## 1. Natures and the four causes



Nature, according to Aristotle, is an inner principle of change and
being at rest (*Physics* 2.1, 192b20-23). This means that
when an entity moves or is at rest according to its nature reference
to its nature may serve as an explanation of the event. We have to
describe how--to what extent, through what other processes, and
due to what agency--the preconditions for the process of change
or of being at rest are present, but once we have provided an account
of these preconditions, we have given a complete account of the
process. The nature of the entity is in and of itself sufficient to
induce and to explain the process once the relevant circumstances do
not preempt it.



Natures as inner principles of change and rest are contrasted with
active powers or potentialities (*dunameis*), which are
external principles of change and being at rest (*Metaphysics*
9.8, 1049b5-10), operative on the corresponding internal passive
capacities or potentialities (*dunameis* again,
*Metaphysics* 9.1, 1046a11-13). When a change, or a state
of rest, is not natural, both the active and the passive potentiality
need to be specified. Natures, then, in a way do double duty: once a
nature is operative, neither a further active, nor a further passive
capacity needs to be invoked. Even so, as will be clear from
Aristotle's discussion, this general thesis will require a host
of qualifications.



Because natures--beside the active and passive
potentialities--are ultimate grounds in causal explanations,
Aristotle sets out how they are integrated with the doctrine of
causation.



An explanation for a state of affairs must specify some feature or
some object (in general, some abstract or concrete entity) which is
responsible for it. The entity responsible is, Aristotle submits, a
cause (*aitia* or *aition*, words used interchangeably
by
 Aristotle).[4]
 Different explanations of a single state of affairs are possible, and
indeed usually necessary, because there are different ways of being
responsible for distinct facets of the same state of affairs. The
varieties of responsibilities are grouped by Aristotle under four
headings, the so-called four causes.



The first two of these are matter and form, what an entity is made up
from according to Aristotle's hylomorphic analysis.
Understandably, both of them can be responsible for the features and
the behaviour of the entity they make up. Hylomorphic analysis,
together with the separation of the material and formal causes as
distinct types, implies that if something is explicable in terms of
matter or form, explanations in terms of form will be different in
kind from those given in terms of matter. As a rule there is a
collaboration between these causes: matter provides the potentialities
which are actualised by the form. Accordingly, these causally relevant
entities give rise to a hierarchic structure of
 explanation.[5]
 In order for a form to be realised, one needs to have suitable
matter. This suitable matter brings with it the features required by a
given hylomorphic composite. These features, then, are on the one hand
the contribution of the matter, and as such the matter is the
(material) cause of these features of the composite entity, whereas on
the other hand they are indispensable presuppositions for the
realisation of the form, and to that extent their presence is prompted
by the
 form.[6]
 Such dependency relations between matter and form are labelled by
Aristotle as cases of hypothetical necessity. Aristotle sometimes
illustrates his point by appealing to the matter required for the
construction of a house. If there is a house to be built, one needs
building bricks, slabs, mortar, etc. Each part provides material with
properties within a definite range of the sort required for a house to
come into being. A house cannot, for example, be made out of liquid
water. That sort of matter would provide potentialities not suited for
the form of house.



Explanations often specify entities beyond the role played by the
matter and the form of the entity itself. These cases are grouped by
Aristotle as efficient or moving causes on the one hand and as final
causes on the other. Efficient causes operate in a straightforward
manner by initiating processes and bringing about their effects,
whereas final causes account for processes and entities by being what
these processes and entities are for, what they objectively intend to
 attain.[7]
 The fact that the role of efficient causes is not identical to that
of the matter and the form of the entity whose features they are to
explain does not require that every instance of efficient causation
must issue from outside the entity moved. On the contrary, an
efficient cause can also be internal. In cases in which the efficient
cause is internal, it will be, in its specific function, one of the
parts, or even the formal aspect, of the entity caused to move.



Natures, understandably, can feature in any of these four causal
functions. However, when the matter of an entity functions as its
nature--i.e., when its natural motion and rest are explained in
terms of the matter it is made of--this matter must possess some
causally relevant features, bestowed upon it by its own formal
aspect.



This role of matter can be contrasted to the causal role of the three
further types of causes--of form, of efficient cause, and of
final cause, respectively. This is so, because, as Aristotle adds,
form and final cause often coincide. Moreover, when a nature is
specified as a first efficient cause, cause and effect are the same in
form (or in species), though this is not to say that one and the same
entity causes itself and is caused through its own causal efficacy
(*Physics* 2.7, 198a24-27, cf. *Metaphysics* 8.4,
1044a32-b1).



