1. Biographical Information

Aristotle was born in Stagira, a Greek colony in Macedonia in 384 BCE and died in 322 BCE.  At Stagira, Aristotle's father was the personal physician to the King of Macedonia, Amyntas.  In 367 BCE, Aristotle became a pupil of Plato at the Academy in Athens, where he remained for over 20 years. Upon the death of Plato in 347 BCE, Aristotle had hopes of being named as Plato's replacement as the director of the Academy, but was disappointed in this.  From 347 to 343 BCE, Aristotle traveled among the Greek islands and Asia Minor.  In 343 BCE, he accepted the invitation of Philip, King of Macedonia, to become the personal tutor to his son, Alexander, who would later become known as Alexander the Great.  In 336 BCE, Philip was assassinated, and Alexander succeeded his father as king.  His tutoring days now at an end, Aristotle left for Athens, where he founded his own school in the gymnasion called the Lyceum; Aristotle's followers were sometimes called the peripatetics, probably because Aristotle used to lecture peripatôs in covered walkways or stoa.  Upon Alexander's death in 323 BCE, Athens revolted against Macedonian rule; Aristotle, being considered pro-Macedonian, fled to the city of Chalcis, where he died the next year.  What remains of Aristotle's writings are his lecture notes, which are extensive; he wrote dialogues, as did Plato, but these have been lost.

2. The Unmoved Mover in Physics

Aristotle resolves the problem of how something can become something else inherited from his predecessors by differentiating between the potentiality (dunamis) and actuality (entelecheia) inhering in a substratum or matter. He defines motion (kinêsis) as "the fulfillment of what exists potentially, insofar as it exists potentially" (Physics 3.1; 201a 10-12). A thing is in a state of actuality, meaning that it is what it is, but it also is potentially something else. Its potentiality is, as it were, an attribute of thing as actual. Aristotle explains further, "It is the fulfilment of what is potential when it is already fully real and operates not as itself but as movable, that is motion. What I mean by 'as' is this: Bronze is potentially a statue. But it is not the fulfilment of bronze as bronze which is motion. For 'to be bronze' and 'to be a certain potentiality' are not the same" (Physics 3.1; 201a). The actuality of a thing as movable, that is to say, its potentiality as moved, is motion. Potentiality inheres in a thing and it is the thing as this potentiality in the process of being actualized that can be said to be in motion. For example, a green tomato has the potentiality to be a red tomato. The actualization of its potentiality to be red is motion, in particular, alteration from being green to being red. Aristotle explains further,

We can define motion as the fulfilment of the movable as movable, the cause of the attribute being contact with what can move so that the mover is also acted on. The mover or agent will always be the vehicle of a form, either a 'this' or 'such', which, when it acts, will be the source and cause of the change, e.g. the full-formed man begets man from what is potentially man. (Physics 3.2; 202a)

A thing as movable is moved by contact with an efficient cause, or a mover. The mover as moved becomes the means by which a form comes to inhere in another moving and then moved thing. It is the actualization of the thing as movable through its contact with a moved efficient cause that is motion. Aristotle would agree with Parmenides and Melissos that being does not come from non-being in an unqualified sense. But he asserts that being comes from non-being in a qualified sense as the actualization of a potentiality; potentiality is qualified non-being. In this way one can say that something both comes and does not come from something else: it comes from the potentiality inherent in something but does not come from what is actually existing.

From his considerations of the nature of motion in Physics, in Book 8, Aristotle concludes that there must be a logically first unmoved mover in order to explain all other motion. In Physics 8.1, he argues that motion is eternal. Motion cannot begin without the prior existence of something to impart motion in another thing, so that there will always be something in motion, since something at rest cannot cause motion in another thing.  In addition, if motion were not eternal, then time would not have always existed, since time is the measure of motion; but, according to Aristotle, no one would be willing to say that time has not always been in existence.  Nor can motion cease, since to do so something must cause it to cease, but then the thing that caused motion to cease would require something to cause its cessation and the process would continue ad infinitum.  Aristotle concludes, "That there never was a time when there was not motion, and never will be a time when there will not be motion" (252b 6-8). Aristotle also objects to the idea that motion may have begun self-caused; he points out that, in those things in which motion is said to be "self-caused," in fact, there is a part of the thing that is already in motion and imparts motion to the whole.  Self-caused means that motion is not imparted from without but from some part of the whole that is already in motion. In such cases, the motion of the part that moves the other parts of a things requires a mover.

