Spinoza Principle

Continuing with my rationalist reconstruction of physics, I elaborate on a point mentioned here, that the Poisson equation $$\nabla^2 \phi=4\pi G\rho,$$ is possible to derive from Descartes' and Spinoza's metaphysical foundation for physics, which I name as Spinoza Principle. To state this principle, as a recap of Descartes' argument, and Spinoza's improvement, consider a single particle, isolated from all other objects in the universe. Descartes argues that if the state of this particle is to be altered from its initial state in which God put it in the creation of universe, God has to `change his mind'; because there is nothing else in the universe except God to change its course of motion, so as God is the cause of everything, any change in the particle's state of motion has to come from `God's decision', but for God to change his mind would be inglorious, therefore the particle continues its path `indifferently'. This is the same as Descartes' famous saying that the right question is not why a particle moves, but why it should not move! To remove vestiges of theology in Descartes' argument, Spinoza's Deus sive Natura implies that for the particle to change its course of motion, something other than Nature (=God) has to intervene, which is impossible for there is nothing other than God/Nature.