Naturalism (philosophy), in philosophy, a movement affirming that nature is the whole of reality and can be understood only through scientific investigation. Denying the existence of the supernatural and de-emphasizing metaphysics, or the study of the ultimate nature of reality, naturalism affirms that cause-and-effect relationships, as in physics and chemistry, are sufficient to account for all phenomena. Teleological conceptions, which suggest design and metaphysical necessity in nature, while not necessarily invalid, are excluded from consideration. The ethical implication, since the naturalist denies any transcendent or supernatural end for humankind, is that values must be found within the social context. It is impossible to determine what is best in an ultimate context, because the ultimate is beyond discovery. Values, therefore, are relative, and ethics is based on custom, inclination, or some form of utilitarianism, the doctrine that what is useful is good.
Naturalism is rooted in British empiricism, which holds that all knowledge is derived from experience, and in European positivism, the doctrine that denies any validity to metaphysical speculation. It reached its peak in the late 19th- and 20th-century works of the American philosophers George Santayana, John Dewey, and their followers.