Ousia in the context of Form of the Good

The Theory of Forms or Theory of Ideas, also known as Platonic idealism or Platonic realism, is a philosophical theory credited to the Classical Greek philosopher Plato.

A major concept in metaphysics, the theory suggests that the physical world is not as real or true as Forms. According to this theory, Forms—conventionally capitalized and also commonly translated as Ideas—are the timeless, absolute, non-physical, and unchangeable essences of all things, which objects and matter in the physical world merely participate in, imitate, or resemble. In other words, Forms are various abstract ideals that exist even outside of human minds and that constitute the basis of reality. Thus, Plato's Theory of Forms is a type of philosophical realism, asserting that certain ideas are literally real, and a type of idealism, asserting that reality is fundamentally composed of ideas, or abstract objects.

Hylomorphism is a philosophical doctrine developed by the Ancient Greek philosopher Aristotle, which conceives every physical entity or being (ousia) as a compound of matter (potency) and immaterial form (act), with the generic form as immanently real within the individual. The word is a 19th-century term formed from the Greek words ὕλη (hyle: "wood, matter") and μορφή (morphē: "form"). Hylomorphic theories of physical entities have been undergoing a revival in contemporary philosophy.

Homoousion (/ˌhɒmoʊˈuːsiɒn, ˌhoʊm-/ HO(H)M-oh-OO-see-on; Ancient Greek: ὁμοούσιον, lit. 'same in being, same in essence', from ὁμός, homós, "same" and οὐσία, ousía, "being" or "essence") is a Christian theological term, most notably used in the Nicene Creed for describing Jesus (God the Son) as "same in being" or "same in essence" with God the Father (ὁμοούσιον τῷ Πατρί). The same term is also applied to the Holy Spirit in order to designate him as being "same in essence" with the Father and the Son. Those notions became cornerstones of theology in Nicene Christianity, and also represent one of the most important theological concepts within the Trinitarian doctrinal understanding of God.

Godhead (or godhood) refers to the essence or substance (ousia) of God in Christianity — God the Father, Son, and Holy Spirit.

Nontrinitarianism is a form of Christianity that rejects the Christian theology of the Trinity—the belief that God is three distinct hypostases or persons who are coeternal, coequal, and indivisibly united in one being, or essence (from the Ancient Greek ousia). Certain religious groups that emerged during the Protestant Reformation have historically been known as antitrinitarian.

According to churches that consider the decisions of ecumenical councils final, trinitarianism was definitively declared to be Christian doctrine at the 4th-century ecumenical councils, that of the First Council of Nicaea (325), which declared the full divinity of the Son, and the First Council of Constantinople (381), which declared the divinity of the Holy Spirit.

In Eastern Orthodox (Palamite) theology, there is a distinction between the essence (ousia) and the energies (energeia) of God. It was formulated by Gregory Palamas (1296–1359) as part of his defense of the Athonite monastic practice of Hesychasm against the charge of heresy brought by the humanist scholar and theologian Barlaam of Calabria.

Eastern Orthodox theologians generally regard this distinction as a real distinction, and not just a conceptual distinction. Historically, Western Christian thought, since the time of the Great Schism, has tended to reject the essence–energies distinction as real in the case of God, characterizing the view as a heretical introduction of an unacceptable division in the Trinity and suggestive of polytheism.

The Theory of Forms or Theory of Ideas, also known as Platonic idealism or Platonic realism, is a philosophical theory credited to the Classical Greek philosopher Plato.

A major concept in metaphysics, the theory suggests that the physical world is not as real or true as Forms (or Ideas, typically capitalized): the timeless, absolute, non-physical, and unchangeable essences of all things, which objects and matter in the physical world merely participate in, imitate, or resemble. In other words, Forms are various abstract ideals that exist even outside of human minds and that constitute the basis of reality. Thus, Plato's Theory of Forms is a type of philosophical realism, asserting that certain ideas are literally real, and a type of idealism, asserting that reality is fundamentally composed of ideas, or abstract objects.

Tomus ad Antiochenos is a letter or mediation proposal written by Bishop Athanasius of Alexandria on behalf of a regional synod he convened in Alexandria in 362, addressed to a group of bishops seeking a solution to the schism between "Eustathians" and "Meletians" in the parishes of Antioch. This letter played a key role in the Trinitarian theological debates between the one-hypostasis model and the three-hypostasis model of the Trinity, anticipating the turning point in this question from the 370s onward.

The central concern is to achieve theological agreement based on the Nicene Creed. By recognizing that certain theological points of contention in the Arian controversy were based not only on differences of belief, but also on different language rules or conceptual differences between Latin and Greek, the Tomus paved the way for the Trinitarian theological language rules of the three Cappadocians from the 370s onward: Basil of Caesarea, his brother Gregory of Nyssa, and their mutual friend Gregory of Nazianzus subsequently established the concept of the one being (Ousia) and the three hypostases of God, which became binding with the decision of the First Council of Constantinople in 381.