Conjoined Twins and the Biological Account of Personal Identity1
Abstract
During the first 16 days after fertilization, the developing embryo has the capacity to
separate into two genetically identical embryos, or monozygotic twins (triplets, etc).
Because of this capacity, philosophers typically argue that the pre-16 day embryo is not a
human being. On a Biological Account of Personal Identity, which considers us human
beings as essentially organisms, the development of the embryo into an organism at 16
(or 21) days marks our origins. The development of an embryo into an organism is also
said to mark the point at which the embryo loses its potential to give rise to monozygotic
twins. This is considered a strength of the Biological Account of our origins. I will argue
here that it is the BAPI’s own criteria that will have to allow cases monozygotic twinning
occurring even after 16 days, and that in virtue of this a Biological Account faces
twinning puzzles usually reserved for earlier accounts of our origins.
I. Introduction
The first 16 days of human embryonic development invite a host of
twinning puzzles. During this time frame, the developing embryo can give rise to two or
more genetically identical human beings, a capacity often considered by philosophers as
sufficient reason for denying that we existed prior to 16 days after fertilization. On a
Biological Account of Personal Identity (BAPI), advanced most famously by Eric Olson,
the first 16 days after fertilization is the period of development in which the conditions
for being a human are not yet satisfied. On a BAPI, human beings are identical to
organisms, and the pre-16 day old embryo is, to many biologists and philosophers, not
An earlier version of this paper was presented at the University at Buffalo’s Metaphysics and Medicine
Conference, Fall 2004. I would like to thank all of the participants, particularly David Hershenov, Alfonso
1
1
yet an organism. At 16 days, however, the embryo is said to satisfy the conditions for
being an organism, and so, according to most BAPIs, a human being. At the same time,
the embryo is said to no longer have the potential to divide into two new genetically
identical embryos, or monozygotic twins. That this is so is considered a strength of a
Biological Account of our origins: at the same time as the organism to which we are
considered to be identical comes into existence, the twinning puzzles that are said to
plague accounts that posit our origins during the first two weeks disappear.
In this paper I will argue that, contrary to most BAPIs, twinning puzzles
persist until at least the 21st day after development. In anomalous cases of monozygotic
twinning, the 16 day old embryo that the most prominent BAPIs claim is an organism,
identical to a human being, can give rise to two organisms. Because of these anomalous
cases, a BAPI faces puzzles that are usually reserved for arguments for our origins prior
to 16 days.
In the next section of this paper I will offer biological and philosophical
accounts of what it is to be an organism. In the third section I will review human
embryology with an eye towards when conditions for being an organism are first met.
The third section will also review the puzzles that twinning is said to pose to our early
(pre-16 day) origins. I will then offer an account of how, in anomalous cases of
monozygotic twinning, a Biological Account is also faced with twinning puzzles and
propose a solution.
II.) Biological Accounts of Our Origins
Eric Olson tells us that it is ultimately the business of biologists to answer the
question “what is it to be an organism?” The biologist Clifford Grobstein, to whom Olson
Gomez-Lobo, John Fisher, Matthew Liao, Jeff McMahan, Barry Smith and Chris Tollefson.
2
refers, tells us that “an organism is a complex macromolecular structure that behaves as a
unit and is capable of replication through a conversion of materials and energies derived
from its environment through a self-controlled interface or boundary” (1964: 61). Olson’s
account is captured in Grobstein’s definition, and so he argues that we can infer three
conditions necessary to being an organism. The first is that in order for an entity to be an
organism, it is necessary that it metabolize, or both exchange matter and energy with its
surroundings and at the same maintain a dynamic stability. Like a flame or fountain, the
organism retains its characteristic form and structure despite this rapid change of matter.
And yet, unlike a flame, whose size depends on the surrounding oxygen and fuel, etc., an
organism must also satisfy a second condition: it must have an internal mechanism, a
“teleology” which will allow it to adjust itself and to take advantage of changes in its
surrounding. The teleological or “goal directed behavior”, Olson claims, is grounded in a
third condition necessary to an organism, “an underlying biochemical structure of
unimaginable complexity” (Olson 1997: 128). This organized complexity, he argues, is
not reducible to having a vast number of parts. Rather, it involves the arrangement and
interaction of these parts, which in turn allows for the goal-directed behavior or teleology
that consists of metabolic activity. In sum, writes Olson, each organism has what Locke
referred to as a “life” or a “special kind of event, roughly the sum of the metabolic
activities the organism’s parts are caught up in” (Olson 1997: 136). The life of the
organism is a well-individuated event that has a definite boundary: “every organism has a
life…and an organism cannot be animated by two lives, at least not at once. This suggests
that lives might match up to organisms one to one” (Olson 1997: 137).
