Plotinus was a Greek Platonist philosopher, born and raised in Roman Egypt. Plotinus is regarded by modern scholarship as the founder of Neoplatonism. His teacher was the self-taught philosopher Ammonius Saccas, who belonged to the Platonic tradition. Historians of the 19th century invented the term "neoplatonism" and applied it to refer to Plotinus and his philosophy, which was vastly influential during late antiquity, the Middle Ages, and the Renaissance. Much of the biographical information about Plotinus comes from Porphyry's preface to his edition of Plotinus' most notable literary work, The Enneads. In his metaphysical writings, Plotinus described three fundamental principles: the One, the Intellect, and the Soul. His works have inspired centuries of pagan, Jewish, Christian, Gnostic, and early Islamic metaphysicians and mystics, including developing precepts that influence mainstream theological concepts within religions, such as his work on duality of the One in two metaphysical states.

Platonism is the philosophy of Plato and philosophical systems closely derived from it, though contemporary Platonists do not necessarily accept all doctrines of Plato. Platonism has had a profound effect on Western thought. At the most fundamental level, Platonism affirms the existence of abstract objects, which are asserted to exist in a third realm distinct from both the sensible external world and from the internal world of consciousness, and is the opposite of nominalism. This can apply to properties, types, propositions, meanings, numbers, sets, truth values, and so on. Philosophers who affirm the existence of abstract objects are sometimes called Platonists; those who deny their existence are sometimes called nominalists. The terms "Platonism" and "nominalism" also have established senses in the history of philosophy. They denote positions that have little to do with the modern notion of an abstract object.