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ARISTOTLE’S THEOR Y OF POTENTIALITY
Aristotle’s theory of potentiality pl ays a large role in his metaphysics and
philosophy of science. In some ways, it is insightful and prophet ic, for (as I shall argue),
it introduces a model of explanation by functional analysis, which is still standard in
some of the cognitive (and arguably) the life sciences. In other ways, it is outmoded, for
explanation by functional analysis is not, as Aristotle thought, applicable to all the
sciences. 1 In this essay, I review some major features of the theory. My aim is to explore,
in a tentative manner, how an Aristotelian should think about T Heather star G Grey Basic shirt Base questions conc e rning the
beginning and end of life.
I want to make it clear at the outset that since I am interested (here) in how
Aristotle’s ideas impact contemporary controversies, historical accuracy is not my on l y
concern. It is of no special interest to be told that Aristotle’ s theory has such and such a
consequence for the que stion of when life begins, if one is inclined to reject the theory
out of hand. It serves both history and philosophy to make a plausible argument using
Aristotelian ideas, even if in order to do so, one is obliged to interpret these ideas with
one’s ey es somewhat more fix ed on contemporary philosophical applications than
historians generally find appropriate. I shall be concerned, therefore, to advance an
interpretation that is fle xible enough to accommodate contemporary que stions and
How does a suitably reconstructed version of Aristotle’ s theory apply to issues
concerning the beginning and end of life? Here, it is even more urgent to distinguish this
question from the one about how Aristotle himself would have spoken about life. As we
shall see, Aristotle had some morally objectionable beliefs about how to assess the moral
entitlements of humans with regard to life. So we should ask how an Aristotelian theor y
1 It goes seriously wrong in physics, for example, where it requires the motion of the sublunary
elements to be defined by reference to a fixed point in space, namely the center of the Universe. (See
Matthen and Hankinson 1993 for discussion.)
of potentiality affects issues concerning the beginning and end of life, given a more
acceptable position on the moral entitlements of humans.
a. Potentiality and Nature in Aristotle’s Physics
a. Kin ē sis and Potentiality
In Aristotle’ s system (Physics III 1-3), a kinēsis is an agent-initiated change that has well-Discounts Discounts Naf Naf Naf Cherry Ecarlate Ecarlate Discounts Scarf Scarf Cherry Cherry wx0qYnTgT
defined starting and ending termini. (The term ‘ kinēsis’ [plural ‘ kinēseis’] is sometimes
translated “motion,” sometimes “change”: both are sometimes misleading. Here, I leave it
transliterated as a term of art.) Standard examples are building a house and teaching a
lesson. These kinēseis begin, respectively, when an agent—a builder or a teacher—begins
to act purposefully upon materials from which the house is to be constructed or an
ignorant student who is to be taught. They must end—there is not way for them to be
extended—when the hous e has been built, and when the student has learned the lesson.
These changes contrast with activities such as hammering or sawing, or speaking to
students. There is no natur al point at which these activities end, no point at which it is
impossible for them to be further extended.
Kin ē sis always has an agent and a patient (or thing acted upon)—the builder acts
upon building materials or unfinished houses; the teacher acts upon ignorant students.
This is where potentiality comes in. The causes of kinēsis are not properly the agent and
patient themselves, but an active potentiality in the agent and a matched passiv e
potentiality in the patient. The builder has a capacity to turn the building materials into a
house; a house comes to be because she comes into contact with building materials that
can be made into a house, and having come into contact with them, exercises her capacity
on them. Analogously with teacher and student—a student comes to know something
because a teacher comes into contact with something able to be taught. 2
As said be f ore, the a gent initiates the kinēsis. When the house has been built, or
the student has learned the lesson, the passive potentiality in that thing has disappeared,
for then there is nothing further for the builder or teacher to do. Both pot entialities are
essentially self-limiting, though in different ways. The patient changes, and thereby loses
2 In this case, too, something comes to be, namely the student’s knowledge. But this thing is not a
substance. (See Matthen 1983.) Thus, the house-building kinesis is a coming-to-be proper, t he coming-to-
be of a substance, but the teaching is not.
ARISTOTLE’S THEOR Y OF POTENTIALITY
its passive potentiality . Once the student learns his lesson, he is no longer ignorant and
cannot be instructed (in this lesson). The teacher, however, does not change. Her teaching
potential has done its job, and stops, but she retains the active potentiality .
b. Nature and Self-Initiated Change
A kinēsis occurs by nature when both active and passive potentials belong to a single
substance in virtue of the kind of substance it is. Growth is natural kinēsis. Over the
period of a year, a newborn infant might triple its birth weight. (Of course, this is just part
of a larger natural kinēsis, which terminates naturally when the individual is full-grown.)
Though growth requires feeding by the mother, an outside source, the potentiality to
convert food into bodily mass is intrinsic to the infant. (More about this in a moment.)
