Quantum Haecceity.

There is an extensive philosophical literature on the interrelated issues of identity, individuality and distinguishability in quantum systems. A key consideration is whether quantum systems are subject to a strong form of individuality termed 'haecceity' (from the Latin for 'this-ness'). I argue that the traditional, strong form of haecceity does not apply at the quantum level, but that in order to properly account for the need for symmetrization in quantum systems, a weaker kind of haecceity must be involved, which I call . In the process, I also question some generally accepted tenets of the current debate, such as the idea that symmetrization of states for identical quanta must be postulated and reflects permutation invariance. Instead, I note that a perturbative Hamiltonian is required for exchange effects, which suggests that the need for symmetrization arises from specific physical conditions. This article is part of the theme issue 'Identity, individuality and indistinguishability in physics and mathematics'.