As internal principles of moving and rest, natures stand in an
exclusive relationship to the efficient or moving causes of the
motions and rests they bring about: in some cases when Aristotle is
not specifying the first moving cause, he can assert the identity of
nature and moving cause. Accordingly, the soul of living beings will
be identified as the substance (i.e., form) and the moving cause of
the organism whose soul it
 is.[8]
 But the identification, even in this restricted sense, will need some
further important qualifications, to which we will return in
 Section 5
 below, on movers and unmoved movers.


## 2. Motion



Because motion or change (*kinesis*) is mentioned in the
definition of nature, any discussion of nature will need to rely upon
the explanation of motion. One might--erroneously--think
that this is an easy task, because Aristotle's categories (as
listed in the *Categories* and also elsewhere) do contain two
related types of entities, action and passion. Aristotle's
discussion of motion in the *Physics*, however, starts out in a
somewhat different manner. When he submits that there is no motion
besides the categories (*Physics* 3.1, at 200b32-201a3),
he does not assign motions to the categories of action and passion.
After mentioning that the entities in the categories come in
oppositions, Aristotle claims a few lines later (at 201a8-9)
that there are as many kinds of motion and change as there are kinds
of being. This means that motions are grouped here with the entities
of the category where they effect
 change.[9]



Nevertheless, when making this claim, Aristotle speaks about four
kinds of motion and change only--those in substance, in quality,
in quantity and in place--whereas the number of the kinds of
being should have remained ten.



Indeed, the *Physics* will later submit its own list of
categories. That list is slightly reduced--it has seven or eight
elements, depending on whether we include or exclude
 time.[10]
 The reduced list also concludes with the claim that there are three
kinds of motion, plus the additional kind of substantial
 change.[11]
 That is to say, even where Aristotle enumerates a fairly complete
list of categories, he will not have motions in every one of these
categories, and he is not content to relegate motions to the
categories of action and
 passion.[12]
 But this is a context where Aristotle stresses another issue: he is
not interested in assigning a separate ontological niche for
motions--regardless of whether that might or might not have been
a feasible task within the categorization of entities. Here Aristotle
is more intent on characterizing the ontological links which motions
have to entities falling into different categories, and to find a
general matrix of undergoing and effecting change. This happens in
several steps. First Aristotle claims that changes of relations are
not changes in their own right; rather they are accidental, as they
occur also in entities in which no change occurs at all, if the entity
which they stand in relation to undergoes some
 change.[13]
 After these considerations the crucial two categories of action and
passion are eliminated. They cannot possibly house motions in the same
way as the other four categories do. This is so because such a motion
would require that there should be motion or change of an action or a
passion. But there are no motions of motions, so even though actions
and passions qualify as motions, or at least are intimately linked to
motions, we can set aside action and passion as types in which motions
can
 occur.[14]
 This leaves us with the shorter list of relevant categories, (1)
substance, (2) quality, (3) quantity, and (4)
 place.[15]



Within the four domains where genuine change can occur, change always
requires the existence of a potentiality which can be actualised. But
change is neither identical to this potentiality, nor to the lack of a
property, nor, without further qualifications, to the actuality which
is acquired when the potentiality is actualised (*Physics* 3.2,
201b33-35). It is a special kind of actuality, the actuality of
the potential in so far as it is potential (*Physics* 3.2,
201b4-5). Aristotle's formulation strongly suggests that
the potentiality actualised in the process of change is not a separate
and independent potentiality for motion, alongside the entity's
potentiality for harbouring the end-state of the process: the process,
say, house-building, and the end result, the house, are different
actualisations of the same potentiality of a set of materials that is
buildable into a house. Not only would Aristotle's definition be
uninformative otherwise, amounting to the tautologous claim that
change is the actualisation of the capacity for change, the further
qualification in the definition, that change is the actuality of the
potential in so far as it is potential, would be completely
 idle.[16]
 This further restriction is easiest taken as selecting between the
different types of the realisations of the same
 potentialities.[17]
 As Aristotle stresses these are the incomplete actualities belonging
to these potentialities, adding also that what is actualised in a
process of realisation is an incomplete potentiality only
(*Physics* 3.2, 201b32-33). Accordingly, potentialities
of change may be admitted into the ontology. But even if they are
admitted as additional potentialities, they]do not need to feature as
potentialities in their own right, but as the incomplete variants of
the fundamental potentiality for an end
 result.[18]