That is, some part or aspect of the infant (its “nutritive soul”) carries an active potential to
transform another part or aspect (its body) by metabolizing nutriment. (Note that even
self-initiated self-directed kinēseis—i.e., natural changes such as growth by metabolism
—conform to the apparatus of active and passi v e potentialities. This will be important in
Sometimes, it is tricky to identify the substance that contains these matched
capacities. Penelope and Odysseus had a bed that was shaped from a live olive tree. The
bed and the olive tree are not one and the same—the tree predates the bed, and the bed
could be destroyed without the olive tree being destroyed. So there are two substances
here. Now, what should we say when the olive tree sprouts? Is it the bed or the tree that is
naturally changing? The obvious answer is that it is the oli ve tree that sprouts by nature,
not the bed. It is the tree, and not the bed, that has a nutritive soul ma tched to its body in
virtue of the kind of thing it is. The bed sprouts, but only because it coincides with the
It is crucial here that the active and passive potential responsible for growth by
metabolism are essential parts of the ba b y—parts of the ba by by virtue of the kind of
thing it is. A doctor too can act upon herself. That is, she can heal herself—her medical
skill is the active potentiality, and her ailment constitutes a passive potentiality in her
body; both are in her, and she acts on herself by virtue of them. However , this is not a
natural change, for medical knowledge does not inhere in her as a consequence of the Basic star G Heather shirt Grey Base T
kind of thing (i.e., substance) she is (namely , a human).
Life consists (among other things) in the capacity to metabolize naturally , i.e., by
means of active and passive potentialities that belong to a thing because of the kind of
thing it is. This indicates the difference between the mother’s act of feeding and the
baby’s act of metabolizing. The first is other -directed and hence non-natural; the second
self-directed by nature. The s econd defines the baby as living; the first is merely made
possible by the life of the baby, but not star Basic G T Grey shirt Base Heather constituted by it. (Note that “non-natural” here
means “not by nature”; it does not mean “against nature”.)
It should now be clear why the self-contained agent-patient kinēsis that constitutes
an embryo’ s nutriti ve soul is different from the mothe r’ s potentiality to nourish it in
utero . The former is constitutive of the embryo’s capacity to act on itself; the latter is not
an exercise of the mother’s self-acting capacity. The embryo’s dependence on the mother
as a source of nutriment does not gainsay its standing as a self-acting metabolizer. At
least as far as Ar istotle is concerned, Jason Eberl is r ight, elsewhere in this volume, to
write “The form of external assistance a uterus provides is analogous to an astronaut’s
spacesuit or an underwater explorer’s submarine.”
Modern science does not, however, support the position that from the moment of
fertilization onward, the embryo possesses a natural potential for metabolism. (Actually,
it’s not clear that Aristotle believes this either.) Early in its development, an embr y o is
dependent on the mother Enjoy Rio Monki Scarf red Black r5zxxXwO0n for metabolism. For in the moments after fertilization, the
embryo is nothing more than a fer tilized ovum. It does not, at this point, possess a
metabolic system. Or to put it in Aristotelian terms, it does not, immediately upon
fertilization, possess a “nutritive soul.” When exactly it comes to possess a nutritive soul
cannot be decided a priori . The best way to look at the matter is that Aristotle provides us
with a criterion f or the beginning of life—life begins when the embryo is able t o
metabolize nutriment on its own, without relying on the assistance of the mothe r’ s
This criterion is evidently very di fficult to apply. For one thing, an embryo cannot
be simply given adult food; it cannot, for instance, metabolize milk or eggs. So the
ARISTOTLE’S THEOR Y OF POTENTIALITY
mother needs to convert what she eats into a form suitable for the embryo to metabolize.
What counts as merely providing nutrition f it for an embryo to metabolize for itself, and
what counts as metabolizing by the mother for the embryo? That is, to with respect to
what kind of nutriment is it appropriate to regard the embryo as a self-acting metabolizer?
Again, only science can answer such questions, or even decide whether they have a
definite ans we r. For what it is worth, it seems that there is a distinction. For metabolism
consists of producing energy for one’s own activities, and matter for one’s own growth.
Regardless of what nutriment is used for producing these things, one can ask whether it is
the embryo’s body that performs the relevant transformation or the mother’s that does so.
And there is clearly a point (or interval) of time at which the embryo’ s body has begun to
do so. This is the point at which the embryo has begun to be alive.
One important point that comes out of this discussion is that the crucial question
for Aristotle is when the embryo is able to sustain its own metabolic activities. The
question is not when it is possible for it to gr ow into an organism capable of performing
characteristically human activities. This distinction cuts two ways. On one hand, the
crucial determinant is not whether a fertilized ovum can ult imately perform human
activities. The question is rather whether it can sustain its own metabolic Grey Heather G Basic Base T shirt star processes. And
clearly there is a period of time during which it is capable ultimately of performing
human activities, but not capable of sustaining its own metabolism.
On the other hand, it is also irrelevant whether or not it is possible for it to
develop. It could be that unfavorable circumstances make it impossible for the embryo to
get nutrition, though it has already developed the capacity to metabolize this nutrition.
For instance, the mother might have developed some kind of illness that makes it
impossible for her body to convert the food she eats into nutriment suitable for the
embryo to metabolize. This situation is, as Eberl rightly suggests, analogous to that of an
astronaut in a ma lfunctioning spacesuit. The astronaut is self-sustaining despite the fact
that the malfunction ma kes it impossible for her to get the external materials needed to
sustain herself. Nonetheless, the astronaut is still alive.
W e should keep in mind that the genesis of a nutritive soul may not constitute t he
right kind of life to be worthy of moral standing. (I’ll return to this question in section
IV .) But if possessing a nutritive soul is regarded as constituting moral worth, then
unfavorable e xternal circumstances would not abrogate its rights. Abandoned and
exposed by i ts parents, a baby cannot find food for itself, and would die without external
intervention. Though it is not, in this sense, self-sustaining, it retains its natural capacity
to metabolize and whatever moral standing it has in virtue of (this form of) li fe . Like the
astronaut in the tragically malfunctioning spacesuit, it is still alive.
c. Potentiality and Functional Analysis
a. Coincident vs. Proper P otentialities
The potentiality theory is sometimes attacked for being vacuous. Moliere’s character
T artuffe pretends to wisdom by saying that opium puts people to sleep in virtue of its
“dormative power.” Y et, he sheds no light on the matter by so saying: dormative power is
trivially the cause of sleep. At times Aristotle sounds like T artuff e, but there is a more
fruitful line of explanation behind the appearance of vacuity .
