Although it was only in the first half of the twentieth century that the term ‘personalism’ became known as a designation of philosophical schools and systems, personalist thought had developed throughout the nineteenth century as a reaction to perceived depersonalizing elements in Enlightenment rationalism, pantheism, Hegelian absolute idealism, individualism as well as collectivism in politics, and materialist, psychological, and evolutionary determinism. In its various strains, personalism always underscores the centrality of the person as the primary locus of investigation for philosophical, theological, and humanistic studies. It is an approach or system of thought which regards or tends to regard the person as the ultimate explanatory, epistemological, ontological, and axiological principle of all reality, although these areas of thought are not stressed equally by all personalists and there is tension between idealist, phenomenological, existentialist, and Thomist versions of personalism.