It is furthermore important to note that potentiality in this
discussion throughout excludes actuality. In a formulation closely
matching the formulation of the principle of non-contradiction,
Aristotle asserts that "some things are the same [=have the same
properties, are the same substances] both in potentiality and in
actuality, but not at the same time or not in the same respect, as
e.g. [a thing is] warm in actuality and cold in potentiality"
(*Physics* 3.1,
 201a19-22).[19]
 Hence the ability of Aristotle's definition to pick out the
paradoxical entity, which is the actuality of a potentiality that can
no longer be present once it has been replaced by the corresponding
property in actuality.


## 3. The principle of causational synonymy



The definition of motion suggests that such processes can be
characterised in terms of a property or state of an entity, acquired
as a result at the end of the process, which can be labelled the
*form* within this process, and an initial *lack* of
this form. Furthermore, Aristotle claims, there is a third component,
which is not changed in the process, the substrate or subject of the
motion (*Physics*
 1.7).[20]



In terms of this threefold division it is the duty of the entity
effecting change to confer the requisite form on the object changed,
as *Physics* 3.2, 202a9-12 puts it. But there are further
important requirements for such a change to occur. First of all, these
motions or changes occur at the interaction of two potentialities.
One, the passive potentiality, is in the object undergoing change,
while the other, the active potentiality, is in the entity initiating
change. The two potentialities need to match each other: when there is
a potentiality for being heated in the object undergoing change, the
process needs to be initiated by another object possessing an active
potentiality for effecting the heating. This is true to the extent
that Aristotle can claim that the definition of passive potentiality
is dependent on that of the active potentiality (*Metaphysics*
9.1, 1046a9-13). These two potentialities need to work in
tandem, and consequently Aristotle can claim that there is only a
single process going on, which is located in the entity moved. Thus,
for example, when a process of instruction is going on, it is
identical to a process of knowledge acquisition, which happens in the
mind of the learner. Hence although action and passion retain their
categorical difference, because their accounts are different, what
they subsist in, the motion, will be the same (*Physics* 3.3,
 202b19-22).[21]



Aristotle already by the introduction of a matching pair of active and
passive potentialities for each causal interaction comes very close to
admitting a separate potentiality for each and every change, something
uncomfortably close to the *vis dormitiva*, ridiculed by
Moliere, according to which a sleeping pill allegedly induces
sleep just in virtue of its power to induce sleep. Aristotle, however,
subscribes to an even stronger principle, that causes in effecting
change transmit the form they possess to the entity they effect change
in. According to this claim the active capacity of the item effecting
change is at its root an actuality, which is synonymous (in the
Aristotelian sense) with the effect that is brought about by it. In
Aristotle's favourite example, only a human in actuality
produces a human from what is a human in potentiality. If this is so,
a sleeping pill need not only possess an active potentiality for
inducing sleep: it needs also to be slumbering
 itself.[22]
 The principle--which we could term the principle of causational
synonymy--comes from Plato (see e.g. *Phaedo*
100B-101D), but Aristotle has his own reasons for endorsing it.
His science attests to the presence and operation of causally active
forms at each level of analysis of the physical
 world.[23]
 Hence, as we shall see, Aristotle's forms are the causally
significant components of the substance effecting a change.
Accordingly, when it comes to specifying the moving cause of an
artefact, Aristotle will refer to the art of the craftsman as the
fundamental component operative in the change. In cases where a living
being is generated, it is the parental form which is transmitted to
the newly emerging living
 being.[24]



But it is not only processes of generation that conform to this
requirement. Instances of qualitative change are often mentioned
alongside substantial generation, and as a crucially important
instance of qualitative alteration--or of qualitative
quasi-alteration, depending on how we interpret Aristotle's
theory of
 perception[25]--Aristotle
 presupposes that the principle of causational synonymy characterises
also the causal link connecting the object of sensation and the sense
organ.