In Physics II 3, Aristotle say s:
In investigating the cause of each thing it is always necessary to seek what is most precise
. . . thus man builds because he is a builder, and builder builds in virtue of his ar t of
building. (195 b 21-24)
And, a little earlier:
All causes, both proper and coincident, may be spoken of either as potential or as actual;
e.g. the cause of a house being built is either house-builder or house-builder building.
These passages seem to describe explanatory potentialities in the same words as the effect
that is being explained. They appear to echo the vacuity of Tartuffe ’s vis dormativa: this
house came into being because a house-builder actualized his house-building skills on
materials that could be turned into a house. This suggests that Aristotle had no more to
say than that if a house came into being at a certain time, there was, earlier, the po ssibility
of there being a house. The contemporary philosopher of science wi ll obje ct: this
possibility is implied by but does not imply t he actuality . How then can it explain the
ARISTOTLE’S THEOR Y OF POTENTIALITY
This impression of vacuity is inco rrect. Throughout this chapter, Aristotle talks
about “coincident” causes. Here is what he says:
Another mode of causation is the coincident and its genera, e.g., in one way Polycleitus,
in another sculptor is the cause of a statue, because being Polycleitus and sculptor are
T o understand some of the implications of this passage, imagine an interlocutor who says:
The s tatue was made by the ac tion of the chisel on the stone. The
sculptor’s action was not necessary, since the very same chisel movements
coincidentally applied would have had the same result.
T o this interlocutor , Aristotle would reply f irst that without the skill of the sculptor, the
sequence of chisel motions would have been a massively improbable random sequence
without any explanation. 3 There is nothing impossible in a non-sculptor executing the
sequence by throwing a bucket of chisels at some marble, but he could do so only by
virtue of an extremely improbable accident, as when a monkey banging away randomly
at a typewriter manages to produce a coherent sentence. But there are many sta tues in the
world—they cannot all have come to be by accident. According to Aristotle, the
frequency of statues can only be explained if in most of them, there is an overarching
coordinating cause—something that ensures that the chisel strokes are executed rightly
and in proper order. (See Matthen 1989 for an account of this argument.) The skill of the
sculptor is canonically this cause. The sculptor ensures the correctness of the sequence by
coordinating it in accordance with her skill. So, to s ay that the skill of the sculptor is
responsible for the star T Base G Grey Heather shirt Basic statue is not vacuous; minimally, it is to say that there is something
that so Basic Base T shirt G Grey star Heather coordinates chisel strokes as to bring about the genesis of the statue non-
accidentally . Obviously , the sculptor’s skill serves this role in the production of statues.
The “coincident” causes are bearers of these and other necessary conditions of
production. Non-vacuous explanation resides in them. In the particular case of
Polycleitus’ s statue, the sculpting-skill was coincident with being-Polycleitus—which is
3 In Physics Book II, chapter 8, he says that a rainstorm during the “dog days” might, but frequent
rain in winter cannot, be the result of “chance and spontaneity. ” This looks like a response to an interlocutor
who argues along the lines indicated in the text—perhaps Empedocles, who is re fe rred to a few lines earlier
as holding (what strikes the modern reader as) a well-worked out theory of chance generation and natural
to say that the sculpting skill has come to reside in the person who is Polycleitus (though
not in virtue of the sort of thing he intrinsically is, which is why it is merely coincident).
The consequence is that this person is able reliably to execute the right chiselling actions
in the right way. By identifying “the being of Po ly cleitus” as coincident with the
sculptor’s skill, we identify the efficient cause of the statue. It was by being coincident
with the being of Polycleitus that the sculpting skill had its effect.
d. Functional Analysis
Aristotle’s strategy of proper and coincident causes is reminiscent of “functional
analysis” (Cummins 1 975). Suppose you want to explain how an adding machine adds.
First, you first specify what “adding p ow er” (or vis addens ) is: it is the potentiality in the
adding machine that ensures that when you sequentially press the keys ‘x’, ‘+ ’ , ‘y’, and
‘=’ (for a n y numerical values of x and y), you get a display of the numerical expression
that stands for the sum of x and y . T o explain this power of the machine, you need to cite
the physical properties of the electronic circuit buried inside the adding machine; you
have to show how these properties yield the co rrect output when a given input is enter ed.
The crucial success condition of your explanation is t hat the sequence of events s et off in
the machine by the key presses should be provably equivalent to adding. In short, you
must show how the power to add decomposes into simpler powers, and you must show
how these simpler powers transform the key-presses T Base Basic star Heather shirt Grey G ‘7’, ‘+’, ‘4’, and ‘=’ lead to the
display ‘11’ (and similarly , for all values of x and y). This is the sine qua non of
Plausibly , this is the kind of analysis Aristotle had in mind when he spoke of
coincident causes. The “proper cause” is identified in terms that are log ically proximate
to the description of the targe t phenomenon: house-building for houses built, teaching for
learning, sculpting for statues, etc. But then you produce another logically more distant
description to sh o w how the logically proximate cause comes to be instantiated in the
case under consideration. Thus consider what he says about a wall in Physics II 9:
[One should not] suppose that the wall of a house comes to be because what is heavy is
naturally carried downwards and what is light to the top, wherefore the stones and
foundations take the lowest place, G Grey Basic star Heather Base T shirt with earth above because it is lighter, and wood at the
top of all as being the lightest. Whereas, though the wall does not come to be without
ARISTOTLE’S THEOR Y OF POTENTIALITY
these, it is not due to these, except as its material cause: it comes to be for the sake of
sheltering and guarding certain things. (200a 1-7)
This suggests a functional analysis of wall-building.
