Merriam-Webster Collegiate Dictionary - naturalism

 noun  Date: circa 1641  1. action, inclination, or thought based only on natural desires and instincts  2. a theory denying that an event or object has a supernatural significance; specifically the doctrine that scientific laws are adequate to account for all phenomena  3. realism in art or literature; specifically a theory or practice in literature emphasizing scientific observation of life without idealization and often including elements of determinism NATURALIST  noun  Date: 1587  1. one that advocates or practices naturalism  2. a student of natural history; especially a field biologist