In philosophy, being means the material or immaterial existence of a thing. Anything that exists is being. Ontology is the branch of philosophy that studies being. Being is a concept encompassing objective and subjective features of reality and existence. Anything that partakes in being is also called a "being", though often this usage is limited to entities that have subjectivity (as in the expression "human being"). The notion of "being" has, inevitably, been elusive and controversial in the history of philosophy, beginning in Western philosophy with attempts among the pre-Socratics to deploy it intelligibly. The first effort to recognize and define the concept came from Parmenides, who famously said of it that "what is-is". Common words such as "is", "are", and "am" refer directly or indirectly to being.