Since everything is moved by something and since motion is eternal, Aristotle concludes that there must be something that imparts motion without itself being moved; otherwise, there would be an infinite regress of movers, the moved and instruments of moving, which is unacceptable (Physics 8.5).  (An axiom for Aristotle is that an infinite regress is impossible.) According to Aristotle, all movable things are only potentially in motion, and require something else to act upon them in order to be set in motion: "So it is clear that in all these cases the thing does not move itself, but it contains within itself the source of motion—not of moving something or of causing motion, but of suffering it." (Physics 8.4; 255b 29-31). Thus, if there were no unmoved mover, there could be no motion, because a moved mover requires a cause of its own motion and no infinite regress is possible. In Physics 8.6, Aristotle argues that, since motion is both eternal and necessary, the first mover must be equally eternal and necessary. Because those things involved in the eternal and continuous process of motion are not eternal and necessary, since they come into being and perish, there must be one or many eternal and necessary thing or things outside the process of motion that imparts or impart motion to the things in motion. This is the only way that there could be any motion, for non-eternal and contingent movers cannot explain all motion, because their own coming into existence needs a cause. He explains, "There is something that comprehends them all, and that as something apart from each one of them, and this it is that is the cause of the fact that some things are and others are not and of the continuous process of change" (Physics 259a 3-5). It is not possible to explain eternal motion by postulating a plurality of unmoved movers capable of imparting motion but that do not exist eternally, for "There must clearly be something that causes things that move themselves at one time to be and at another time not to be" (Physics 258b 21). Aristotle determines that there is only one unmoved mover, not only because many unmoved movers are unnecessary, but because only one mover could produce a continuous motion, in the sense of being an interconnected system of causes and effects.  Moreover, since it is continuous, motion is one; one effect requires a single cause, so that the unmoved mover must also be one. He concludes that an unmoved mover causing eternal motion must likewise be eternal (Physics 260a 1-2).

Aristotle identifies locomotion as the primary source of motion, because the other two types of motion—increase and decrease and alteration—presupposes locomotion (He offers several arguments in support of this position) (Physics 8.7; 260a 20 - 261a 28). There are two types of locomotion: continuous and successive. The former is circular motion, whereas the latter is all other types of locomotion. (In fact, only locomotion can be continuous because increase and decrease and alteration represent change from one contrary to the contrary, which is not continuous, but has a beginning and an end. The contraries, in other words, are states of rest.) Continuous motion is said to be primary, as opposed to the other types of motion—rectilinear or a composite of rectilinear and circular motion—because, "It is more simple and complete" (Physics 8.9; 265a 17). There can be no infinite rectilinear motion because there cannot be an inifinite line; thus continuous or complete rectilinaer motion cannot be one motion but must be a composite motion because what is moving along a finite line must stop and reverse its course if it is to be a continuous motion. Motion along finite line that does not reverse itself is discontinuous or incomplete motion, since its motion ceases. Only circular motion can be both a continuous or complete and simple motion. Aristotle adds that only circular motion can be eternal, since other types of motion have a starting-point, a middle-point and finishing-point of its motion, whereas circular motion is unending; what admits of being eternal is prior to that which does not and so is primary motion. He then argues that the first mover first imparts continuous circular locomotion to the heavenly spheres, which then transmit motion to other things. Earlier, at the conclusion of Physics 8.6, Aristotle affirms that the hypothesis of an unmoved mover explains why the motion of the outermost heavenly sphere is simple and one: "But the unmoved mover, as has been said, since it remains permanently simple and unvarying and in the same state, will cause motion that is one and simple" (Physics 8.6; 260a). The unmoved mover, according to Aristotle, is infinite, since it causes infinite motion. It follows that it is also without magnitude, since an infinite force cannot reside in a finite magnitude (and there can be no infinite magnitudes); having no magnitude means that the first mover is indivisible, having no parts (Physics 8.10; 267b 18-26). In fact, it exists at the circumference of the cosmos where it imparts motion to the outermost sphere, that of the fixed stars. The first mover cannot exist at the center of the outer sphere, the other possible option for its placement because center and circumference are the two principles of a sphere. This last point is proven by the fact that the things nearest to the movent have the quickest motion and the outermost sphere (of the fixed stars) rotates most quickly of all the spheres (Physics 8.10; 267b 5-9).