Goal Definition What is needed
T o build a house House = Stable shelter-
W eight-bearing walls to
support sheltering roof
T o build a wall W all = Stable weight-
Upper parts of wall should
be supported stably by
T o build structure in which
lower parts stably suppo rt
Stable support Ł upper parts
should not disturb position
of lower pa rts on which
Upper parts light in order to
exert less pressure on lower
parts; lower parts strong and
heavy to support upper parts
Strong and heavy lower part
coincides with Ł Stone
Thus, the art of house-building will seek stone for the lower part of the wall, and
the potentiality to stand stably will be actualized by the stone below .
The arts that govern the matter and have knowledge are two, namely the art that uses the
product and the art that directs the production of it. That is why the using art also is in a
sense directive; but it differs in that it knows the form, whereas the art which is directive
as being concerned with production knows the matter. For the helmsman knows and
prescribes what sort of form a helm should have, t he other from what wood it should be
made and by means of what operations. ( Physics II 2, 194b1-6)
The success condition of a more distant description being the correct explanation
is that the “proper cause” should “coincide” with it. The capacity to provide stable shelter
coincides with structures that have stone below . Similarly , the adding potential coincides
with the underlying electronic operations, not with the whine of the machine. And
sculpting capacity explains the sequence of chisel strokes, and it coinci des with
Polycleitus, not the chisel he wields.
Bringing this back to natural kinēsis, it is relatively vacuous to be told that a baby
grows because it has within it an active potential to convert nutriment into bodily mass.
T o provide a more explanatory theory, Aristotle must cash this out in terms of coincident
causes. He must, in other words, show how the baby is so put together that it is able to
metabolize food. There are places where he takes a stab at doing something like this, but
obviously his science was not up to G shirt Base T Basic star Heather Grey this task. H owever , Max Delbr ück (1971)—a Nobel
Laureate and James W atson’s teacher at CalTech—provides a brief appreciation of the
progress Aristotle made toward delineating the parameters of such a functional analysis.
According to Delbr ück, Aristotle realized that:
The form principle is the information stored in the semen. After fertilization, it is read out
in a preprogrammed way; t he readout alters the matter on which it acts, but does not alter
the stored information, which is not, properly speaking, part of the finished product. (54)
There is a long distance, and many centuries, to go before this analysis could be taken to
its ultimate conclusion; nevertheless, Delbr ück suggested that “if that committee in
Stockholm, which has the unenviable task of pointing out the most creative scientists, had
the liberty of giving awards posthumously, I think they should consider Aristotle, for the
discovery of the principle implied in DNA.”
e. Potentiality and Possibility
a. Possibility Insufficient for Potentiality
I have gone into some detail concerning Aristotelian potentialities in order to make three
points about them.
First, Aristotle’ s attributions of potentialities are tied to functional analysis. Thus,
the potentialities are not merely vacuous verbal re-descriptions. In fact, Aristotle here
anticipates an important explanatory tool in modern cognitive science—functional
analysis. One should not, h owever , exaggerate his prescience here. Aristotle’s use of
functional analysis is ubiquitous—he defines the elementary building blocks of the
Universe functionally . For example, in the De Caelo Book I, he defines earth, which is an
element in his s y stem, in terms of its power to actualize itself at the centre of the
Universe. But we have seen that functional analysis is ul timately dependent on an
ARISTOTLE’S THEOR Y OF POTENTIALITY
analysis of powers in terms of other kinds of properties. Aristotle’ s over -use of functional
analysis robs his system of the causal foundation it requires.
Second, Aristotelian potentialities are causes, not merely earlier possibilities of
later realities. When Aristotle says that this building material has a potential to be mad e
into a house, he is not merely saying that it is possible for the material to be worked on so
as to yield a structure that affords shelter to people. In the first place, even if t he
possibility of p is materially equivalent to the potentiality in something of p, potentialities
are causally efficacious attributes and reside in substances; possibilities are not.
Third, and crucially for my ar gument, the possibility is not even materially
equivalent to the potentiality . Consider water and cement powder . It is possible for this
stuff first to be made into cement blocks and then to be made into a house, so it is
possible for the water and cement p owder to be worked into a house. However, water and
cement powder do not have the potentiality to be made into a house. This is, first, because
water and cement powder do not have the appropriate potentialities—they do not, for
instance, keep their shape when subjected to pressure. In order to get things with the
appropriate potentialities from cement powder and water, these things must be mixed
together and allowed to set. (This supplements the point I made toward the end of
section I, namely that the point of in terest is when embryo comes to acquire the
potentiality to metabolize. This potentiality is more than a possibility .)
Plausibly , the new potentials that mixed-and-set concrete blocks possess indicate
that some new thing has come to be from cement powder and water. This would suggest
that these materials must first go out of existence and cement blocks must come to be
from them. If this is correct, the house-building potentiality cannot be exerted on cement
powder and water as such. It must be exerted on cement blocks.
