The unmoved mover (Ancient Greek: ὃ οὐ κινούμενος κινεῖ, ho ou kinoúmenos kineî, "that which moves without being moved") or prime mover (Latin: primum movens) is a monotheistic concept advanced by Aristotle, a polytheist, as a primary cause or "mover" of all the motion in the universe. As is implicit in the name, the "unmoved mover" moves other things, but is not itself moved by any prior action. In Book 12 (Greek "Λ") of his Metaphysics, Aristotle describes the unmoved mover as being perfectly beautiful, indivisible, and contemplating only the perfect contemplation: itself contemplating. 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 Quinque viae.
Contents
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 πρῶτον κινοῦν ἀκίνητον: 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.
Many of Aristotle's contemporaries complained that oblivious, powerless gods are unsatisfactory. Nonetheless, it was a life which Aristotle enthusiastically endorsed as one most enviable and perfect, the unembellished basis of theology. As the whole of nature depends on the inspiration of the eternal unmoved movers, Aristotle was concerned to establish the metaphysical necessity of the perpetual motions of the heavens. It is through the seasonal action of the Sun upon the terrestrial spheres, that the cycles of generation and corruption give rise to all natural motion as efficient cause. The intellect, nous, "or whatever else it be that is thought to rule and lead us by nature, and to have cognizance of what is noble and divine" is the highest activity, according to Aristotle (contemplation or speculative thinking, theōrētikē). It is also the most sustainable, pleasant, self-sufficient activity; something which is aimed at for its own sake. (In contrast to politics and warfare, it does not involve doing things we'd rather not do, but rather something we do at our leisure.) This aim is not strictly human, to achieve it means to live in accordance not with mortal thoughts, but something immortal and divine which is within humans. According to Aristotle, contemplation is the only type of happy activity which it would not be ridiculous to imagine the gods having. In Aristotle's psychology and biology, the intellect is the soul, (see also eudaimonia).
First cause
In book VIII of his Physics, Aristotle examines the notions of change or motion, and attempts to show by a challenging argument, that the mere supposition of a 'before' and an 'after', requires a first principle. He argues that in the beginning, if the cosmos had come to be, its first motion would lack an antecedent state, and as Parmenides said, "nothing comes from nothing". The Cosmological argument, later attributed to Aristotle, thereby draws the conclusion that God exists. However, if the cosmos had a beginning, Aristotle argued, it would require an efficient first cause, a notion that Aristotle took to demonstrate a critical flaw.
But it is a wrong assumption to suppose universally that we have an adequate first principle in virtue of the fact that something always is so … Thus Democritus reduces the causes that explain nature to the fact that things happened in the past in the same way as they happen now: but he does not think fit to seek for a first principle to explain this 'always' … Let this conclude what we have to say in support of our contention that there never was a time when there was not motion, and never will be a time when there will not be motion. (Physics VIII, 2)
The purpose of Aristotle's cosmological argument, that at least one eternal unmoved mover must exist, is to support everyday change.
Of things that exist, substances are the first. But if substances can, then all things can perish... and yet, time and change cannot. Now, the only continuous change is that of place, and the only continuous change of place is circular motion. Therefore, there must be an eternal circular motion and this confirmed by the fixed stars which are moved by the eternal actual substance substance that's purely actual.
In Aristotle's estimation, an explanation without the temporal actuality and potentiality of an infinite locomotive chain is required for an eternal cosmos with neither beginning nor end: an unmoved eternal substance for whom the Primum Mobile turns diurnally and whereby all terrestrial cycles are driven: day and night, the seasons of the year, the transformation of the elements, and the nature of plants and animals.
Even though the foregoing might have suggested that generation of substances is fundamental for all the other kinds of changes, in fact locomotion will have a privileged status. All other changes depend on locomotions, because any two entities involved in change, with their active and passive potentialities respectively, need to come into contact in order for the interaction to occur… Moreover locomotion is the form of change which can occur in isolation of generation, perishing and the other forms of change. (Physics 8.7, 260b26-29)
Aristotle argues at the opening of Physics bk. 8 that motion and change in the universe can have no beginning, because the occurrence of change presupposes a previous process of change. With this argument Aristotle can establish an eternal chain of motions and refute those who hold that there could have been a previous stationary state of the universe. Such an eternal chain, Aristotle argues, needs to rely on a cause which guarantees its persistence: if each of the constitutive processes in the causally connected web were of finite duration, for every one of them it can be the case that it is not present in the world, indeed, at some later time it will not be present any longer. But then the whole causally connected series of events, Aristotle submits, would also be contingent. Hence Aristotle postulates that the processes of the universe depend on an eternal motion (or on several eternal motions), the eternal revolution of the heavenly spheres, which in turn is dependent on one or several unmoved movers (Physics 8.6, 258b26-259a9).
The priority of the eternal celestial revolutions, furthermore, guarantees the causal finitude of the universe. This is so, even though there are infinite causal chains: behind every single individual of an animal species there is an infinite series of male ancestors, each causally responsible for the subsequent members in the series, because Aristotelian species are eternal and male parents are the efficient causes of their offspring. Left to its own devices, the finite universe on its own would swiftly reach a dissolution, a state of complete separation of the elemental masses into their concentrically arranged natural places. In view of the fact that such a complete segregation of the elemental masses is avoided through the constant excitation caused by the celestial motions, producing heat in the sublunary domain, especially around the regions of the Sun, Aristotle will be entitled to assert that the cause of the human being is in the first instance his or her father, but is at the same time the Sun as it moves along its annual ecliptic path. Between celestial revolutions and the individual natural processes there is always a finite causal chain, as these natural processes could not possibly have continued without the celestial motions. The infinite causal chains passing through male parents cannot subsist on their own without this constant external support, and this dependence can always be analysed in terms of finite causal chains.
Substance and change
Aristotle begins by describing substance, of which he says there are three types: the sensible, which is subdivided into the perishable, which belongs to physics, and the eternal, which belongs to "another science." He notes that sensible substance is changeable and that there are several types of change, including quality and quantity, generation and destruction, increase and diminution, alteration, and motion. Change occurs when one given state becomes something contrary to it: that is to say, what exists potentially comes to exist actually. (See Potentiality and actuality.) Therefore, "a thing [can come to be], incidentally, out of that which is not, [and] also all things come to be out of that which is, but is potentially, and is not actually." That by which something is changed is the mover, that which is changed is the matter, and that into which it is changed is the form.
Substance is necessarily composed of different elements. The proof for this is that there are things which are different from each other and that all things are composed of elements. Since elements combine to form composite substances, and because these substances differ from each other, there must be different elements: in other words, "b or a cannot be the same as ba."
Number of movers
Near the end of Metaphysics, Book Λ, Aristotle introduces a surprising question, asking "whether we have to suppose one such [mover] or more than one, and if the latter, how many." Aristotle concludes that the number of all the movers equals the number of separate movements, and we can determine these by considering the mathematical science most akin to philosophy, i.e., astronomy. Although the mathematicians differ on the number of movements, Aristotle considers that the number of spheres would be 47 or 55. Nonetheless, he concludes his Metaphysics, Book Λ, with a quotation from the Iliad: "The rule of many is not good; one ruler let there be."