  Quasi-set theory $\cal Q$ allows us to cope with certain collections of
objects where the usual notion of identity is not applicable, in the sense that
$x = x$ is not a formula, if $x$ is an arbitrary term. $\cal Q$ was partially
motivated by the problem of non-individuality in quantum mechanics. In this
paper I discuss the range of explanatory power of $\cal Q$ for quantum
phenomena which demand some notion of indistinguishability among quantum
objects. My main focus is on the double-slit experiment, a major physical
phenomenon which was never modeled from a quasi-set-theoretic point of view.
The double-slit experiment strongly motivates the concept of degrees of
indistinguishability within a field-theoretic approach, and that notion is
simply missing in $\cal Q$. Nevertheless, other physical situations may
eventually demand a revision on quasi-set theory axioms, if someone intends to
use it in the quantum realm for the purpose of a clear discussion about
non-individuality. I use this opportunity to suggest another way to cope with
identity in quantum theories.