3. The Unmoved Mover in Metaphysics

3.1. Metaphysics 6.1

Aristotle asserts that if there is a type of substance that is unchangeable, then first philosophy would deal with this primarily, since the unchangeable substance would be prior to changeable substance, presumably because the former would be the cause of latter. Theoretical science is more to be desired than practical science, and the theoretical science the most desired is the science occupying the highest genus (dealing with the highest type of substance), that which has the unchangeable as its subject matter, what Aristotle calls first philosophy (hÍ prŰtÍ philosophia) or theology (theologikÍ), as opposed to physics, which has the changeable for its subject matter, and mathematics. He writes, "The first science deals with things that both exist separately and are immovable" (hÍ de prŰtÍ kai peri chŰrista kai akinÍta) (Metaphysics 6.1; 1026a 15). Thus, the first science has for its subject matter not only that which is substance and for that reason exists "separately" or independently, as all substances do, but also that type of substance that is unchangeable and therefore has no matter, for whatever has matter has potentiality and is therefore changeable.

3.2. Metaphysics 12

3.2.1. Metaphysics 12.1-2

Aristotle says that the subject of his inquiry (theoria) is substance (ousia). As he has argued elsewhere in Metaphysics, substance is primary and therefore to have theoria, or to understand Reality, is to know substance, or as he put it, to know the principles and causes of substances: "For the principles and causes we are seeking are those of substance" (tŰn gar ousiŰn hai archai kai ta aitia zÍtountai) (Metaphysics 12.1; 1069a 1). (The exact difference between the terms "cause" [aition] and "principles" [arche] is difficult to determine from Aristotle's use of these terms.) (In Metaphysics 1.1, Aristotle defines wisdom as a knowledge of the first causes and the principles of things [ta prota aitia kai tas archas].) Although he recognizes that this term has several meanings (Metaphysics 5.1), Aristotle, in this context, seems to mean by "principle" "that from which a thing can first be known" (hothen gnoston to pragma proton) (Metaphysics 5.1; 1013a 15). A principle of a susbtance is that by which the substance is first known or known fundamentally. A substance is an individual thing; to know this individual thing is to know its principle, which means that one has understood it generically, no longer as an individual. Aristotle uses the term "cause" to denote the four causes of a thing (the material, formal, efficient and final causes) (Meta 5.2). Presumably, to know the cause of a substance is to know one or more of these four causes.

In Metaphysics 4.1-2, Aristotle affirms that there is a science that investigates being as being:  "There is a science which investigates being as being and the attributes which belong to this in virtue of its own nature" (Metaphysics 4.1; 1003a 17-18).  There are as many parts of philosophy as there are types of things or substances, types of being.  There is, however, a first philosophy, which is the genus of all the parts of philosophy and which studies that which is common to everything that exists; it abstracts from all the differences among things and studies all things as existing, i.e., what everything has in common insofar as it exists:  it studies being as being, to use Aristotle's words.  In other words, first philosophy (or philosophy) is the study of the principles or causes of all things, that which makes everything what it is. The study of being as being turns out to be the study of substance for two reasons.  First, what is primary in the order of being is the individual thing:  all other categories of attribution are dependent on substance:  something must first exist to stand under the attributes predicated of it.  Therefore, what is in the most basic sense is substance.  Second, what all things have in common insofar as they exist is that they are individual things or substances; to be is to be a substance.  The study of being as being, therefore, concerns itself with the principles and causes of substance.  Aristotle adds that an essential attribute of being is unity, which is related to sameness and the similar, so that what is one is these things also.  Each  individual thing or substance is one thing, so that unity is predicated necessarily of every substance.  Moreover, philosophy is to give an account of that which can be predicated of substance.  Aristotle explains that it is the work of philosophy to investigate opposites, and what is opposite of unity is plurality (including otherness and dissimilarity), which Aristotle seems to take to denote the contraries, that which is predicated of substance. 

