 However, this view is certainly not without its  problems.[13] Indeed, one of its strengths—helping to ground inertia in the persistent activity of bodies — is a weakness. We saw earlier that it is not easy to square this active reading of inertia, grounded as it is in the idea of motion pertaining to the action proper to a body, with a good deal of what Spinoza says in the first and second parts of the Ethics about how no body can be determined to produce an effect unless is it so determined externally. Another, related problem for this interpretation is that it represents Spinoza as, not an avant-garde thinker anticipating modern physics, but as a rear guard defender, despite his official anti-scholastic stance, of the traditional neo-Aristotelian doctrines of essence and substantial form, open to the same charges of ad hoc theorizing and of appeal to occult powers that Modernity and the Scientific Revolution levelled against that tradition in their rise to intellectual dominance. Indeed, the notion of the individual essence of a body conceived as a power to maintain itself is of dubious intelligibility. Spinoza clearly believes that an individual's power to persevere can increase or diminish. But if this power is to constitute the thing's essence and identity, such changes in degree cannot alter the identity of the power. But what then constitutes the individuality and identity of a power? In this light, the account of individuation by essence seems unexplanatory: either it amounts to an elaborate name for the problem of the individuation of bodies it is supposed to solve, or it simply displaces the same sort of problem from bodies to powers.