This make s f or a complicated link bet ween x’s potentialities (where x is a
substance) and possibilities for x (Krizan 2006). Aristotle says: “a thing is capable of
doing something if there is nothing impossible in it h a ving the actuality of that which it is
said to have the capacity” ( Met. 1047a24-26). Water and cement are not actually present
in the material of a house. Hence, they do not have the relevant potentiality . (Remember,
though, that even if water and cement were present in the house, as modern chemist ry
would have it, they do not have the potentiality to support w eight except when
thoroughly mixed together.) On the other hand, it is possible for the cement blocks to
have the actuality of being parts of a house. So, it is a possibility for them. It is on the
basis of s imilar reasoning that Aristotle says that the possibility for x to be made into a
house implies the potentiality to be made into the house.
f. Possibility Unnecessary for Potentiality
Possibility is not necessary for potentiality either. (Here I am indebted to John Lizza for
probing questions.) Consider an embr yo whose mother dies during pregnancy. This
embryo possesses a na tural capacity to metabolize nutriment and develop into an adult.
However, it is now not possible for this embryo to develop into an adult, because it
cannot get nourishment. For at least a short time after the mother dies, the embryo retains
the natural potentiality for development, though there is, in fact, no possibility of it
developing any further.
This situation contrasts with that of a genetically or developmentally defective
embryo. Embryos sometimes suffer a catastrophic d ev elopmental event, which causes
them to be born without a skull, or even a brain. (This is known as anencephaly .) Such
embryos lack the innate potentiality to develop into intellectually functioning adults. In
Aristotle’s way of looking at things, this is very dif f erent from the situation where the
Cement + water ——> Cement blocks ——> House
ARISTOTLE’S THEOR Y OF POTENTIALITY
embryo has the potential but lacks the possibility for development because of the absence
of external sources of nutriment.
If natural (i.e., self-acting) potentiality is the basis for moral standing, then the
embryo whose mother has died has moral standing, and the anencephalic baby lacks it.
g. Genesis and Nature
In the case of the arts, it is, to some extent, arbitrary how potentialities are assigned.
Consider a stone quarry . It is not a builder’s job—not part of his art—to cut stone from a
quarry and make it into blocks suitable for building. Consequently, the stone in the quarry
does not have the potential to be made into a building; a builder could not work on it
(though he would have a say , as the last quot ation above shows, in the form that the cut
stone would take). But (whether Aristotle took this into account or not) the division of
labour between the quarry-man and the builder is contingent. If it had been considered
part of the builde r’ s job to excavate and cut stone, then the rock in the quarry would have
had the potential to be fashioned into a house.
Things are different with natural substances. The kuēma, or fertilized union of
male and female genetic material, develops more or less spontaneously into a mature
organism. But this does not mean that either the male or the female genetic material has
the potential to become a mature human being all by itself. Left alone, neither will
develop. First they must be made into a kuēma (see below) and then t his spontaneously
develops. There are (at least) two kinēseis here, the second one of which is natural.
This is an important point, for as Alfonso G ómez-Lobo (2004) repor ts, some have
argued as follows:
The potentiality argument is understood as moving ba ckwards in the following way: i f a
human person deserves respect, then a potential human person . . . also deserves respect.
But. . . by virtue of the transitivity of potentiality, . . . the sperm and ovum also deserve
respect. [But this is] “a position almost no one f inds plausible,” to quote an elegant
understatement by Professor [R. M.] V eatch. (200)
Of course, G ómez-Lobo is right. Nothing of the sort follows. Potentials are not transitive
in the way t hat the “moving backwards” argument assumes. The ovum is not a potential
human being at least until fertilized. And in Aristotelian science, similarly, the genetic
material of each parent does Base star Grey Heather T Basic G shirt not have the potential to develop naturally into a mature
human by itself.
h. Soul: The Capacities That Define Human Life
W e are now ready to consider some questions about the beginning of life. According to
Aristotle, sexual intercourse leads to pregnancy when male and female genetic material
intermingle and join in such a way that the form of the male can work on the matter
provided by the female. At this point, we have a Foulard Aigner Persian Aigner Aigner Exclusive Blue Foulard Blue Persian Exclusive Exclusive 4qqSZWHwa6 kuēma, or fertilized union of male and
female genetic mate rial; thi s i s the v ery first stage of the embryo. Embryonic
development, or ontogenesis, is the natural kinesis that results in the baby born nearly
forty weeks later. (It is possible to regard t his kinesis as terminating a couple of decades
later, when the child becomes a man.) Aristotle’s account of ontogenesis is thus an
application of his theory of kinēsisExplore Scarf Explore Antracite Versace oro Scarf Versace zxzqfTn.
Several questions can now be posed:
1. At what stage does the embryo become a human being?
2. At what stage is the embryo a potential human being?
3. At what stage is the embryo alive?
First, in agreement with G ómez-Lobo above, fertilization is, on anybody’s
account but especially A ristotle’s, the very earliest point at which there could be said to
be a new life or new being. (As I argued earlier, the Aristotelian account probably implies
that life, or natural metabolism, starts later.) Prior to fertilization, the active potential of
ontogenesis, the human form provided by the Base G T Grey Heather shirt Basic star father is not in contact with the matter
provided by the mother. Consequently , though there is the possibility of the female matter
being part of a continuous process that culminates in a mature human being, this genetic
material lacks the potentiality to become a mature human being naturally.4 And as we
have seen, possibilities and potentialities are quite different conditions.
