In his first philosophy, later called the Metaphysics, (or “after the Physics”), Aristotle discusses the meaning of being. He refers to the unmoved mover (hyperagent), and assigns one to each movement in the heavens and tasks future astronomers with correlating the estimated 47 to 55 motions of the Eudoxan planetary model with the most current and accurate observations. According to Aristotle, the unmoved mover continuously contemplates its own contemplation; the planets and stars, which have their source of motion within themselves (by virtue of aether, Aristotle's fifth element) aspire to emulate the uniform circular motion of the unmoved mover. Thus captivated, their tireless performance is entirely the result of their own desire. This is one way in which the mover is said to be unmoved. Likewise, it must have no sensory perception whatsoever on account of Aristotle's theory of cognition: were any form of sense perception to intrude upon its thoughts, in that instant it would cease to be itself, because actual self-reflection is its singular essence, its whole being. Like the heavenly bodies in their unadorned pursuit, so the wise look, with affection, toward the star; and hence as a role model, they inspire those who look up to them, and by whom others still, will yet find themselves enthralled, and so on, creating the enduring natural order of aeon, season, animal and plant.
Aristotle concludes the unmoved mover is God. Metaphysics 12 1072b. "Hence it is actuality rather than potentiality that is held to be the divine possession of rational thought, and its active contemplation is that which is most pleasant and best. If, then, the happiness which God always enjoys is as great as that which we enjoy sometimes, it is marvellous; and if it is greater, this is still more marvellous. Nevertheless it is so. Moreover, life belongs to God. For the actuality of thought is life, and God is that actuality; and the essential actuality of God is life most good and eternal. We hold, then, that God is a living being, eternal, most good; and therefore life and a continuous eternal existence belong to God; for that is what God is."
In the Metaphysics, Aristotle discusses actuality (entelecheia, Greek: ἐντελέχεια) and potentiality (dynamis, Greek: δύναμις). The former is perfection, realization, fullness of being; the latter imperfection, incompleteness, perfectibility. The former is the determining, the latter the determinable principle. The unmoved movers are entirely actual, Actus Purus, because they are unchanging, eternal, immaterial substance.
Contrary to the later so-called "traditional" view of prime matter (prima materia in Latin), Aristotle asserts that there can be no pure potentiality without any actuality whatsoever. All material substances have unactualized potentials.
Aristotle argues that, although motion is eternal, there cannot be an infinite series of movers and of things moved. Therefore, there must be one, which is not the first in such a series, that inspire the eternal motion without itself being moved "as the soul is moved by beauty". Because the planetary spheres each move unfalteringly for all eternity in uniform circular motion with a given rotational period relative to the supreme diurnal motion of the sphere of fixed stars (or First Heaven), they must each love and desire to mimic the one unmoved mover corresponding to the given periods.
Because they eternally inspire uniform motion in the celestial spheres, the unmoved mover must itselve be eternal and unchanging. Because it is eternal, it has already had an infinite amount of time in which to actualize any potentialities and therefore cannot be a composition of matter and form, or potentiality and actuality. It must always be fully actual, and thus immaterial, because at all times in history it has already existed an infinite amount of time, and things that do not actually come to fruition given unlimited opportunities to do so cannot potentially do so.
The life of the unmoved mover is self-contemplative thought ("νοήσεως νόησις (noeseos noesis)", i.e. "thought of thought"). According to Aristotle, the gods cannot potentially be distracted from this eternal self-contemplation because, in that instant, they would cease to exist.
Aristotle's principles of being (see section above) influenced Anselm's view of God, whom he called "that than which nothing greater can be conceived." Anselm thought that God did not feel emotions such as anger or love, but appeared to do so through our imperfect understanding. The incongruity of judging "being" against something that might not exist, may have led Anselm to his famous ontological argument for God's existence.
Many medieval philosophers made use of the idea of approaching a knowledge of God through negative attributes. For example, we should not say that God exists in the usual sense of the term, all we can safely say is that God is not nonexistent. We should not say that God is wise, but we can say that God is not ignorant (i.e. in some way God has some properties of knowledge). We should not say that God is One, but we can state that there is no multiplicity in God's being.
Aristotelian theological concepts were accepted by many later Jewish, Islamic, and Christian philosophers. Key Jewish philosophers included Samuel Ibn Tibbon, Maimonides, and Gersonides, among many others. Their views of God are considered mainstream by many Jews of all denominations even today. Preeminent among Islamic philosophers who were influenced by Aristotelian theology are Avicenna and Averroes. In Christian theology, the key philosopher influenced by Aristotle was undoubtedly Thomas Aquinas. There had been earlier Aristotelian influences within Christianity (notably Anselm), but Aquinas (who, incidentally, found his Aristotelian influence via Avicenna, Averroes, and Maimonides) incorporated extensive Aristotelian ideas throughout his own theology. Through Aquinas and the Scholastic Christian theology of which he was a significant part, Aristotle became "academic theology's great authority in the course of the thirteenth century" and exerted an influence upon Christian theology that become both widespread and deeply embedded. However, notable Christian theologians rejected Aristotelian theological influence, especially the first generation of Christian Reformers and most notably Martin Luther. In subsequent Protestant theology, Aristotelian thought quickly reemerged in Protestant scholasticism.