Therefore we'll pass to the next, the consideration of Bodies, which though we see, and feel, and continually converse with; yet its constitution, and inward frame is an America, a yet undiscovered Region.
Joseph Glanvill, The Vanity of Dogmatizing, 1661
This chapter addresses the conflict between two rival theories of the internal structure of matter and the ontological status of its parts. The one theory is central to the mechanistic world-view of Galileo and Newton, and is overwhelmingly popular with partisans of the new science. The other is the classical doctrine of Aristotle, St Thomas, and the schools. Interestingly, new variants of the scholastic theory are also sponsored by Hobbes and the later Kant, who stand opposed to the mainstream of the new science on this question.
The first of our rival theories is the doctrine of actual parts, according to which the parts of bodies are so many distinct existents. These concrete parts are all already embedded in the architecture of the whole: division merely separates them, it does not create them. The whole body is an aggregate of these independently existing parts, and its structure is best described with count terms: it is a composite of so many distinct parts, so many independent beings. The second of our rival theories is the doctrine of potential parts, according to which the parts of bodies are merely possible or potential existents until they are generated by division. They represent ways in which the whole could be broken down, but do not exist other than as aspects of the whole until a positive act of division actualizes them as so many independent entities. Here the whole is a metaphysically simple (noncomposite) structure best described with mass terms: prior to division,