  As we have seen, Locke develops some central theses of his Essay in connection with the corpuscular hypothesis. In his theory of ideas, corpuscles provide at least a structural basis for simple ideas, and depending upon one’s interpretation, there may be a causal relationship as well. Further, and of particular interest here, Locke often appears to identify a material substance’s real essence with the set or some subset of its component particles’ primary qualities. In the following well-known passage, for instance, he points to the primary qualities of a body’s parts—their bulk or solidity, motion, and shape—as the causal ground of the qualities we perceive.