 Aristotle identifies three types of substances: 1. sensible (aisthÍtÍ) and perishable (phthartÍ); 2. sensible and eternal (aÔdios); 3. immovable (akinÍtos), i.e., non-sensible and eternal. Sensible and perishable substances are those with which human beings are most familiar, being everything in the world, such as plants and aninals. There are some sensible and eternal substances, however; these are the heavenly bodies, which move, but do so eternally. All sensible substances are the subject matter of physics, which is the science that studies substances as moving. The study of the immovable type of substance belongs to another science (theology). What sensible and perishable substances have in common is that they are changeable in four ways:

Now since changes are of four kinds—either in respect of the 'what' or of the quality or of the quantity or of the place, and change in respect of 'thisness' is simple generation and destruction, and change in quantity is increase and diminution, and change in respect of an affection is alteration, and change of place is motion, changes will be from given states into those contrary to them in these several respects. (Metaphysics 12.2; 1269b 8 - 18)

A sensible and perishable substance may change with respect to the 'what' (ti), or in other words, it may become something else. Aristotle identifies this type of change as change with respect to "thisness" (to tode) or absolute generation and destruction: something comes into being from something else that now no longer exists. Change with respect to quantity is that of increase or decrease, whereas change in quality is alteration with respect to non-essential attributes. Finally, change of place is locomotion. Aristotle defines change as from contrary to contrary (or an intermediate between the two contraries); so defined, it follows that there must be a third element in the change process, that which changes from one contrary to another, what he calls "the matter" (hÍ hulÍ). The matter is the thing that undergoes change from contrary to contrary. Of the two contraries, one is definition and form, whereas the other is the negation of this. (It seems that the definition is a statement of what something is, its form.) He adds that "the matter" must be capable of receiving both contraries. This means that being (to on), exists in two senses, as existing potentially or actually; change is the actualization of potentiality in being. It follows from his understanding of change that all things come into being from being as potential; nothing comes from absolute non-being.

3.2.2. Metaphysics

In all change, Aristotle says that neither the matter (to hulÍ) nor the form (to eidos) comes into existence (Metaphysics 12.3; 1069b 35). His point is that for something to change it must already be something, in which case the matter of the process of change pre-exists its change. In other words, that which changes, the matter, must already exist for it to be capable of change. The form is that into which the matter changes; as such, it likewise must already exist for there to be change. (That by which change occurs is the immediate mover.) Because change presupposes matter and form (and an immediate mover), the process of change will regress to infinity, because every change presupposes matter and form, which pre-exist the process of change. It follows that there must be a terminal point in the process of change: "Therefore there must be a stop" (anankÍ dÍ stÍnai) (Metaphysics 12.3; 1070a 4). But this is not a temporal terminal point, because change or motion is eternal; rather it is a logical one. Aristotle holds it as an axiom that there cannot be an infinite regress of causes and effects, movers and the moved. That which is the logical starting point of infinite change must be an unchanging substance, causing change but not being subject to change.

In addition, in Metaphysics 12.3, Aristotle identifies three meanings of the term "substance" (ousia).  It can mean "matter" (hulÍ) in the sense that it is that which underlies the change, or it can mean nature (phusis), in the sense of the form or essence of the thing, the "positive state towards which movement takes place" (Metaphysics 12.3; 1070a 11).  Finally, one could call the individual thing consisting of matter and form a substance, since it is a "this." Aristotle says that in a general and analogous sense the causes of substances—i.e., all things, since substances are primary—are the same: matter (substratum), form, privation and efficient cause, but in another sense universals, such as these are, cause nothing, because only individual things can cause and be caused. Unlike Plato, Aristotle refrains from naming universals as causes.


