 Thus the form or essence of a thing, which individuates it, and whose retention through change constitutes the persistence of an individual, just is its inherent and individual power of striving to retain just form, hence to resist those extrinsic determinants that would diminish its power and destroy it. In the case of body, striving is expressed by the active tendency of a pattern of relative motion and rest, in their ordinary sense, of its parts to persist. The active tendency of that pattern to persist is the essence of the individual body. That pattern is determined in its existence transiently and externally; now this way, and now that, now larger, now smaller, now swifter, now slower, now with these parts, now those. Such transient, external determination may give it accidental properties that oppose and hinder the power of action it has in virtue of what inheres in it, and hence limit its power of action. Another virtue of the idea that essence as active striving is what individuates bodies is that it would provide a basis in the nature of body for the principle of inertia, in such a way that inertia could be taken as a ground for the PLMM we earlier saw was needed to underwrite the collision laws.