Cultures and religions do not exist apart from the individual minds that constitute them and the environments that constrain them, any more than biological species and varieties exist independently of the individual organisms that compose them and the environments that conform them. They are not well-bounded systems or definite clusters of beliefs, practices, and artifacts, but more or less regular distributions of causally connected thoughts, behaviors, material products, and environmental objects. To naturalistically understand what "cultures" are is to describe and explain the material causes responsible for reliable differences in these distributions.

Introduction: an evolutionary riddle, p. 10 - In Gods We Trust: The Evolutionary Landscape of Religion (2002)