 Philosophers of science have explored whether mental illnesses might be classified as natural kinds, the claim made about particular diagnostic categories or symptom clusters (depression, schizophrenic disorder, and autism, for example), as well as about separate symptoms (Cooper 2007; Samuels 2009). Bolstered by recognition that even the category of biological species would not meet an essentialist criterion for classification as natural kinds, a “homeostatic property cluster” account of natural kinds has been proposed (Ellis 2001; Cooper 2007; Murphy 2007; Samuels 2009). (See entry on Species.) The expectation here is merely that a natural kind must be able to ground explanations and inductive inferences, and enable effective human intervention. Linked by shared causal properties such as genes, on this view, their homeostatic aspect ensures that property clusters making up separate disorders (or symptoms) will remain stable enough to behave the same way in response to the same conditions, due to a similarity-generating mechanism explaining their co-occurrence. (That comparable stability and consequent predictive properties could arise not from shared causal properties such as genes, but as the result of feedback loops binding symptoms together in stable clusters has also been proposed. See essays in Kincaid and Sullivan 2014.)