  Drawing on his commitment to panorganicism, his talk of corporeal  substances, and his explicit invocation of the Aristotelian notions  of form and matter, it has been suggested that in his middle years  Leibniz held to an essentially Aristotelian ontology according to  which the fundamental level of reality is occupied by organisms  composed of substantial forms and matter. As Daniel Garber –  the reading’s most influential defender – puts the view, it is  “a world whose principal inhabitants are corporeal substances  understood on an Aristotelian model as unities of form and matter,  organisms of a rudimentary sort, big bugs which contain smaller bugs, which contain smaller bugs still, all the way down” (1985, 29).  According to this picture, the derivative forces studied by  physicists are grounded in the active and passive natures of the organisms from which they are composed. Organisms strive and are  acted upon at the most basic level of ontology, and those strivings  and passions serve as the ground for the active and passive  derivative forces examined, for example, in collisions between  elastic spheres. Although Leibniz’s panorganicism remains striking,  this picture promises a relatively straightforward account of the foundations of  Leibniz’s physics, and squares tolerably well with many of his texts, especially from his middle years. 