 On this interpretation, Aristotle must deny the core of (4), which is that the behaviour of complexes derives necessarily from the primitive causal powers of the elements and their spatial relations—this is a ’bottom up’ atomist principle., and presupposes that the parts retain their identity within composites. But without this core idea in (4), it is difficult to see how there can be ‘closure under physics’, for if the atoms did not retain their identity, how could the nomic structure that rests on them be preserved?