It is, nevertheless, important to note that Aristotle restricts the
principle of causational synonymy in different and subtle ways. Most
significantly, an important domain of cases where a property of an
object is actualised is exempted from the requirements of this
principle. The actualisation of a property can be the continuation of
a previous causal process to the extent that Aristotle claims it is a
*second actuality*, following upon a previously acquired
*first actuality*. In these cases the emergence of the second
actuality does not necessarily require an additional external
efficient cause. The operation of this first actuality, through which
it reinforces and completes itself, can be the mere extension of the
operation of the original efficient cause (this will be
Aristotle's claim about the natural locomotion of the elements,
see
 Section 5
 below), or the entity which has acquired this first actuality can be
already causally responsible for its own activities, including the
ones which bring it to a level of higher
 actuality[26]
 (Aristotle's examples for this case are the soul of the embryo
or of the newborn cub, which commands and effects the nourishing and
the activities of the animal; or the actual application of a piece of
knowledge one has acquired beforehand). It is important to note that
these claims are far from trivial: they rest on further claims that
the very definitions of these first actualities (what it is to be an
element, an animal, or knowledge, respectively) inseparably include
references to these activities.



Second, the principle of causational synonymy is couched in terms
which do not include locomotions: it is substantial, qualitative or
quantitative form which is claimed to be transmitted through the
efficacy of the cause in *Physics* 3.2, 202a9-12. One of
the reasons for this is that locomotion, as Aristotle claims, affects
the least the substance, the *ousia* of the object undergoing
motion (*Physics* 8.7, 261a20f). Unlike the other types of
change, locomotion does not change the being of the moved object at
all. To some extent that should mean that the predication of place
should remain extrinsic to the being of the entity that is at a
particular
 location.[27]
 Hence the fundamental presupposition of causation, that it is
intrinsic characterisations of entities which are conferred on the
object moved cannot be in full force in cases of
 locomotion.[28]
 Accordingly, Aristotle will have a more intricate account for natural
and forced locomotions.



Third, the principle of causational synonymy is restricted to
substances at the end of *Metaphysics*
 7.9,[29]
 and in the first half of the same chapter the non-standard presence
of some causally relevant forms may also be envisaged.
Aristotle's example there is the heat in motion, which produces
heat in the body when the doctor rubs the patient in the appropriate
manner. This heat in the motion can be the presence of an active
potentiality in the motion which is able to elicit heat in the body,
without heat being predicable of motion itself. But even if such
non-inherential subsistence of properties is only hinted at, and not
expressly envisaged in this passage (on a more detailed description
the heat in motion is the active capacity of motion to produce heat in
the skin of the patient--and in the skin of the
doctor--which as far as the treatment is concerned enters into
the inner recesses of the patient's body, becoming heat in the
body), some similar sort of presence is required in two large classes
of cases: natural generations and artificial productions.



Aristotle claims that in a chain of efficient causes, where the first
element of the series acts through the intermediary of the other
items, it is the first member in the causal chain, rather than the
intermediaries, which is the moving cause (*Physics* 8.5,
257a10-12). Then, both in cases of natural generation and
artificial production, it is only this first efficient cause which has
to satisfy the requirement of synonymous causation. Aristotle's
prime example, that human generates human, is also such a case. Here,
the causal efficacy of the paternal human form is transmitted through
the generative potentialities of the semen of the father. The semen,
however, although it acts as an efficient cause in the process of the
formation of the embryo, is not a human; it does not possess the form
it transmits in the same way as the male parent does.
Aristotle's discussion makes it clear that this is not an
isolated instance of an exception from the general principle. He
compares the case to the activity of a craftsman, where the form of
the product of the artistic production is in the soul of the
craftsman, and then through the motions of the instruments this form
can get imposed on the material manufactured into an artefact. The
instruments and their motions are efficient causes of the process, but
they do not contain the form in the same way as the soul of the
craftsman (*On the generation of animals* 730b14-23 and
740b25-29, for further discussion see the entry on
 Aristotle's biology).[30]



All these restrictions notwithstanding, Aristotle can claim that the
principle of causational synonymy remains universally valid. This is
so, because all the three restrictions above specify cases where
Aristotle can claim that a preceding, more prominent cause has already
satisfied the requirement: in the case of second actualities the first
actuality was called into existence by a synonymous cause in the first
place; locomotions, qualitative and quantitative changes, even if not
caused by a synonymous entity, can be part of a larger pattern of
causation, in which a substance is caused by a substance of the same
kind; and causal chains producing substances can be claimed to start
out invariably from synonymous substances.



