Middle Platonism is the modern name given to a stage in the development of Platonic philosophy, lasting from about 90 BC – when Antiochus of Ascalon rejected the scepticism of the new Academy – until the development of neoplatonism under Plotinus in the 3rd century. Middle Platonism absorbed many doctrines from the rival Peripatetic and Stoic schools. The pre-eminent philosopher in this period, Plutarch, defended the freedom of the will and the immortality of the soul. He sought to show that God, in creating the world, had transformed matter, as the receptacle of evil, into the divine soul of the world, where it continued to operate as the source of all evil. God is a transcendent being, who operates through divine intermediaries, which are the gods and daemons of popular religion. Numenius of Apamea combined Platonism with neopythagoreanism and other eastern philosophies, in a move which would prefigure the development of neoplatonism.

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.