Philosophy can be defined as the study of the fundamental nature of knowledge, reality, and existence. Philosophy has a long history. The traditional branches of philosophy are metaphysics, epistemology, logic, aesthetics, and ethics. In the 20th Century, the philosophical foundations of science and linguistic analysis occupied the work many Anglo-American philosophers; whereas in Europe, critical theory and post-modernist interpretations of philosophical problems emerged. The following is a select list of reference and research materials in philosophy that is aimed at both undergraduate and graduate study. All resources selected are available in the reference section of the Hoose Library of Philosophy. Many of the resources are also available in the Reference Center of the Doheny Library and the Information Commons of the Leavey Library.