  With his two criteria in hand, Leibniz next argues that the bodies of Cartesian physics fail to meet the standards for created substances  twice over. For on the one hand, every Cartesian body must be  extended and divisible, and so must be composed of parts. But this,  Leibniz maintains, is sufficient by itself to show that Cartesian  bodies are not fundamental created entities: for if every Cartesian  body is composed of parts, then by the independence criterion, the  parts will be more fundamentally real than the wholes which they  compose. No body whose essence is simply extension could therefore be a genuine created substance according to Leibniz. On the other hand,  according to Descartes’s physics the only principle available to  unite bodies – at least as they are studied by physicists  – is motion. But it is clear that Leibniz thinks that the sort  of unity that might be provided by mere common motion is insufficient for genuine, substantial unity. Even if an army were to always march  in perfect step, it would still not be a unity per se, and  thus would not be “truly one being either” (G II 76/AG 79). Thus measured either by the standards of the independence  criterion or the unity criterion, Cartesian bodies can be shown to be not fully real in the deepest sense available to created beings. In  this thought, Leibniz sees not only an important contrast with human  minds – which as partless simples are plausibly independent and unified – but also another indication that mechanistic physics  must itself rest on a deeper metaphysical reality quite different  from the “manifest image” with which we are familiar from everyday experience. 