  Boyle, along with his fellow mechanical philosophers, argued for his position on the grounds that it was clear and intelligible compared to rival systems such as Aristotelianism and those developed in chemical and related contexts by the likes of Paracelsus. The argument operated at the level of the fundamental ontology of the rival philosophies. Boyle insisted that it is perfectly clear what is intended when shape, size and degree of motion are ascribed to an impenetrable atom and when arrangements are ascribed to groups of such atoms. That much can surely be granted. But Boyle went further to insist that it is unintelligible to ascribe to atoms properties other than these primary ones, that is, properties other than those that atoms must necessarily possess by virtue of being portions of matter, such as the forms and qualities of the Aristotelians or the principles of the chemists. ‘Nor could I ever find it intelligibly made out’, wrote Boyle, ‘what these real qualities may be, that they [the scholastics] deny to be either matter, or modes of matter, or immaterial substances’ (Stewart, 1979, 22). If an atom is said to possess elasticity, for example, then Boyle is saying that the ontological status of whatever it is that is added to matter to render it elastic is mysterious, given that it cannot be material. This is not to claim that attributing elasticity and other secondary properties to gross matter is unintelligible. For such properties can be rendered intelligible by regarding them as arising from the primary properties and arrangements of underlying atoms. Secondary properties can be ascribed to the world derivatively but not primitively. So the stark ontology of the mechanical philosopher is established a priori by appealing to a notion of intelligibility.