John Locke holds that matter is solid, the soul thinks, and for all we know the soul may be a material substance divinely endowed with a power to think. Though he openly admits to nothing stronger than the bare possibility of thinking matter, Locke grants that what thinks in us occupies a definite spatial location to the exclusion of other souls. Solidity is the quality that prevents other things from occupying a spatial location. Locke’s general criterion for identity is spatiotemporal exclusion of other things of the same kind. To meet these conditions for identity, souls must be solid. Although Locke refuses to declare that souls really are material things, taking the solidity of souls to be a condition for their identity is consistent with the following of Locke’s other important commitments: (1) nominalism about the essences by which substances are classified, (2) agnosticism about the underlying reality of what supports such “nominal essences,” and (3) the identity of persons is distinct from the identity of any substance. Locke ignores the implication that souls are solid because the solidity of souls is irrelevant to those three aims. Nevertheless he could allow for the solidity of souls without giving up on any of his other important and explicitly held commitments. There is therefore no need for Locke’s commentators to refrain from employing solidity in their accounts of Locke’s general criterion for identity from fear of attributing to Locke the position that souls would be solid.
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