In the functional mental disorders, where conjointly definitive pathophysiology and etiology are unknown or nonexistent, a disease entity is specified syndromically, by the statistical relations of signs, symptoms, traits, tests, and other data. The very concept of co-morbidity becomes problematic, and its application to individual patients is difficult. Coherent cut kinetics, the taxometric method composed of multiple procedures (e.g., MAMBAC, MAXCOV, MAXSLOPE, MAXEIC, L-Mode) developed by Meehl and colleagues is a partial objective solution. The importance of the INKS condition, a concept from philosophy of science, is explained. Four situations of putative comorbidity are defined with explanations of how taxometric results would distinguish them.