In Categories 5, Aristotle discusses the category of substance in more detail. He says, "Substance, in the truest and primary and most definite sense of the word, is that which is neither predicable of a subject nor present in a subject; for instance, the individual man or horse" (Categories 2a 11-13). That which is predicable of a subject is its higher classifications, the species and genera of a subject; these Aristotle calls secondary substances, whereas the individual thing is a primary substance.  For example, the species "man" and the genus "animal" are predicable of the individual Socrates. That which is in a subject is an attribute of the subject; these predicates are not part of the definition or essence of the subject, i.e., not predicable of the subject, and do not exist without the subject.  Aristotle gives the example of the predicable "white": "For instance, 'white' being present in a body is predicated of that in which it is present, for a body is called white:  the definition, however, of the color 'white' is never predicable of the body" (Categories 2a 31-34).  Aristotle remarks about primary substances, the individual things to which other things are predicated: "Primary substances are most properly called substances in virtue of the fact that they are the entities which underlie everything else, and that everything else is either predicated of them or present in them" (Categories 2b 15-18).  Another feature of a primary substance is the fact that each is numerically one and capable of admitting contrary qualities at different times (see Categories 3b 24-26; 4a 10-12).  An individual man can be hot one day and cold the next; contrary qualities can be predicated of him, the individual, which underlies its predicables.


3.2.4. Metaphysics 12.6

Earlier, in Metaphysics 12.1, Aristotle identifies three types of substances, one of which was the non-sensible and eternal, which, unlike the sensible substances, is unchangeable, since it is immaterial (The other two types of substances are called "physical" [physikai] in this 12.6, because they liable to change.) Aristotle argues that it is necessary that there be such a substance (or substances), which is an eternal, unmovable mover, fully actual and good. He begins from the premise that there is eternal and continuous motion: "But it is impossible that movement should either have come into being or cease to be (for it is eternal), or that time should" (all' adunaton kinÍsin Í genesthai Í phtharÍnai (aei gar Ín), oude chronon) (Metaphysics 12.1; 1071b 6). (The existence of eternal motion implies that time is eternal, because time is imseparable from motion, according to Aristotle.) He then argues that there could be no eternal motion unless that which causes motion does so necessarily: eternal motion is necessary motion, since it cannot be otherwise, for it were, it would not be eternal. (It is axiomatic that what is eternal is necessary, for in eternity every contingency will be realized, so that if it were possible for there to be motion there would be no motion!) It should be noted that, for Aristotle, continuous motion is locomotion (for the other types of change must have a beginning and an end), and the only continuous locomotion is circular motion: "And there is no continuous motion except movement in place, and of this only that which circular is continuous" (kinÍsis d' ouk esti sunechÍs all' Í hÍ kata topon, kai tautÍs hÍ kuklŰi) (Metaphysics 12.6; 1071b 11). This then implies that what first causes motion has no potentiality, but is fully actual, because eternal motion could not be necessary, if it were possible for the mover not to move. He writes, "If there is something that is capable of moving things or acting on them, but is not actually doing so, there will not necessarily be motion: for that which has potentiality need not exercise it" (alla mÍn ei esti kinÍtikon Í poiÍtikon, mÍ energoun de ti, ouk estai kinÍsis: endechetai gar to dunamin echon mÍ energein) (Metaphysics 12.6; 1071b 11-13). If it could not have been otherwise that there is eternal motion, then the cause of eternal motion must be fully actual, because only the fully actual substance can cause necessary motion. In other words, only the fully actual can produce a necessary effect; potentiality introduces the notion of contingency, that things can be other than they are. This means that, if the mover potentially is not a mover, given eternity, that possibility will be realized.  In one sense, potentiality is prior to actuality, insofar as the actual is first potential and not everything that can become actual becomes so (see Metaphysics 12; 1071b 22-26). But with respect to the first cause of eternal motion this is not so, because the first cause must be fully actual and have no potentiality; otherwise there could not be eternal motion.