4 Sagan and Singer suggest, elsewhere in this volume, that in vitro embryos are different from
naturally produced ones because the former have to be sustained in certain ways from the outside. On my
interpretation of Aristotle, this is not the crucial difference. Rather, we must ask whether the embryo is able
to perform the activities characteristic of life by nature—whether it requires outside intervention to acquire
the materials for self-sustenance is not important. (See my comments on Eberl’s astronaut-and-spacesuit
ARISTOTLE’S THEOR Y OF POTENTIALITY
Of course, Aristotle would allow that the both the female and the male genetic
material had the potentiality to interact with one another to become first a fertilized ovum
and ultimately a mature human being, in the way that stone and builder have
potentialities to interact with one another to produce a house. There are two reasons why
this is i rrelevant. First, there is no particular human being it will produce: both male and
female genetic material require something of the other kind, but prior to fertilization,
there is no particular thing of the complementary kind that provides the potentiality to
produce the offspring. 5 Thus, there is, be f ore fertilization, no substance that is potentially
human or potentially alive. Second, and perhaps more importantly, there is not, prior to
fertilization, a nything that can initiate change within itself—and as we have seen, this is
the hallmark of li fe. Thus, alluding to a more modern notion of “respect” or “regard,”
there is, as G ómez-Lobo says, no plausibility in thinking that sperm and ovum deserve
respect. In answering questions 1-3, we should, there f ore, consider only stages of the
embryo, and nothing earlier.Blend Red K Extremely Filippa Scarf 0RwE7wFq
Embryonic development is the acquisition of some capacity that defines life.
(We ’ ll come in a moment to what capacity this might be. I am assuming for now that
“capacity” and “potentiality” are interchangeable, though as we’ll see this is too simple.)
The acquisition of the life-defining capacity takes place in something, X, that did not
possess it at the start of the process. Does this mean that X (whatever it is) has the
potentiality to be alive? Or is some new thing, Y, created in the process (in t he way that
cement blocks were created out of cement powder and water), such that Y is alive—but is
not the same as X? If Y is a new thing, it would be wrong to say that X is potentially alive
simply in virtue of the fact that X becomes, or becomes a constituent of Y. Thus:
A. If X did not initially have the life-defining capacity, but only later came to have it,
is X potentially alive at the start of the process (in virtue, perhaps, of potentially
possessing the life-defining capacity)?
5 John Lizza asks whether intracytoplasmic sperm injection (ICSI) would challenge this point.
After ICSI, as he points out, it is already determined which sperm conjoined with which egg produced the
embryo. My position on ICSI is of a piece with what I say at the end of section I and in note 3. In
Aristotle’s theory, the origin of the embryo, and the source of its nutriment, are not the important points.
Rather its current status determines its status. Are its life-functions performed by nature? If they are, then it
has whatever moral standing is concomitant upon this kind of life.
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B. If a thing that has life (e.g., X above) is necessarily distinct from that which does
not have life ( Y), then could X be one and the same as Y?
These difficult questions are crucial to any determination of the starting point (and end-
point) of life for a human. They are not explicitly dealt with by Aristotle, but one can
speculate as to what his answers might be.
In order to answer these questions, I turn now to A ristotle’s conception of life. In
Aristotle’s system, as in most modern ones, life is defined by certain capacities: that is, a
living thing is defined as one that is capable of certain activities (e.g., metabolism). As we
have noted, cap acities or potentialities inhere in substance. So the capacities that define
life inhere in living things. This thing lives in virtue of having certain capacities. It does
not live in virtue of the presence in it of a separable soul (as Plato had it in the Phaedo ).
In Aristotle’ s ontology , souls depend for their existence on the living substances in which
they reside, not the other way around. Soul is a capacity in virtue of which its possessor is
alive; but this capacity would not exist but for the thing in which it inheres.
Aristotle’s conception of soul is of particular interest because he defines three
kinds of living. All living things, including plants, are capable (i) of metabolizing
nutriment and growing. T his c apacity is sometimes known as the “nutriti v e” soul.
Animals have a nutritive soul, but in addition they are capable (ii) of perception and self-
movement; this is the “sensitive” soul. Finally , humans h ave both a nut ritive and sensitive
soul, but are, in addition, capable (iii) of thought and reason. This is the rational soul.
Now , in modern ways of thinking about life, the emphasis is on what is common
to all living things. If A ristotle had followed this line of thought, he would have identified
life (hence, soul, which is the principle of life) with the nutritive soul. This, however , is
not how he proceeds. He says:
Living is spoken about in different ways. And should even one of these belong to
something, we say it is ali ve: reason, perception, m otion and rest with respect to place,
and further the kinesis of nourishment, decay, a nd growth. ( De Anima II, 413a22-25)
Here, Aristotle acknowledges that we say of anything at all that it is alive if it merely
nourishes itself. H owev er, he says that life is spoken about in different ways. The same
word is used, but it is used to describe different kinds of thing. P lants, which have on l y
ARISTOTLE’S THEOR Y OF POTENTIALITY
nutritive soul, have a different kind of life than animals, which have nutritive as well as
sensitive soul. (Notice that this kind of position would support subs uming respect for
animals to respect for the kind of life that animals possess. Respect for animal life would
not automatically imply that respect be paid to plants.)
Aristotle’s idea here is, I believe, something like this. Plants nourish themselves
statically: they stand still and draw up nutriment from the earth and air around them.
Animals can sense and move, and they use these capacities to find food. The very activity
of self-nourishment is modified by the manner in which animals engage in it. Thus, it is
(according to Aristotle) in a way true, and in a way false, to say that plants and animals
share a life-defining capacity. Plant and animal self-nourishment are di fferent in kind. In
a similar vein, one might note that humans use reason (among other things) to get T shirt Grey Basic star G Base Heather food.
This fundamentally modifies their way of going about an activity that, viewed from a
certain perspective, they share with plants and animals.
