Discernibility and Qualitative Difference

The Principle of the Identity of Indiscernibles (PII), according to which two objects are identical if they share all the same properties, has come in for much criticism. Michael Della Rocca has recently defended PII on the grounds that it is needed to forestall the possibility that where there appears to be only one object present, there is actually a multiplicity of exactly-overlapping such objects. Katherine Hawley has criticized this approach for violating a plausible “ground rule” in applying rules of indiscernibility to questions of identity: where there is putative duplication, it must be qualitatively significant. Hawley further suggests that with this rule in hand, one can tell the difference between the presence of one and two indiscernible objects without recourse to either PII or brute, nonqualitative individuation. In this paper, I critically examine Hawley’s contention and find that her appeal to “qualitatively significant duplication” fails since its application to distinct indiscernibles involves a difference that is primarily quantitative anyway. The upshot is a different proposed set of “ground rules” for applying the criterion of qualitative difference when seeking a grounding or explanation for distinctness and identity.