Aristotle also rejects the Platonic notion of eternal forms as first movers because these do not necessarily cause motion:

Nothing, then, is gained even if we suppose eternal substances, as the believers in the Forms do, unless there is to be in them some principle which can cause change; nay, even this is not enough, nor is another substance besides the Forms enough; for if it is not to act, there will be no movement. Further even if it acts, this will not be enough, if its essence is potency; for there will not be eternal movement, since that which is potentially may possibly not be. (Metaphysics 12.6; 1071b 14-19)

(Aristotle also criticizes Plato for asserting that the soul is self-moving and therefore immortal [Phaedrus 245c], because the soul cannot be the first cause since it comes later in the order of creation [Timaeus 34b].) That which first causes eternal motion must be fully actual, which means that it cannot have any matter, since to have matter is to have potentiality:  "There must, then, be such a principle, whose very essence is actuality. Further, then, these substances must be without matter" (Metaphysics 12.6; 1071b 19-22). A fully actual mover, without potentiality, cannot have matter because, for Aristotle, whatever has matter has potentiality. This is because change is possible because matter receives form: matter is the necessary substratum of change.

For Aristotle, although time is not identical to motion, since motion is many and varied, whereas time is always one, nevertheless, time is inseparably connected with motion. When something moves from one contrary to the other, the motion is continuous because magnitude (body) is continuous. For example, when an body becomes hot, the change in temperature is continuous. There are no minimum units of temperature change that are traversed one by one as the body heats up; rather the motion is infinite in the sense of being infinitely divisible. Now, the concepts of "before" and "after" related primarily to place, but these concepts can be applied also to motion.  Thus, since time is intimately connected to motion, the concepts of "before" and "after" also apply to time.  This leads to Aristotle's definition of time as "Number of motion in respect of before and after" (219b 2).  To put it differently, time is the numerical aspect of motion.  Although motion is a continuous process, because magnitudes are continuous, one can still distinguish a series of phases of the process, which one can identify as a series of "nows."  ("Now" is the division and link between past and future [222a 10-11].)  (Since motion is continuous, the division of motion into a series of "nows" represents the arbitrary division of an infinitely divisible process.)  Time is that by which change is measured, and there can be no measure without enumeration of units of the process of change.  Aristotle points out that, in one sense, there is not a series of "nows," but one "now" that is associated with different events that produces the experience of before and after:  it is as if the "now" were a substratum that takes on different properties as it becomes associated with different events in the process of motion.   It is clear that time is secondary to change or motion and presupposes change; there can be no time without change.


3.2.5. Metaphysics 12.7

For Aristotle, the first heaven moves in unceasing, circular motion, which means that the first heaven is eternal: "Therefore the first heaven must be eternal" (hŰst' aÔdios an eiÍ ho prŰtos ouranos) (Metaphysics 12.7; 1072a 24); the first heaven then communicates motion to all other things. What is eternally in motion, however, requires an unmoved mover: "There is therefore also something which moves it. And since that which moves and is moved is intermediate, there is something which moves without being moved, being eternal, substance, and actuality" (esti toinun ti kai ho kinei. epei de to kinoumenon kai kinoun [kai] meson, toinun esti ti ho ou kinoumenon kinei, aÔdion kai ousia kai energeia ousa) (Metaphysics 12.7; 1072a 23 - 26). The first mover is an eternal, fully-actual substance that moves the first heaven without itself being moved, either self-moved or moved by something else. (Being unmovable, it is fully actual, because, otherwise, it would have potentiality and therefore not be unmovable.) Aristotle points out that the object of desire and of thought move in this way, for they cause motion in those who desire and think, but do not themselves move: "For the apparent good is the object of appetite, and the real good is the primary object of rational wish" (Metaphysics 1072a27). This leads him to conclude that the unmoved mover moves by being the final cause of the motion of the first heaven, insofar as it is the object of love: "The final cause, then, produces motion as being loved, but all other things move by being moved" (kinei dÍ hŰs erŰmenon, kinoumena de talla kinei) (Metaphysics 12.7; 1072b 4). The unmoved mover as final cause causes motion by being loved, whereas all other (moved) movers cause motion by first being moved. The first heaven is subject to change with respect to place (locomotion), though not with respect to substance (ousia), since it is eternal; locomotion is the primary type of change and motion in a circle is the primary type of locomotion (Metaphysics 12.7; 1072b 7-8). God could not impart motion as the first efficient cause, because to do so God would have to be in motion, and if God were in motion, then God would be moved and movable.  Besides, there is no beginning to the process of eternal motion, no creation. What is implicit in Aristotle's argument is that the first heaven has intelligence, or soul, in order to love the unmoved mover and so allow the latter to function as final cause. The circular motion of the first heaven is an expression of a love of the unmoved mover, because such motion is the attempt to imitate the eternal and unchanging first cause: circular motion stands closest to motionless eternity, because, in a sense, in rotation no real locomotion occurs, since that which is moving in a circle always returns to where it started. It follows that the unmoved mover cannot be otherwise than it is: "But since there is something that moves while itself unmoved, existing actually, this can in no way be otherwise than it is" (epei de esti ti kinoun auto akinÍton on, energeiai on, touto ouk endechetai allŰs echein oudamŰs) (Metaphysics 12.7; 1072b8-9). This means that it exists as necessary (ex anankÍs ara estin on) and therefore is good (kallos). The unstated assumption is that what is necessary is good. Its necessity consists in the fact that it cannot be otherwise but can exist only in a single way; in other words, its necessity is a result of its lacking all potentiality. The first mover is also a first principle (archÍ), for the first mover explains everything else because it causes all motion. Aristotle writes, "On such a principle depend accordingly depend heaven and nature" (ek toiautÍs ara archÍs ÍrtÍtai ho ouranos kai hÍ phusis) (Metaphysics 12.7; 1072b 14).