Christopher Shields (2009) puts this point well. He contrasts two models of the
human soul. There is first the “layer cake” model, with the nutritive soul forming a
distinct layer under the sensitive soul, which in turn is separate and distinct from the
rational soul. The second model emphasizes the unity of life-activities.
A being with a rational soul has a perceptual capacity neither more nor less than a being
with a perceptual soul. Still, the manner in which the perceptual faculty is present is
distinct. A rational soul is not formed by the layering of a rational faculty upon the top of
an actually existing perceptual faculty . Rather, a rational soul subordinates a perceptual
faculty to its own ends, thereby integrating it into a unified, single soul. ( ibid, 306;
The italicized words above are significant: the point is that the “perceptual faculty” is not,
as such, sitting there deep in our souls, a separate entity that is the same as what animals
possess. Rather, we have faculties that, but for their integration with reason, would be an
animal perceptual faculty, pure and simpl e. Given this integration, our percept ual
faculties do not constitute a separable animal soul.
This explains Aris totle’s w ords:
What holds in the case of the soul is similar to what holds concerning figures: for both
figures and the ensouled, what is prior is present as a capacity in what follows in the
series, for example, the triangle in the square, and the nutritive in the perceptive. We must
investigate the reason why they are thus in a series. For the perceptive faculty is not
without t he nutritive, though the nutritive is separated from the perceptive in plants.
Triangles are present in squares, but not as actual triangles—they are on ly potentially
present awaiting their generation from the square by the div ision of the latter. Similarly ,
the plant soul is present in us, but not as actual. We say that things are alive because they
engage in “nourishment, dec a y, and growth”: nevertheless, we speak of life in different
ways when we do so.
Aristotle’ s notion of the human soul, then, is something like this: it is the
capacity to self-nourish and to perceive in ways that are subordinate to reason. Now, God
has reason, but does not have self-nourishment or motion, and therefore does not have a
human soul. It is not very much noticed in the literature that because humans are unique
with respect to the kind of soul th ey possess, Aristotle’ s discussion of soul in the early
parts of De Anima II actually provide a definition of our species. This is a bit of a
surprise, and i t is germane to question 1, above. The question is: “At what stage does the
embryo become a human being?” The ans wer is: “When it becomes capable of rational
When is something capable of rational thought? Is an embryo capable of rational
thought when it is at its very earliest stage of male genetic form united with the female
genetic matter? T o understand this, we h av e to take Aristotle’s notion of a first actuality
into account. Consider a sleeping adult. She is capable of thought and speech, but is not
exercising this capacity . This is a first actuality; the second or complete actuality is
present in somebody only when t hey are actually speaking. Now, Aristotle posits a
potentiality that is prior to the first actuality. Babies are in this stage with respect to
speech. That is, babies are not like sleeping adults who are capable of speaking when they
awake. There is something they have to acquire in order to be capable of speaking.
Babies, however , are unlike puppies because they will (in quite short order) acqu ire the
capacity to speak. They have a “first potentiality”. The f irst potentiality to speak and
think marks them as human, even though they are still not, in the sense of the sleeping
adult, capable of speaking and thinking.
ARISTOTLE’S THEOR Y OF POTENTIALITY
First potentiality Second Potentiality/First
Baby (to speak): must learn
language to move to second
Asleep adult (to speak) Adult giving a speech
Embryo (to think): what
must happen to move to
second potentiality? Is a
new entity created?
Asleep adult (to think) Adult actively thinking
The question remains. Is it the first potentiality or the first actuality that bestows
upon an embryo the status of a human? Or to put it anot her way, when does human life
begin, this being conceived as a different question than that concerning life as such. Here
it should be noted that Ar istotle ’s notion of human rationality is even more demanding
than we have encountered so far. When a person reasons something out—for example,
the solution to a math problem or a difficult course of action—she is not engaging in t he
highest form of rationality. Reasoning something out, whether in the practical or the
theoretical realm, is a kinēsis. One starts with some premises, and arrives at a conclusion.
Once one has arr ived at the conclusion, the reasoning terminates. When God thinks,
however, It does not en gage in kinēsis. God simp ly thinks; this is a so-called energeia, a
non-goal oriented activity without a natural terminus. God thinks in a non goal-oriented
way; Its Heather shirt Basic Base star Grey T G potential to think does not disappear when the goal is a chiev ed. (This is the
reason why I said earlier that potentiality and capacity might not be exactly the same: we
have a capacity for non-goal-oriented thought, but not a potentiality , as I have explained
the latter, i.e., relative to kinēsis.)
Now , according to Aristotle, it is part of human essence to emulate God in this
form of non-goal-oriented, non-terminating thinking, and though (of course) humans
cannot achieve it, it is (as we learn in Nicomachean Ethics X), the highest good for t hem.
Here, it is perhaps helpful to contrast the rotational movement of the stars with the
movement of the sublunary elements to their natural places. The latter kind of motion is
kin ēsis : it terminates when an element such as earth reaches its natural place at the centre
of the universe. Similarly , solving a problem has a natural terminus, but thinking per se
This throws some doubt on Jason Eberl’ s approach elsewhere in this volume.
I follow Aquinas in contending that all that is required for something to be a person is for
it to have at least an acti ve potentiality to perform self-conscious rational operations .
The actual performance of such operations is accidental to a person’ s existence.
