This paper shows that so-called "threshold concepts" have been defined in a way that makes it impossible, even in principle, to empirically isolate them. It continues by proposing an alternative theoretical framework, and argues: (1) that concepts are not reducible to abilities; (2) that acquisition of a given concept can be necessary, but not sufficient, for the possession of an ability; and (3) that being "threshold" is an extrinsic property, such that what is threshold for one person is not for another. It closes by outlining two resultant problems for related empirical research. First, how is it possible to test for concepts, rather than abilities? Second, how can we tell if there is more than one possible conceptual route to the same ability?