 The problem we have been addressing is to find in Spinoza a satisfactory ground for the idea that modes of extension consititute bodies to which dynamical laws like the laws of collision should apply.  We have characterized that problem in terms of the “robust persistence” of bodies – their resistance to change.  Barry (2021) presents a detailed and complex treatment of possible ways Spinoza might be thought to account for the resistence to change bodies manifest in the dynamics of collision.  He argues that neither Spinoza's account of inertia nor the conatus doctrine can support resistance, and that a more direct appeal to modes as expressions of God's power or activity cannot do so either.  He suggests tentatively that, via Spinoza's parallelism (EP27), the resistance characteristic of bodies might instead be read off of the resistance relatively adequate ideas have to alteration in the face of relatively less adequate ideas.  This reverses the more usual direction of reading off features of ideas from those of their modal counterparts in extension; but, despite Spinoza's remark in letter 27 (to de Vries) that ethics is to be founded on metaphysics and physics, turnabout here should be fair play.  As Barry recognizes, however, since not all features of modes of one attribute can be applied to the counterparts under the other attributes, there must be something identifiable in bodies that corresponds to the adequacy of ideas in virtue of which they resist other ideas if the strategy is to work. The extended counterpart of adequacy Barry fixes on is the degree to which the collection of component bodies whose relation consitutues a composite individual body communicate their rest and motion to one another in a fixed manner, as discussed in the PI.  One might argue that, had Barry taked that idea seriously enough in his discussion of inertia and the PI, the detour through Thought via the parallelism would have been unnecessary.  In any event, Barry worries here that appreal to adequacy explains at best resistance to change i.e., the ability of a body to retain the same ratio of motion and rest in a collision, but not the ability to produce a change; i.e., the power of a body to move or alter another, both of which powers Spinoza clearly attributes to bodies.