Given his commitment to causal synonymy, Aristotle needs to invoke
considerations through which a chain of efficient causes of some
entity can be meaningfully compared in terms of causal efficacy. These
considerations will on each occasion describe synonymous causes not
only as temporally prior, but also as having priority in terms of
causal efficacy over the intermediate causes, which are responsible
only for the transmission of the forms of the original locus of causal
efficacy.



This allows, then, that in the two major paradigms of such
causation--in natural generation and in artificial
production--the nature of the natural entity, and the
 art[31]
 of the craftsman exercising his art respectively, the relevant form
is the causally operative entity initiating change. This has wide
ranging consequences for the status of forms in several respects.
First, the causal relevance of these forms shows that not any
arrangement or configuration can qualify as a full-fledged form. While
it is true that privations are also forms in some sense
(*Physics* 2.1, 193b19-20), this is not the sense in
which the causally operative forms, describable in evaluative terms,
can be called forms. Moreover, the causal relevance of forms allows
Aristotle to switch (e.g. in *De generatione et corruptione*
1.7) without notice between the craftsman and the craft itself as the
appropriate specification of the efficient cause in these cases. We
should note that in the latter cases, Aristotle specifies causes which
are unmoved. They do not effect motion by being in motion themselves,
in so far as they are the causally effective forms within the causal
framework; hence they are not under any reactive influence during this
process either.


## 4. Priority among motions



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.[32]
 Contact, however, as a rule needs to be established by locomotion:
either the entity to be moved, or the mover, or both, need to proceed
so that they meet (*Physics* 8.7, 260a26-b7). 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). Other changes are independent kinds of change
insofar as they can occur in an entity which does not perform any
other change. Nevertheless all these forms of change include or
presuppose that some other entity engages in
 locomotion.[33]



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 claims, would also be
 contingent.[34]
 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).[35]



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.[36]
 Left to its own devices, the finite universe on its own would
certainly 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,[37]
 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.[38]
 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.


## 5. Movers and unmoved movers



The definition of motion as the actuality of a potentiality of the
entity undergoing motion in so far as it is potential requires that in
each case the passive potentiality for the change is present in the
changing object. The presence of the potentiality can, nevertheless,
be in accordance with the nature of the object--in which case the
change is *natural* (*phusei*) or *according to
nature* (*kata phusin*), or can happen in the face of a
contrary disposition on the part of the nature of the entity--in
which case the change is *forced* (*biai*) or
*contrary to nature* (*para phusin*). A major
presupposition on Aristotle's part is that this division is
exhaustive: there are no changes to which the nature of the entity
would be indifferent or
 neutral.[39]
 The major consideration behind such a presupposition is that natures
regulate the behaviour of the entities to which they belong in a
comprehensive manner, and not merely partially. Any influence the
entity is exposed to interacts with its nature in a substantive
manner. The entity does not possess potentialities for change which
would not be directly related to the tendencies emerging from its
nature.



Note, however, that even if we endorsed the exhaustiveness of the
dichotomy of natural and forced motions, and accepted the thesis that
simple bodies possess a unique natural motion (*De caelo* 1.2,
269a8-9), we would not need thereby to accept Aristotle's
further major claim, that natural and forced motions come in pairs of
opposites, with the result that if a motion is contrary to the nature
of an entity, the opposite motion will be its natural motion (*De
caelo* 1.2, 269a9-18). Where there is room for some more
complex relationships among the targets of changes than a simple
opposition along an axis of a single dimension--and this is
eminently so between locomotions along rectilinear and circular paths,
respectively--there can be several forced translations in
contrast to the single natural motion of the elements endowed with
rectilinear natural motion, as Aristotle also admits in some passages
of the *De caelo* (see 1.2, 269a30-b2 and
269b10-12).



Although this allows for several different motions that are contrary
to the nature of the same entity, the natural motion will still have a
single opposite motion, the one which is directed to the opposite
location. Consequently, natural circular motion will have no motion
that is opposite to it (*De caelo* 1.4,
 270b32-271a5).[40]



Aristotle's classification of motions into those contrary to
nature and those according to nature applies not only to the motions
of the moved objects, but transfers also to the movers effecting
motions. A mover can effect a motion which is contrary to its own
nature. Aristotle's example of such an unnatural mover is the
lever, an object heavy by nature, with which loads can be lifted
(*Physics* 8.4, 255a20-23). Although such movers can
effect motions in the contrary direction to the motion at the head of
the causal chain (levers are operated by the downward push of
something heavy at the other end), the crucial consideration for
Aristotle in this case is that the original, initiating cause of the
causal chain should effect the motion according to its nature. Taken
together, these considerations imply that we have a complete account
of the physical domain once we have a thorough description of what is
natural to the entities in that domain, together with a specification
of all the circumstances in which they
 operate.[41]