On Smith and Brogaard’s BAPI, the organism is an Aristotelian substance and
3
so a bearer of change, with a unified boundary, that persists through time as one and the
same entity. What separates an organism from other substances such as footballs or tables
is that “it is a unified causal system that is relatively isolated from its surroundings”
(Smith and Brogaard 2003: 49). In order for an entity to be an organism, then, it must be
the case that:
1.) the external boundary of the entity is established via a physical covering that
extends continuously across all or almost all of its surface (“almost all” because it
typically contains small apertures – such as pores, mouth or nostrils—which allow
interchange of substances such as air and food between exterior and interior).
2.) The events transpiring within the entity are subject to a division between those
whose characteristic magnitudes (of temperature, pressure, chemical composition
and so on) fall within a certain spectrum of allowed values and those where
corresponding magnitudes fall outside this spectrum. The former fall within a
restricted family of types of sequences of events (for example, digestion) which
are often cyclically repeated. The latter are distinguished by the fact that they will,
in cumulation, lead to the entity’s ceasing to exist.
3.) The external boundary or membrane serves as a shield to protect the entity
from those causal influences deriving from its exterior which are likely to give
rise to events which are outside the spectrum of allowed values.
4.) The entity contains within itself its own mechanisms which are able to
maintain (or in cases of disturbance reestablish) sequences of events falling within
the spectrum of allowed values. The entity also contains within itself mechanisms
for reconstituting or replacing its external membrane or covering in case of
4
damage (Smith and Brogaard 2003: 49-50).
These conditions are not sufficient, however, for an entity to be a higher
organism, of which human beings are a kind. It is maintained that since human beings
and other higher organisms are unitary individuals “in the strong sense that they cannot
be subject to division into two or more entities which are similar to themselves” an
additional condition must be satisfied (2003: 66). And so, “In order for an entity to be a
human organism, the parts of the entity must be integrated together in such a way that it
is not able to divide and goes out of existence and is replaced by two or more entities
which themselves are organisms” (2003: 66). This additional condition offers a
distinction between higher organisms, such as human beings, and the lower organic
entities that can divide into two of their kind, such as amoebas.
III.) Human Embryology and the Origin of the Human Organism
When we turn to the details of human embryology, the unicellular zygote is an
organism, but it fails to satisfy the condition for Smith and Brogaard’s condition unitary
individuality. This is because the unicellular zygote is destined to undergo mitotic
division into two cells that are said to be similar to it.2 And, the details of the next period
of embryonic development strengthen the claim that the zygote is not the kind of
organism that we are. Advocates of a BAPI maintain (as do many biologists) that after
the first mitotic division of the zygote, the multi-cellular so-called pre-embryo is but a
loose aggregate of cells that have minimal inter-communication. The pre-embryo does
not have the necessary common life that all of its parts share in; rather, each individual
cell metabolizes and functions as a single cell rather than as part of a multi-cellular entity.
2
While Olson does not specifically advance the condition of unitary individuality, he denies that the
unicellular fertilized egg that is an organism is identical to a human being (See Human Animal p. 90)
5
Even the membrane (the zona pellucida) that covers the pre-embryo is not a part of the
pre-embryo, but the membrane of the fertilized egg from which the pre-embryo
developed. This membrane serves as a protective jacket to the developing embryo, but is
shed at approximately 6 days after fertilization. And so the outer boundary of the early
pre-embryo does not appear to be the sort to which Grobstein or Smith refer: it is not a
self-controlled boundary or interface but merely a vestige of the original unicellular
zygote.
Towards the end of week one, the cells begin to specialize and migrate to areas
that are destined to become the future embryo proper (which will develop into the fetus),
and extra-embryonic membranes and tissues. But, even if it should be argued, as some
do, that the pre-embryo is an organism at this stage, the possibility of twinning still exists.
