With regards to ‘naturalism’ as a philosophical and scientific outlook rather than the literary and artistic movement proffered by Zola, the doctrine of ‘naturalism’ is characterised by a refusal to idealise experience and by the belief that everything is composed of natural, physical entities. In this respect, proponents of ‘naturalism’ reject spiritual, supernatural or teleological explanations.
Naturalism is closely tied to materialism and the development of ‘mainstream’ scientific methodology and authority. For example, Darwin’s theory of evolution had a big impact on the advancement of scientific naturalism and by the end of the nineteenth-century, naturalist philosophies began to dominate where idealism had hitherto reigned unchallenged. Naturalism’s ranks included figures such as, H. Spencer, J. Tyndall and T. H. Huxley.
Robert Audi argues that philosophy in the twentieth-century was primarily anti-naturalist. For example, both phenomenology and analytic philosophy united in a rejection of psychologism, a strand of naturalism on which empirical discoveries about mental process are crucial for understanding the nature of knowledge, language, and logic.
T. Honderich, The Oxford companion to philosophy (Oxford, 2005), pp.604-6.
R. Audi, The Cambridge Dictionary of Philosophy (Cambridge, 2015), pp.517-8. https://www.cambridge.org/core/books/the-cambridge-dictionary-of-philosophy/50389231FC1A5DF1B1BF0F4140264792