According to Aristotle, the unmoved mover, now identified as God (ho theos), eternally does one thing (but this is not self-movement), which is the best thing: God thinks.  Likewise, God thinks about the best thing, which is thought (since thinking is the best of activities), so that thought and its object are the same: God's thinking about his own thinking. In addition, Aristotle says that, because God thinks, God is alive: "And life also belongs to God; for the actuality of thought is life, and God is that actuality; and God's self-dependent actuality is life most good and eternal" (kai zŰÍ de ge huparchei: hÍ gar nou energeia zŰÍ, ekeinos de hÍ energeia: energeia de hÍ kath' hautÍn ekeinou zŰÍ aristÍ kai aÔdios) (Metaphysics 12.7; 1072b 25-27). What Aristotle means by life's being the actuality of thought is that only living substances can think, so that, if he actually thinks, God must be alive. What it means for God to be alive—apart from the fact that God thinks—is not , however, clarified; certainly, for God to be alive is different for other substances to be alive, since God has no matter. Aristotle concludes, "We say therefore that God is a living being, eternal, most good, so that life and duration continuous and eternal belong to God; for this is God" (phamen dÍ ton theon einai zŰion aÔdion ariston, hŰste zŰÍ kai aiŰn sunechÍs) (Metaphysics 12.7; 1072b 28-29).

Aristotle calls God, the unmoved mover, a substance (ousia), but differentiates this substance from all other substances, insofar as it is "eternal, unmovable and separate from sensible things" (aÔdios kai akinÍtos kai kechŰrismenÍ tŰn aisthÍtŰn) (Metaphysics 12.7; 1073a4). God is separate from sensible things because God has no magnitude (megethos), meaning that God is without a body or a spatial existence. The reason that God can have no magnitude is that God produces motion through infinite time, which means that God must be infinite, since an infinite effect requires an infinite cause; but there cannot be such a thing as an infinitude magnitude. As being a substance without magnitude, God is without parts and, therefore, indivisible (magnitudes are divisible).