In fact, approximately two- thirds of monozygotic twinning cases occur during the
second week of development; most of the other third occur during the first week.3 This
not only renders implausible a claim that the embryo is a human being (since we
allegedly do not have this capacity to divide) but also poses difficulties to the persistence
of the same individual from fertilization onwards. In cases of monozygotic twinning (or
tripletting, etc.), a single pre-embryo divides into two or more pre-embryos. The
identification of the original pre-twinning pre-embryos with one of the “twins” is
arbitrary and arguably an ad hoc concession made so as to preserve an account of our
early origins. Nor is it at all likely that the original pre-embryo is now a scattered object,
or that the pre-embryo survives as both of these “twins”. More likely than survival is that
when the pre-embryo divides into twins it “fissions out of existence”. And a problem
3
Discerning this figure is made possible by examining what membranes are shared by the twins and at
what point these membranes begin to develop. Twins that have almost no membranes in common originate
during the first 6 days after fertilization.
6
with this is that now the timing of our origins varies: some human beings (those that are
not identical twins) come into existence at an earlier point than others. (Although
“varying origins” does not render an account of our origins incoherent, a theory that
entails this problem is usually considered less appealing than one that gives a categorical
account of our origins). What accompanies fissioning and varying origins is perhaps a
more difficult obstacle: if it is argued that the pre-embryo is a human being, then every
set of monozygotic multiples has a tragic aspect, since they came into the existence at the
cost of the life of a genetically identical sibling. I will refer to these three problems as
“varying origins”, “fissioning out of existence” and “the tragedy of the multiples”.
One might argue, however, that twinning is an exception, and so an account of
our origins need only offer an explanation for those normal cases, when the pre-embryo
does not twin. The problem with this is that it is said that during the first 16 days after
fertilization, although only some pre-embryos may twin, every pre-embryo has the
potential to twin, or can be forcibly twinned. Smith and Brogaard tell us that “we know
that at every pre-gastrular stage that the foster (or the pre-embryo) is able to undergo
division in such a way as to give rise to two or more distinct human individuals” (2003:
67). One explanation for this capacity is that during the first 4 days after fertilization,
each of the cells of the pre-embryo is said to be totipotent: it is argued that if any one of
the cells is removed and placed in a proper environment, it can become an embryo that
will develop into a fetus. During the first 4 days, then, the pre-embryo is potentially a
human being and each of its cells potentially a human being. The very young preembryo, Smith and Brogaard tell us, is “actually one but potentially many” (25). These
problems that accompany potential twinning persist, it is said, even after the cells are said
7
to no longer be totipotent, since a more general sense of totipotency applies to the preembryo as a whole. Olson tells us that “during the first two weeks after fertilization, if
you separated the cells (of the pre-embryo) into two clumps of cells you would end up
with identical twins” (1997: 90). And so, if it is argued that each of the to-be-separated
clumps is a human being and the pre-embryo composed of these cells is a human being,
the multi-cellular pre-embryo is, again, actually one but potentially many.
One problem that arises with this is that since it is prima facie evident that we are
not “actually one but potentially many”, it seems implausible that the pre-embryo is one
of us. It is, at best, like a lower organism (e.g., an amoeba or a plant), that can be divided
into many of the same kind. A second problem that arises with this is that if were put
forth that the pre-embryo is a human being, then each of its cells or clumps of cells would
also be a human being. On this account, then, every single birth would have a tragic
aspect: it would entail the subsumption of two or more human beings that are genetically
identical siblings of the survivor. This is the converse of the “tragedy of the multiples”:
the survival of the embryo as a whole would entail the demise of those human beings of
which it was composed. I will refer to this as the “subsumption problem”.
These puzzles persist for as long as the pre-embryo has the capacity to twin, and
are argued to be sufficient reason for denying our having existed while twinning is still
possible. And so, ideally, the origin of the human organism will occur at a point that
twinning is no longer possible. Many advocates of the Biological Account argue that this
is, in fact, what happens. On Olson’s and Smith and Brogaard’s accounts, it is with the
process of gastrulation, which begins at 14 days and is completed at 16 days, that the
human organism comes into existence. Olson tells us that
8
many embryologists believe that a genuine human embryo…the multicellular organism that later becomes a fetus, an infant and an
adult…comes into being about sixteen days after fertilization, when the
cells that develop into the fetus (as opposed to the placenta) become
specialized and begin to grow and function in the coordinated manner.
They develop bilateral symmetry around the “primitive streak”, the
ancestor of the spinal cord. At this point twinning is no longer possible:
cutting away half the cells would not result in two smaller living embryos
but would simply cause death (1997:91).
And Smith and Brogaard tell us that
…it is with gastrulation that the foster ceases to be a cluster of
homogenous cells and is transformed into a singular heterogeneous entity,
a whole living multi-cellular individual living being. It is with a
gastrulation that the embryo’s cranial axis and its dorsal and ventral
surfaces come into existence, and it is from this point that the boundaries
of a discrete, coherent entity have been formed. (2003: 62-63).
