will so as to acquire a definitive knowledge of the nature of material bodies.

The second transformation has to do with a new addition to the list of attributes of natural body, namely the force of attraction. According to the definition of Musschenbroek,

those things which we find to be in all bodies, we call their attributes.… Among these attributes there are some, which can never be intended or remitted, and others, which are capable of intension and remission. The former are extension, solidity, inactivity, mobility, a capacity of being at rest or having a figure. The latter are gravity and the power of attraction. (Musschenbroek, p. 10)

It is true that in the course of time, this addition gave much trouble to the proponents of Newtonian philosophy. Even in the early eighteenth century, it was not clear whether attraction was an inherent active principle of the matter, or a force transmitted through an ethereal substance filling the whole universe. As a result, the supporters of this view were accused of reverting to the "occult qualities," which had been banished from philosophy thanks to Cartesian philosophy. Concerning this issue, the Newtonians attempted to articulate a moderate philosophical thesis maintaining that attraction was simply a force of unknown origin that dominated the interactions between material bodies:

And lest any one should think, because we do not assign the Cause of the abovemention'd attracting and repelling Forces, that they too are to be reckon'd among the Occult Qualities: We shall say, with the great Newton, we do not consider those Principles, as Occult Qualities, which are imagin'd to arise from the specifick Forms of Things; but as the universal Laws of Nature, by which Things themselves are form'd; for the Phaenomena of Nature shew us, that such Principles do really exist, tho' no one hath explain'd yet what are the Causes of them. ('sGravesande, p. 24)

Concerning the idea that attraction might be an inherent quality of matter, however, things were more troublesome. Such an interpretation of Newtonian dynamics by some supporters of Spinozistic philosophy, like John Toland (1670–1722), favored materialism, which was much repudiated by the "orthodox" Newtonians, as we shall see below.