Bk. 8 of the *Physics* argues for the additional thesis that
for each motion, whether natural or contrary to nature, there needs to
exist a
 mover.[42]
 In cases of forced motion, movers are present in a conspicuous way.
This need not be so, however, in cases of natural motion. Apart from
the cases where the nature of the entity is at the same time a moving
and efficient cause--i.e., apart from living beings, whose
nature, the soul, is both formal and efficient cause--the mover
may be inconspicuous. This is eminently so in the remaining large
class of natural motions, the natural motions of the elements. The
nature of these elements, their inner principle of motion and rest is
not the moving cause of the motions of the elements, Aristotle claims.
If it were, then it would be up to the elementary masses to determine
when they should perform their motions, but plainly this is not so.
Moreover, the principle of causational synonymy rules out that any
homogenous mass, without an internal demarcation into separate
components, one moving and the other being moved, could move itself
(*Physics* 8.4, 255a5-18). This is so because, on the
assumption that one part of a homogeneous body could move another
part, the active component of change would be, in every aspect,
indistinguishable from the part in which change is effected, and this
in turn would mean that change would occur even though there would be
no transmission of a causally relevant property from the active part
to the passive. This implies that even though we may answer the
question as to why the elements move to their natural places--the
light bodies up and the heavy ones down--by an appeal to their
respective natures as causes ("that it is simply their nature to
move
 somewhere,[43]
 and this is what it is to be light and to be heavy,"
*Physics* 8.4, 255b13-17), we do not thereby specify
their moving causes. Their thrust being in a single direction, the
elements cannot circumvent even rather simple obstacles they may
encounter on their way (a sealed container can retain air under water,
the roof stays put pressing down on the walls of a building etc.).
Hence, whoever removes an obstacle to an element's motion is
causally responsible for the ensuing elemental motions. But even such
a causally responsible agent will not qualify as *the* moving
cause, without yet further qualifications. For the identification of
the moving cause of these locomotions Aristotle invokes his
distinction of two potentialities. Some heavy material can be
potentially light, as it can be transformed into a light material in a
process of generation, whereas the emerging light material is still
potential in a sense until it has acquired its full-fledged status,
which involves its having arrived at that region of the cosmos which
is its natural place. This analysis, then, describes the natural
locomotion of the elements as a possibly postponed, completing stage
within a single overarching process, and hence in these cases
Aristotle can identify the cause of the second stage of the process
with the efficient cause of the first stage, the entity which
generated the element in the first place (*Physics* 8.4,
256a1).



Once it is established that there is a mover for each change, the
finite causal
 chains[44]
 can be followed up to the primary instance of motion, which are
eternal circular motions. (In particular the Sun's course along
the ecliptic is responsible for many sublunar changes, the cycle of
the seasons being foremost among them.) Whether these circular motions
require external movers, and ultimately, whether the universe is
causally closed or needs some external causal influence for its
preservation, depends on the status of these revolutions.



In this regard the very first thing to establish is that they cannot
be constrained
 motions.[45]
 But natural motions are also in need of movers, as *Physics*
8.4 argues. This does not apply only to the natural motions of living
beings, which are performed under the causal influence of their soul.
The natural motions of the four sublunary elements are also caused by
specific external causes responsible for these motions, and on the
basis of these considerations Aristotle feels entitled to assert that
every motion whatsoever--including also, then, the eternal
circular motions--require a mover.



But the entities performing these eternal circular motions cannot
possibly be moved by moving causes of the same kind as the movers of
sublunary elemental motions. These entities are eternal and
ungenerated bodies. Consequently, there is no entity that could
produce them. Nothing can be responsible for the circular motion on
account of generating these eternal and ungenerated bodies. Moreover,
as they do not encounter any hindrance during their revolutions, there
is no room for an accidental mover which would remove any obstacle in
their way.



