 Boyd (1999a) applies homeostasis to the analysis of kinds, both natural and social. According to Boyd, kinds are clusters of entities that stably have similar properties, with these similarities sustained by a causal homeostatic mechanism. Marriage, for instance, is a kind because there are many particular entities with similar properties (such as being formed by ceremonies, involving couples paired up, and so on), and because there are mechanisms causing entities to have and keep these properties. Like Millikan, Boyd argues that kinds are a product of actual tokens in the world and the causal processes in which those tokens are involved. Since his account involves actual causal patterns over time, kinds are historical, but they do not need to involve functional roles or evolution.