 Theoretical identity statements are of interest across the gamut of philosophy. Various writers appeal to rigid designation to support views about the essences not only of kinds from chemistry or biology but also kinds from philosophical subdisciplines apparently far removed from empirical science: ethics and aesthetics, for example (see, e.g., Carney 1982; Funkhouser 2014, pp. 4, 164; Gampel 1997; Leddy 1987; Noonan 2020). The reader interested in pursuing any of these traditions should beware that attempts to apply rigidity in the foregoing subdisciplines, especially early attempts, sometimes confuse rigidity with other phenomena recognized by popular theories of reference: see clarifications in section 2.2. 