3.2.6. Metaphysics

In Metaphysics 7, Aristotle speaks of only one unmoved mover, but in in Metaphysics 12.8, he considers the question of whether there are more than one unmoved movers and, if so, how many. He concludes that there is a plurality of unmoved movers:

Since we can see that besides the simple spatial motion of the universe (which we hold to be excited by the primary immovable substance [hÍ prŰtÍ ousia kai akinÍtos]) there are other spatial motions—those of the planets—which are eternal (because a body which moves in a circle is eternal and is never at rest—this has been proved in our physical treatises); then each of these spatial motions must also be excited by a substance which is essentially immovable and eternal. (Metaphysics 12.8; 1073a 28-34)

Presumably, the unmoved mover that causes the motion in the first heaven, the sphere of the fixed stars, is the unmoved mover referred to in Metaphysics 12.7; however, since there are celestial movements other than the rotation of outer sphere of fixed stars, there must be other unmoved movers, substances eternal and without magnitude. According to Aristotle, there are fifty-five movements that require unmoved movers (Metaphysics 12.8; 1074a 10-11), which means that, in addition to the outermost sphere, there are fifty-five spheres. Yet, later Aristotle argues in such a way to lead one to believe that he thinks that there could only be one unmoved mover. He writes, "But the primary essence (to ti Ín einai) has not matter; for it is complete reality (to prŰton). So the unmovable first mover (to prŰton kinoun akinÍton on) is one both in definition and in number; so too, therefore, is that which is moved always and continuously; therefore there is one heaven alone" (Metaphysics 12.8; 1074a 36-39).  God, or "the primary essence," has no matter, which means that there can only be one God, since it is matter that differentiates one form or definition into many manifestations of that one form or definition.  Since God has no matter, then God is one not only formally or in definition, but also numerically.  In addition, there can be only one unmoved mover, because there is only one heaven:  continuous motion is one motion, since such motion is a system of moving parts.

3.2.7. Metaphysics

Aristotle considers the nature of God's thought. It must be of what is most divine and precious, for anything else is unworthy of God.  Likewise, there can be no change in divine thought because that change would be change for the worst, since God thinks only of the most divine and precious; to think of anything but the best, however, would be unworthy of God, and, therefore, impossible. Aristotle also rejects the notion that divine thought is a potentiality, since, if it were, it would involve effort to actualize the potentiality and would mean that, for God, thinking would be laborious, as for finite intelligences, which cannot be true. He writes, "If thought is not the act of thinking but a potentiality, it would be reasonable to suppose that its continuous thinking would be wearisome to it (ei mÍ noÍsis estin alla dunamis, eulogon epiponon einai to suneches autŰi tÍs noÍseŰs: epeita dÍlon) (Metaphysics 12.9; 1074b28-29). In other words, when speaking about God as thinking, one must not imagine that God can begin to think about something, so that thought is a potentiality realized in the act of thought. Moreover, if thought were a potentiality for God, the object of thought would be greater than the thought, for otherwise God would not think about it. This would mean that something would be greater than God who thinks, but this is impossible. Similarly, distinguishing thought from its objects allows for the possibility of thinking "the worst thing in the world," which is unworthy of God. Thus, Aristotle attempts to avoid positing a distinction between divine thought and the object of divine object. He concludes that divine thought thinks of itself as its object, which means that God thinks about thinking: "Therefore it must be of itself that the divine thought thinks (since it is the most excellent of things), and its thinking is a thinking on thinking" (hauton ara noei, eiper esti to kratiston, kai estin hÍ noÍsis noÍseŰs noÍsis.) (Metaphysics 12.9; 1075b 34). What he means is that, since God is nothing but intelligence or thought, for God to think of himself is to think of thinking. This would imply that God has no awereness of the cosmos. How thinking can be an object of thought, however, is not clear.



Questions for Discussion

1. How does Aristotle's God differ from the gods of Greek polytheism? Does Aristotle's God differ the God of the Bible? If so, how?

2. Is Aristotle correct in his assertion that what exists primarily is substance?  How else might you account for the fact that human beings think and speak in terms of substances? Should one call God a substance?  Would his argument for the existence of an unmoved mover be possible without his assumption of the existence of substance?

3. Does eternal motion presuppose the existence of an unmoved mover?  Is Aristotle's proof from motion for the existence of an unmoved mover so tied to his erroneous cosmology that it stands or falls along with it? Does Aristotle's argument still work if one assumes that motion is not eternal? 

4. Is it necessary that God be assumed to be fully actual, and have no potentiality? If so, does God really have no potentiality?



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