IV.) The Origin of the Human Organism
Gastrulation, which occurs from days 14-16 in human embryonic development
is said to be
…the truly important event in your life…during gastrulation, the
orientation of the body is laid down, and the embryo is converted into a
structure of three germ layer: ectoderm, endoderm and mesoderm, all of
which are derived from the epiblast. The three germ layers are the
9
progenitors of all the tissues of the organism (Strachan and Read: 88).
At gastrulation, several physical changes take place which many philosophers (following
the lead of biologists) submit as indicative of the first point in embryonic development
that the human being exists. The process of gastrulation begins with a massive migration
of cells that will form the precursor to the spinal cord; this is called the primitive streak.
As the streak lengthens by addition to cells to its caudal (tail) end, the cells at the cranial
end multiply rapidly and form the primitive node. These physical markers are said, by
many biologists, to be the first signs of the embryo’s body axes: the head and tail and the
dorsal/ventral surfaces, marked by the primitive streak; and the right and left sides,
relative to the discernable head and tail of the embryo (Moore and Persuad: 46). Because
of these happenings, the philosopher Norman Ford, whom is referred to by both Olson
and Smith and Brogaard as sharing their view for our 16 day origins, tells us that
…as gastrulation is getting underway, a convergence of epiblastic cells
converges in the posterior part of the embryonic disc. This is called the
primitive streak. This appears to be the first stage when the cells become
organized through this primitive streak into one whole multi-cellular
individual living human being, possessing for the first time a body axis
and bilateral symmetry’ (1988: 172-172).
While most advocates of the Biological Accounts of our origins subscribe to the
16 day view, there are other BAPIs that argue that a different development stage marks
the point of our origins. Hershenov argues that the conditions that are necessary to being
an organism are not satisfied until approximately 21 days after fertilization, when the
heart begins to beat and the circulatory system is established. On his account, “what
10
matters is that the human animal functions as a unit that metabolizes food, excretes waste,
assimilates oxygen and maintains homeostasis until it maintains itself” (2002: 511). The
first point at which these conditions are satisfied is, he argues, not gastrulation but rather
when systematic functioning is established. “Thus in human beings”, he writes, “it is only
when the heart forms and the primitive circulation begins (at 21 days) that the clump of
cells forming for the past three weeks constitute a biological system” (2002: 507). Apart
from a lack of systemic functioning, he also rejects a 16 day account because those
conditions argued as sufficient for the origin of the human organism—the development of
the primitive streak and cell differentiation—are found in a corpse and so are not
sufficient for being alive.
On either the 16 day or the 21 day account, however, it appears that if the details
of human embryonic development are correct, the origin of the organism either coincides
with or succeeds the point at which twinning is no longer possible. In fact, Smith and
Brogaard claim that the prohibition of twinning does not merely coincide with the
developments that take place at gastrulation, but that gastrulation brings this about; the
loss of potential to twin is a function of the development of the organism. “Gastrulation”
it is argued, “brings a new type of integration which is manifested in the fact that
twinning is no longer possible” (2003: 63). And since the capacity to twin is said to end
with gastrulation, Hershenov’s account of our origins after gastrulation does not have to
attend to twinning puzzles.
It is true that embryonic data (as far as I know) does not offer cases of twinning
in which a 16 day old embryo divides fully into two separate embryos. However, I will
offer cases of twinning in which the 16 day old organism gives rise to two organisms.
11
These cases, I will argue, pose the same puzzles that are considered sufficient grounds for
denying that we were once a pre-embryo: like the pre-embryo that gives rise to two preembryos, the 16 day old organism can also rise to two of its kind. This is the case not
only for the 16 day old organism, but for the 21 day old organism that is offered on
Hershenov’s account. It is also possible on a third alternative account which would posit
the origin of the organism, at 18-22 days, at when primordial brain begins to form.
Because of these cases, I will argue that the organism that is said to be the human being
has twinning capacities (and twinning problems) that are typically considered sufficient
grounds for denying that the pre-embryo is one of us.
V.) Conjoined twinning and the Human Organism
Monozygotic twinning is a relatively rare event, occurring in only 4 out of
every 1000 births. In most cases of monozygotic twinning, the pre-embryo divides into
two separate pre-embryos. However, in some cases, the twins fail to separate and share
body parts to greater or lesser extents. These twins that fail to separate are conjoined
twins; conjoined twins are always genetically identical, having developed from the same
egg.
