Classical Theism in the Three Abrahamic Traditions

The classical concept of God — simple, eternal, immutable, omnipotent, omniscient, the necessary creator — was not the product of any one religion but the joint achievement of philosophers working in Judaism, Christianity, and Islam, in close and often explicit conversation across the three. The medieval synthesis of biblical monotheism with Greek (chiefly Aristotelian and Neoplatonic) philosophy was a shared project: Jewish, Christian, and Muslim thinkers read, translated, criticised, and built upon one another, and the arguments and attributes treated throughout these notes bear the marks of all three. This page traces that cross-pollination — the falsafa tradition of Islam, the negative theology of Judaism, and the Christian scholastic synthesis.

It complements the history page (chronological) by focusing on the interaction of the traditions, and the concept of God and attributes pages (which it historically grounds).

Islamic philosophy: kalām and falsafa

Islamic thought developed two distinct modes of theological reasoning:

Kalām (dialectical theology). The Mu'tazila ("rationalist") and Ash'arī schools debated God's attributes, justice, and human freedom. The Mu'tazila stressed divine justice and unity — arguing that reason can know good and evil, that God must do what is just (a near-intellectualist view), and that humans have real freedom (else God's punishments are unjust). The Ash'arīs (al-Ash'arī, al-Ghazālī) countered with a stronger voluntarism and occasionalism: good and evil are what God commands (divine-command ethics), and God is the sole true cause (occasionalism) — human "acquisition" (kasb) of God-created acts, a distinctive position in the free-will debate. The kalām also produced the Kalām cosmological argument (al-Kindī, al-Ghazālī): the world cannot have an infinite past, so it began, so it has a cause.
Falsafa (philosophy proper). The Arabic Aristotelians transmitted and transformed Greek philosophy:
- al-Fārābī and especially Ibn Sīnā (Avicenna, d. 1037) built a grand Neoplatonised-Aristotelian system. Avicenna's signature contribution is the distinction between essence and existence and the contingency argument: beings are divided into the possible/contingent (whose essence does not entail existence — they are "possible in themselves, necessary through another") and the Necessary Existent (wājib al-wujūd), whose essence is existence, which exists by its own nature and is the ground of all else. This is one of the most important arguments in the history of natural theology and directly shaped Aquinas's essence/existence metaphysics and Leibniz's contingency argument.
- al-Ghazālī (d. 1111), in The Incoherence of the Philosophers, mounted a powerful critique of the falāsifa — attacking their claims about the world's eternity and necessary causation, and defending occasionalism and the possibility of miracles (his analysis of causation as mere constant conjunction strikingly anticipates Hume).
- Ibn Rushd (Averroes, d. 1198) replied to al-Ghazālī in The Incoherence of the Incoherence, defending Aristotle and a more robust natural causation, and reflecting on the relation of philosophy and revelation (the "double truth" attributed to him, and the view that philosophy and scripture are two routes to one truth). Averroes's commentaries on Aristotle became the authority — "the Commentator" — for the Latin scholastics.

Jewish philosophy: Maimonides and negative theology

Medieval Jewish philosophy reached its height with Moses Maimonides (Rambam, d. 1204), writing in Arabic and deeply engaged with the falsafa. His Guide of the Perplexed sought to reconcile Torah with Aristotelian philosophy, and his most influential contribution to the philosophy of religion is a rigorous negative theology (the via negativa):

Because God is absolutely one and simple (divine simplicity), and utterly unlike creatures, no positive attribute can be predicated of God literally without compromising his unity and transcendence. To say "God is wise" or "God is powerful" cannot mean God has wisdom or power as properties (that would introduce plurality into the divine).
So positive predications must be reinterpreted as negations (God is "living" = God is not inanimate; "one" = not multiple) or as statements about God's effects/actions (God is "merciful" = God acts as a merciful person would), not about God's essence. We can know that God is and what God does, but of God's essence we can only say what it is not.

Maimonides' negative theology is the most thoroughgoing in the Abrahamic traditions, a direct influence on Aquinas (who tempered it with analogy) and a key text for the religious-language debate. Earlier Jewish thinkers — Saadia Gaon (kalām-influenced) and the Neoplatonist Ibn Gabirol (Avicebron, whose Fons Vitae was read by Christians as the work of a Muslim or Christian) — show the same cross-traditional traffic.

Christian scholasticism

Latin Christendom inherited this rich Arabic and Jewish material — Aristotle and Avicenna, Averroes, and Maimonides — chiefly through the 12th–13th century translations, and synthesised it:

Anselm (d. 1109), earlier and working from Augustine, gave the ontological argument and the perfect-being method.
Thomas Aquinas (d. 1274) achieved the great synthesis. His Five Ways draw on Aristotle (the unmoved mover), Avicenna (the contingency argument, essence/existence — God as ipsum esse subsistens), and Maimonides; his theory of analogy mediates between Maimonides' pure negation and a naive univocity; his account of faith and reason (reason establishes the preambles, faith the mysteries) is the classic synthesis model; and his treatment of divine simplicity, eternity, omniscience, and providence is the touchstone of classical theism. Aquinas is, in a real sense, unintelligible without his Muslim and Jewish predecessors — he cites "Rabbi Moses" (Maimonides) and "the Commentator" (Averroes) constantly.
The later scholastics — Duns Scotus (the univocity of being, against Aquinas's analogy; the modal refinement of the ontological argument) and William of Ockham (nominalism, the hard/soft-fact solution to foreknowledge, divine-command voluntarism) — carried the debates forward and set the stage for the early-modern transformations.

A shared project

The historical lesson is that classical theism is interreligious. The same arguments and attributes were forged in a centuries-long, cross-confessional conversation:

The cosmological argument passed from Aristotle through the Muslim falāsifa (Avicenna's contingency argument, al-Ghazālī's kalām) and Maimonides to Aquinas and on to Leibniz.
Divine simplicity and negative theology were developed jointly by Neoplatonists, Maimonides, Avicenna, and Aquinas.
The foreknowledge, omnipotence, and free-will debates ran in parallel (Ash'arī occasionalism and kasb; Molinist middle knowledge; Ockhamist soft facts) across all three traditions.
The faith-and-reason question (Averroes, Maimonides, Aquinas) received its classic synthesis answers from thinkers in all three faiths.

This shared inheritance is also why the problem of religious diversity is internal as well as external: the three traditions agree on a vast common philosophical theology while dividing sharply on revelation and salvation — a structure that the pluralism debate must address.

Where the Abrahamic traditions sit

The classical concept of God and the arguments and attribute-debates that fill these notes are the deposit of a shared Abrahamic project — Greek philosophy wedded to biblical/Quranic monotheism through the joint labour of Muslim falāsifa (Avicenna's contingency argument and essence/existence metaphysics, al-Ghazālī's Kalām and occasionalism, Averroes's Aristotelianism), Jewish philosophers (Maimonides' negative theology and simplicity), and Christian scholastics (Anselm's ontological argument, Aquinas's grand synthesis, Scotus and Ockham's refinements). Recognising this cross-pollination corrects the parochial impression that natural theology is a Christian invention, situates the Western canon in its true multi-confessional setting, and frames the diversity problem as a division within a shared philosophical monotheism as much as between it and the non-Western conceptions. The mysticism of the three traditions — Kabbalah, Christian and Sufi mysticism — is the experiential counterpart to this shared philosophical theology.