For just as it is enough to know how the different disposition of its wheels can produce the various motions in a watch, and to have besides this no distinct idea of what could be the cause of this motion, although several of them might be had from logic as this is enough to convince us that a watch is not something different from the matter of which it is composed, so these people are convinced that extension is the essence of matter because (a) they have no distinct idea of what could be in matter if extension were taken from it;