A principle feature of Platonism, which refers to the doctrines or philosophies influenced by Plato, is the belief in the existence of a distinction between the world that appears to the senses and a real realm that can be grasped only by the intellect. This latter realm contains the transcendental Ideas or Forms which, although existing independently of both the empirical world and human consciousness, may be apprehended through a human capacity of central import to Platonism, namely, that of reason. Reason, however, is not posited solely as a capacity possessed by humans, but is also understood more expansively as an ordering principle that extends throughout the entirety of the universe.
Platonism can be said to begin at Plato's death in 347 B.C.E. when his nephew Speusippus (c. 410–c. 339 B.C.E.) succeeded him as head of the Academy (founded by Plato in 386 B.C.E.). Under Speusippus, a Pythagorean metaphysics linking thought and numbers was drawn to the fore. The influence of the Epicurean and Stoic schools eventually superceded the preeminent role of the Academy, which turned toward the tradition of the Skeptics in its attempt to counter Stoicism. Confluent in many ways with the critical stance of Plato's teacher Socrates, the initial turn toward the Skeptic influence is generally associated with Arcesilaus (c. 316–c. 241 B.C.E.), while the further amplification of this tendency is associated with Carneades (213–129 B.C.E.).
With the decline of the Stoic influence, Neoplatonism, a new and highly significant strain of Platonism, emerged in the third century C.E. The influence of Pythagoreanism, already strongly imbued in Platonist thought, was here combined with features of Aristotelianism—although with Neoplatonism both the mathematical and quasi-religious features of Pythagorean thought were greatly emphasized. Traceable to Ammonius Saccas (175?–c. 242), Neoplatonism was founded by his student Plotinas, the Egypto-Roman philosopher (205–270). In its broad fundamentals, Neoplatonism appears as an effort to articulate a relation between the two spheres that comprise the premier division in Platonism, that of the world of sense and that of the transcendent realm of Ideas. In Neoplatonism, the dichotomy between the two spheres emerged as a hierarchy of Being: in descending order, the One, Intellect or Mind, and the Soul, the Soul serving as the mediating functionary. Among those whose writings helped prepare the way for Neoplatonism was Philo (c. 20 B.C.E.–c. 50 C.E.), a notable member of the Egyptian Jewish diaspora whose writings on Hebrew scripture were based on Jewish religious revelation and highly influenced by the theory of Forms. Philo located the Forms within the Divine Mind; as preexisting features, these Forms serve as the pattern for the sensible world.
The significance exerted by Neoplatonist thought on Christian theology is traceable to Origen (185?–254? C.E.), who took from the Platonist tradition the doctrines of the preexistence of the soul and reincarnation and for whom, as for Philo, reason and revelation were not clearly distinguished. The relation between Neoplatonist philosophy and Christianity is most fully and richly pronounced, however, in the writings of St. Augustine (354–430). Augustine, a student of Neoplatonism before his conversion to Christianity, maintained the importance of this influence in his evolution toward the Christian faith. The Neoplatonist features of Augustine's thought emerge in his linking of human knowledge to an illumination by the Divine Mind and the positing of truth as existing in the mind of God. As with Philo and Origen, no absolute demarcation between reason and spiritual advancement exists in the writings of Augustine, for whom the highest level of philosophic knowledge is also at once a state of beatitude.
Neoplatonism exerted an influence not only upon Christianity, but also upon Islamic philosophy. Al-Kindi (c. 801–873) was the first of the Islamic philosophers influenced by Neoplatonism; he combined Neoplatonist thought with Aristotelian philosophy. Another significant figure was Avicenna (980–1037), who, like al-Kindi, is generally classified as an Aristotelian and was likewise influenced by Neoplatonism in his attempt to combine Platonist and Aristotelian traditions. The Neoplatonist proclivity for melding reason with spiritual insight emerged in the Middle Ages in the writings of St. Anselm (1033?–1109) and St. Bonaventure (1221–1274); the rationalist writings of the former were carried out under the Augustinian motto of credo ut intelligam ("I believe in order to understand").
Despite the fact that by the thirteenth century Aristotle had largely supplanted Plato as the preemininent source of philosophic influence, a revival of Platonism occurred during the Renaissance. Inspired by Plato's Athenian Academy, the Florentine Academy was founded with the support of Cosimo de' Medici (1389–1464) in the mid-fifteenth century. Among the more influential figures populating the Academy were Marsilio Ficino (1433–1499) and Giovanni Pico della Mirandola (1463–1494); Ficino's rendering of Platonic love exerted considerable influence on the literature of the sixteenth century and beyond. In the European literature of successive centuries, both the doctrine of Platonic love and the notion of an unchanging transcendent true world resonated strongly, as, for example, in the sixteenth-century writings of Edmund Spenser (1552–1599) and in the Romanticism of the nineteenth century as found in the writings of Percy Bysshe Shelley (1792–1822) and William Wordsworth (1770–1850).
In the latter portion of the seventeenth century, the Neoplatonist tendency toward religiosity and mysticism emerged again in a group referred to as the Cambridge Platonists, for whom divine authority asserted the transcendent existence of ethical ideas. The premier enemy of the Cambridge Platonists was materialism; particularly that variation articulated in the writings of Thomas Hobbes (1588–1679). Benjamin Whichcote (1609–1683) and Henry More (1614–1687) number among this group, although Ralph Cudworth (1617–1688) is generally considered to be the most significant figure. Platonism was utilized not only as a counter to materialism but as a means to rethink empiricist philosophy, as found in the response of Richard Price (1723–1791) to the philosophy of John Locke (1632–1704). Among the German idealists, a Platonic inflection is found in the writings of Immanuel Kant (1724–1804), while similar tendencies condition the work of Georg Wilhelm Friedrich Hegel (1770–1831). In the late nineteenth and early twentieth centuries, a strikingly Platonic metaphysics occurred in the work of the Hegelian John Ellis McTaggart (1866–1925), for whom reality is a system of spiritual substances that reveal the unreality of both matter and temporality.
The influence of the Platonist rejection of the immediacies of sensible existence is likewise found in the writings of G. E. Moore (1873–1958). A common-sense realist, Moore rejected both materialism and idealism in favor of a philosophic realism. Moore was among the more energetic critics of idealism at the turn of the century, and his realism maintained a separation between the mental act of knowing and the objects of knowledge, the latter existing as interconnected objects independent of thought. Finally, Alfred North Whitehead (1861–1947) has been called the last and greatest of the Cambridge Platonists. A critic of positivism who sought a unified scientific religion, Whitehead articulated universal absolutes as eternal objects—and it was Whitehead who famously commented that all philosophy was "a series of footnotes to Plato."