I was reading the Wikipedia page on Dynamism in order to get an idea of the motivation and thinking behind Liebniz's physics. In it there is this paragraph: In the opening paragraph of Specimen dynamicum (1692), Leibniz begins by clarifying his intention to supersede the Cartesian account of corporeal substance by asserting the priority of force over extension. Additionally, while browsing through PMM Duhems text, Evolution of Mechanics he discusses how force is something irreducible to geometry and extension: And finally, yet other, pursuing to the end the ideas of Leibniz, did not hesitate to see in force a notion irreducible to extension and motion, a first and essential property of material substance. I'm confused on what the thought is behind this concept of extension. My understanding is that these philosophers were attempting to reduce to first principles and to keep things as geometric as possible and so they attempted to explain mechanics and dynamics only through direct physical contact and thus required things like gravity where action at a distance is observed to be due to an unobserved extension of the physical bodies actually making contact with one another -- or like the Cartesians who imagined space to be filled with vortices which actually pushed and pulled on pieces through direct contact. is this the meaning of extension in these concepts and philosophies?