Apart from the fictional case of the extreme dicephalus, which is a single
organism that has separate cerebrum, developed conjoined twins are not usually
considered problematic to BAPI.4 On mainstream Biological Accounts, the “Siamese
Twins” Chang and Eng were two organisms joined together; what they shared was
merely a stretch of cartilage at the sternums. Even with more extreme cases, such as the
non-fictional dicephalic Hensel twins, it may be argued that there are two organisms and
4
See McMahan( 2002: 35-39) for an account of the dicephalus. For a defense of how the Biological
Account can handle the extreme dicephalus, see Hershenov 2004.
12
two human beings, or persons. This is because although these twins share two arms, two
legs and several organs, they have separate heads and nervous systems. On Olson’s
account, the separate brain stems of the Hensel girls are sufficient for individuation; each
stem is a control center for a separate organism.
The problem that conjoined twins pose to a Biological Account is that in some
cases of conjoined twins, although one organism (defined on either the 16 or 21 day
account) appears to exist, two organisms arise from this single organism. The evidence
for this is found with cases of developed conjoined twins that share everything that is
presented as biologically significant at developmental points at which, on a BAPI, the
human being is said to come into existence. This would be either at gastrulation and
formation of a definitive body plan, indicated at 14-16 days by the development of the
primitive streak, or at the 21 day commencement of the circulatory system, indicated by
the beating of the heart. The physical features or parts that these twins do not share
develop after 16 or 21 days and I will argue that this poses a puzzle to a BAPI.
A type of twins that can develop after the 16 day old organism comes into
existence, even should there be cell restriction and a single primitive streak, are
pygopagus twins. These twins are the second most common type of conjoined twins
(comprising approximately 18 %) and involve sharing (to different extents) the spine,
rectum, urinary tract and reproductive organs (see McDowell, et al. 2003; Wirt et al.
1995). Although the extent to which pygopagus twins are joined varies, some have
posterior union that involves sharing a lower spinal cord. What is philosophically
interesting about twins that share a lower spinal cord is that while each developed twins
satisfies conditions for being an organism, at 16 days there would be by all appearances a
13
single embryo. On a 16 day account, then, there is a single organism, indicated by a
single primitive streak (shared by the twins, if both are present), a single boundary (since
even when developed there is a shared boundary) and so by all appearances a single
“life” or “relatively isolated causal system”. In fact, Ford (to whom Olson and Smith
refer) tells us that “the appearance of a single primitive streak signals that only one
embryo proper and human individual has been formed and begun to exist” (1988: 172).
As development continues, however, these twins will eventually develop into
what, on mainstream BAPIs, are two organisms. This becomes evident as body parts that
are not shared come into existence. In fact, pygopagus twins are frequently surgically
separated, and so a theory that would suggest doubting that they are each a human being
would have many metaphysical puzzles with which to contend. And so we have a case in
which an entity that satisfies the conditions for being an organism (the 16 day old
embryo) can give rise to two organisms. An obvious problem that this poses to a BAPI is
that the 16 day old organism is said to be a human being, and yet this 16 day old embryo
can give rise to two human beings.
The same puzzle is found on Hershenov’s 21 day account of the origin of the
human being, which necessitates systematic functioning, a condition satisfied by the
commencement of the beating heart. The most common form of conjoined twins
(approximately 74%), are thoracopagus twins. These twins involve sharing (to different
extents and not necessarily all organs) a heart, liver, thorax and gastrointestinal tract (see
Gilbert-Barness et al. 2003). Thoracopagus twins that are philosophically relevant to this
paper are those that share a heart. In a case of thoracopagus twinning, Hershenov’s
criterion for a single organism would be met at 21 days, since there would be a single
14
circulatory system established, individuated by the single beating heart. But as the
organism developed, two human beings (and so, on a BAPI, two organisms), would
emerge. (Although twins that share a single heart are not successfully separated, we can
imagine that with improvements in medical science they could be). And so with
thoracopagus twins we again have a case in which although only one organism may be
apparently present, a second twin will either develop or is already present but does not
yet have a distinguishing feature.
One could also claim that the organism does not come into existence until the
developmental process of neurulation is underway (those that are “brain oriented” might
argue that a brain is necessary for the existence of the human organism). And so they
might offer that the human being, identical to the organism comes into existence at
approximately the 18th day after fertilization, when the primordial brain begins to form.