This would be correct if human life was defined as the first actuality of thinking. My
point here is that there are places in the corpus, notably in Nicomachean Ethics X and
Metaphysics XIII, where Arisototle suggests that it is a failing not to be actually in a state
This is an extremely powerful and telling result. It implies that human life is
defined by an activity t hat the apparatus of potentiality and actuality does not perfectly
fit. Eberl (footnote 5) helpfully lists a number of attempts to say what activities define
human life, and concludes “th e y all include the criterion of either rationality or self-
consciousness .” He goes on to say: “any being who possesses the capacity for both would
undoubtedly qualify as a person .” This is dubious. It could be argued that it is essential to
humans that they are always actually self-conscious, and that a cessation of such self-
consciousness was a cessation of life itself. Of course, much depends on how self-
consciousness is defined—for example, whether it is to be understood in such a way as to
include sleep. I do not want to deny that Eberl has strong considerations on his side. My
point here is mere l y that Aristotle may well not agree. The cessation of self-conscious
thought is eo ipso a failing with regard to human essence.
Put non- kinetic reason, i. e., non-goal-oriented thinking, to one side. There is
another reason why Eberl’ s position is too undemanding. Aristotle defines full reason as
the capacity to set goals, not merely work toward them. (Thus understood, full reason is
kinetic .) If humans lack full reason, he says, they are deficient. And in his most explicit
consideration of moral standing, he asserts that deficiencies in this regard rob humans of
ARISTOTLE’S THEOR Y OF POTENTIALITY
their right to autonomy. To illustrate the point, think of the morally repugnant
conclusions he draws about slaves in Politics I 5:
For he who can belong to another (and that is why he does belong to another), and he
who participates in reason so far as to apprehend it but not so far as to possess (for the
other animals obey not reason but feelings), is a slave by nature. The use made of them
differs little; for from both—slaves and tame animals—comes bodily help in the supply
of essentials. (1254b20-26)
The passage occurs in the context of a justification of slavery. Slaves are deficient in
reason, Aristotle says, and this is why it is right to treat them as instruments, or tools.
Ta me animals are by nature better than wild, and it is better for all of t hem to be ruled by
men, because it secures their safety. Again, the relationship of male to female is that the
one is by nature superior, the other inferior, and the one is ruler, the other ruled. And this
must hold good of all mankind. (1254b10-15)
It is reasonably clear, then, that Aristotle identifies human living with some quite
demanding conc eptions of reason, and argues that those who (as he thinks) are deficient
with respect to reason are also deficient with respect to being human. Such deficient
humans lose their moral entitlements, for they are subject to being used, ruled, and owned
by others. Inasmuch as embryos are similarly deficient with respect to the activity that
defines human life, they too do not have a right to respect. V ery much the same holds for
those who suffer irreversible loss of cognitive faculties.
Apparently , then, Ar istotle ’s notion of potentiality and natural change combined
with his own notion of the human soul leads to a very negative answer concerning the
beginning of human li fe, at least in its full form. His answer sanctions infanticide and
euthanasia T Basic Grey star G Heather Base shirt for pe ople with irreversible senile dementia (and not just for those in a
persistently vegetative state). And this is not merely a projection of his arguments. Gi ve n
his position on slavery and the position of women, there is no good reason to think he
would have rejected the position that I have attributed to him.
i. Pulling Back
Let me now brief l y consider some ways of pulling back from the harsh consequences of
Aristotle’s positi on.
One might drop the idea that rationality is the threshold for moral regard. I offer
two very brief comments.
a. First, one must do this in a way that preserves a reason for moral regard. That is,
we should not be content simply to identify some beings as worthy of moral regard and
others as not. This identification should, in addition, rest on some relevant difference
between beings of the first sort and those of the second. We do not accord animals the
same moral regard as we accord humans. One might ask: what do a-rational humans
possess that is deserving of moral regard? How are th ey different from the animals from
which we withhold regard? I am not suggesting that this question be answered by saying
“Nothing.” I simply observe that it poses a problem. Second, this is an utterly un-
Aristotelian path, and lies beyond the scope of my remit.
b. One might drop the idea that rationality defines the human species. In general, the
idea that any intrinsic characteristic defines a species is suspect today . (See Ereshefsky
and Matthen 2005 for an up-to-date version of the argument.) The quick reason is that
according to contemporary Darwinian biology, species display variety, bot h synchronic
and diachronic. Consequently , many have Sports Black Onzie Sports Manchester Shirt Onzie Black Manchester Shirt FwPRqYw adopted a relational notion of species. One
example of such a notion is the Biological Species Concept, which (to simplify greatly)
implies that the offspring of two members of any species belongs to that species
regardless of what characteristic it might Base G star Basic Grey Heather shirt T have. This conc eption pulls us back from the
worst in Aristotle’s theory: it accords human status to slaves and women, and pushes
back against the idea that being a human is a matter of degree. However, because such
species concepts undermine the idea that certain capacities define the human species, it
also casts doubt on the idea that humans are worthy of moral respect because th ey have
certain potentialities. Thus, it gives us no help with the questions posed at the start of the
last section. We are left in the dark with regard to when an embryo deserves moral
Base Grey shirt T star Heather G Basic respect.
T o sum up. Aristotle’s notion of natural kinēsis implies that we should not treat
the entity at the beginning of embryonic development as human, or indeed as the same as
the one that is born. This leads us to ask: When does the embryo turn into a human?
Aristotle’s own answer to this question is very harsh. Bracketing the views that lead to
ARISTOTLE’S THEOR Y OF POTENTIALITY
this harsh answer , his theory of kinēsis still gives us reason for searching for a
replacement answer . Aristotle’ s own work unfortunately gives us no help in finding this
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Blackwell Guide to Aristotle Oxford: Blackwell: 292-309.