It is maintained in this treatise, that neither matter nor spirit (meaning by the latter the subject of sense and thought) correspond to the definitions above mentioned. For that matter is not that inert substance that it has been supposed to be; that powers of attraction or repulsion are necessary to its very being, and that no part of it appears to be impenetrable to other parts. I therefore, define it to be a substance possessed of the property of extension, and of powers of attraction or repulsion.
From what I understand, both Descartes and Newton believed matter is "inert", or has "no power of its own" until an outside force causes it to go into motion. Priestley talks about both men in his essay. Was Priestley describing fundamental interactions the only way he knew how?