Although Aristotle's physical treatises do not discuss in what
particular way their movers are causally responsible for these eternal
circular motions, the general framework set out apparently applies to
them as well. In accordance with this, motion is an incomplete
actuality of the potentiality of what is movable (*Physics*
8.5, 257b6ff). This incomplete actuality is always dependent on the
causal efficacy of the corresponding actuality of a mover. The example
presented here is the process of heating a potentially hot item,
caused to be hot by an actually hot thing. The division of roles
guarantees that the same item cannot possibly be the mover and the
moved entity at the same time. Consequently, these eternal circular
motions are envisaged as having a cause responsible for their motion,
the causal influence of which produces (and upholds) this motion
continuously in them. Aristotle does not specify in any detail what
exactly the actuality of the mover is; nor does he indicate what
relationship there is between this actuality and the actuality that is
being continuously communicated by the mover to the entities in
eternal circular motion.



A further requirement is that the mover of these eternal circular
motions has to be
 unmoved.[46]
 But then the actuality of this mover should not be restricted to a
causally efficacious single property. Rather, the mover should be
actual all way through. Any potentiality it might have would carry
with it the jeopardy that it might undergo possible changes of its
 own.[47]
 This is ruled out to the extent that such a first cause cannot house
motion even
 accidentally.[48]
 In other words, the mover cannot be related to the moving object in
the way the soul of an animal is related to the moving animal; in the
case of an animal wherever the living being, the animal, moves, its
soul will follow suit and will also move,--accidentally--to
the same
 place.[49]



Moreover, the entity causally responsible for the eternal circular
motions in the world has to possess an infinite
 power,[50]
 which it communicates to the bodies it moves. As a result, this
entity cannot be divisible and cannot have extension. Moreover, it
must be located where the motion is quickest, at the periphery of the
physical realm (*Physics* 8.10).



All this testifies to the exceptional status of the first movement,
and behind it, of the first mover in the universe. The mover of these
spheres possesses nothing but actuality, but this actuality is not
what is transmitted in the process of causation. As we have seen in
 Section 3
 above, this would not be exceptional as such: locomotion need not be
caused on the transmission model of causation. But locomotions,
although caused without the transmission of an actuality of being
located in any particular location, were understood to be embedded in
larger patterns of causation which observe the principle of
causational synonymy. It is precisely any such larger pattern of
causation which is missing in the case of celestial
 motions.[51]
 Note furthermore that what we hear in *Metaphysics* 12.6, that
the first mover moves as an object of love and
 striving,[52]
 comes perilously close to abandoning the claims of *Physics*
bk. 8 to the effect that there is an unmoved mover serving as the
efficient cause of the motions of the cosmos. Such doubts, however,
should be dismissed. In these two contexts Aristotle gives distinct
descriptions of some principle that is beyond the realm of physics.
First, still connecting it to the framework of the *Physics*,
he is characterizing a supra-physical entity without which the
universe could not function or persist, and then, second, in the
*Metaphysics*, he is offering an account of the causal efficacy
of the supra-physical substance in terms of what this entity itself
is. Small wonder that these two accounts situate the mode of operation
of this entity in two different frames of *physical*
causation.


## Glossary of Aristotelian terms


* action: *poiein*
* activity: *praxis*
* actuality: *energeia* or *entelecheia*
* art, craft: *techne*
* capacity: *dunamis*
* cause: *aitia* or *aition*
* change: *kinesis* or *metabole*
  

to effect change or motion: *kinein*
  

to undergo change or motion: *kineisthai*
  

qualitative change: *alloiosis*
  

quantitative changes--growth: *auxesis;* decay:
*phthisis*
  

locomotion: *phora*
* to come to be: *gignesthai*
* coming to be: *genesis*
* force: *bia*
  

forced: *biai*
* form: *eidos* or *morphe*
* in so far as: *hei*
* genus, kind: *genos*
* goal: *telos*
* kind, species: *eidos*
* known, knowable: *gnorimon*
  

more known, more knowable: *gnorimoteron*
* matter: *hule*
* magnitude: *megethos*
* motion: *kinesis*
* nature: *phusis*
  

natural: *phusikos, phusei*
  

according to nature: *kata phusin*
  

contrary to nature: *para phusin*
* passion: *paschein*
* to perish: *phtheirein*
* perishing: *phthora*
* place: *pou* (as one of the categories, literally: where)
or *topos*
* potentiality: *dunamis*
* power: *dunamis*
* quality: *poion*
* quantity: *poson*
* substance: *ousia*
* time: *pote* (as one of the categories, literally: when) or
*chronos*