The unmoved mover () or prime mover () is a concept advanced by Aristotle as a primary cause (or first uncaused cause) or "mover" of all the motion in the universe. As is implicit in the name, the moves other things, but is not itself moved by any prior action. In Book 12 () of his Metaphysics, Aristotle describes the unmoved mover as being perfectly beautiful, indivisible, and contemplating only the perfect contemplation: self-contemplation. He equates this concept also with the active intellect. This Aristotelian concept had its roots in cosmological speculations of the earliest Greek pre-Socratic philosophers and became highly influential and widely drawn upon in medieval philosophy and theology. St. Thomas Aquinas, for example, elaborated on the unmoved mover in the .
First philosophy
Aristotle argues, in Book 8 of the Physics and Book 12 of the Metaphysics, "that there must be an immortal, unchanging being, ultimately responsible for all wholeness and orderliness in the sensible world".
In the Physics (VIII 4–6) Aristotle finds "surprising difficulties" explaining even commonplace change, and in support of his approach of explanation by four causes, he required "a fair bit of technical machinery". This "machinery" includes potentiality and actuality, hylomorphism, the theory of categories, and "an audacious and intriguing argument, that the bare existence of change requires the postulation of a first cause, an unmoved mover whose necessary existence underpins the ceaseless activity of the world of motion". Aristotle's "first philosophy", or Metaphysics ("after the Physics"), develops his peculiar theology of the prime mover, as : an independent divine eternal unchanging immaterial substance.
Celestial spheres
Aristotle adopted the geometrical model of Eudoxus of Cnidus, to provide a general explanation of the apparent wandering of the classical planets arising from uniform circular motions of celestial spheres. While the number of spheres in the model itself was subject to change (47 or 55), Aristotle's account of aether, and of potentiality and actuality, required an individual unmoved mover for each sphere.
Final cause and efficient cause
Despite their apparent function in the celestial model, the unmoved movers were a final cause, not an efficient cause for the movement of the spheres; they were solely a constant inspiration, and even if taken for an efficient cause precisely due to being a final cause, the nature of the explanation is purely teleological.
Aristotle's theology
The unmoved movers, if they were anywhere, were said to fill the outer void, beyond the sphere of fixed stars:
The unmoved movers are, themselves, immaterial substance (separate and individual beings), having neither parts nor magnitude. As such, it would be physically impossible for them to move material objects of any size by pushing, pulling, or collision. Because matter is, for Aristotle, a substratum in which a potential to change can be actualized, any and all potentiality must be actualized in a being that is eternal but it must not be still, because continuous activity is essential for all forms of life. This immaterial form of activity must be intellectual in nature and it cannot be contingent upon sensory perception if it is to remain uniform; therefore, eternal substance must think only of thinking itself and exist outside the starry sphere, where even the notion of place is undefined for Aristotle. Their influence on lesser beings is purely the result of an "aspiration or desire", and each aetheric celestial sphere emulates one of the unmoved movers, as best it can, by uniform circular motion. The first heaven, the outmost sphere of fixed stars, is moved by a desire to emulate the prime mover (first cause), in relation to whom, the subordinate movers suffer an accidental dependency.