At this stage, a “nueral plate” begins to develop to give rise to the brain and the upper
nervous system; at 40 days, the developed upper nervous system will meet the lower
spinal cord to which the primitive streak was an ancestor. On most Biological Accounts,
the conditions for an organism have already been satisfied. However, should the
primordial structure of the brain be considered necessary to an organism, what would
appear to be a single organism, in virtue of an apparently singular process of neurulation,
could give rise to what are arguably two organism. This can occur in the extremely rare
but non-fictional case of janiceps twins, which share a head and parts of the brain, but
have separate lower bodies (see Biswas et al. 2003), including separate brainstems.
What I contend is that pygopagus and thoracopagus (and janiceps) twins pose to
mainstream Biological Accounts the same puzzles that monozygotic twins pose to pre-16
15
days accounts of our origins. With these 3 kinds of conjoined twins, the metaphysically
significant biological characteristics of the 16 day old or 21 day old embryo can be
shared; an embryo would have the same metaphysically significant biological
characteristics, whether or not twins will arise from it. And so even if it is argued that
each twin exists then, sharing these characteristics, a BAPI must claim that they are
either not identical to the single organism that is there (contrary to what is claimed on a
Biological Account, on which human beings are identical to organisms) or there are two
organisms present despite appearances. Furthermore, in these cases the developed twins
are, on any mainstream account, two organisms. So if it is argued that it is a single
organism present at 16 or 21 days that gives rise to two organisms then, like the preembryo, the organism can give rise to two of its kind and so invites the twinning puzzles
that accompany this capacity. I will offer below how the puzzles that accompany each of
these solutions are essentially the same as those in section II above. I will then offer a
solution.
VI.) Conjoined Twinning Puzzles and Solutions
If it is argued that, in some cases of conjoined twins, a single 16 day or 21 day
old organism gives rise to two organism, as in the case of the pre-embryo, it is more
likely that the original 16 day old organism “fissioned out of existence”. This seems more
plausible than it surviving as both organisms that are the conjoined twins, or as a
scattered object. And so should it be claimed that the 16 day old organism is a human
being, then every set of conjoined twins entails the “tragedy of the multiples”: the twins
came into existence at the cost of a genetically identical sibling. “Fissioning out of
existence” also invites the problem of “varying origins” since now human beings come
16
into existence at different times.
One solution to the problems of “fissioning out of existence” and “the tragedy of
the multiples” is to argue that in the case of conjoined twins which (physically)
differentiate after the 16 or 21 day mark, one organism is present at 16 days; the second
twin does not yet exist, but originates at a later date, when what does physically
differentiate it develops. The difficulty with this, again, is “varying origins”, since we do
not have single point at which human beings come into existence.
These puzzles might be avoided if it were argued that the human organism does
not come into existence at 16 or 21 days, but at a later date when conjoined twinning no
longer poses these problems. This may be appealing to an account, such as Smith and
Brogaard=s, which offers a condition of unitary individuality: in virtue of the organism
being able to give rise to two organisms, it cannot be a human being. One problem with
this solution, though, is that it seems to depart from a biological account: most biologists
would tell us that an organism is present at the 21 day mark, if not the 16 day mark. In the
face of a redefinition of “organism”, a biologist may make the charge that philosophers
are defining an organism as a human being, rather than a human being as an organism,
and while this is perhaps legitimate, it is no longer a ‘biological account’ of human
beings. And, this solution seems to go against the spirit of both Olson’s and Smith and
Brogaard’s projects. Olson tells
According to the Biological Approach…you began to exist roughly
fourteen to seventeen days after fertilization. This is purely a factual
matter, to be settled empirically…It is not an ethical question but a
scientific one (91).
17
And Smith and Brogaard write “when does a human being exist? We argue that it is
possible, through a combination of biological fact and philosophical analysis to provide a
definitive answer to this question” (45).5
Another set of problems arises when we look at potential twinning and the
puzzles that it entails. On most BAPIs (and even non-BAPIs) it is claimed that any entity
that naturally twins can be induced to twin. The post-16 day natural twinning which gives
rise to pygopagus, thoracopagus and janiceps twins implies, then, that post-16 day
induced twinning could also occur. In fact, what this implies is that every human
organism, at least at the early stages, can be induced to twin, in virtue of the claim that
some can twin. If this is the case, then it is difficult to argue that the human organism
which comes into existence on the given Biological Account is identical to the individual
human being. In fact, Biological Accounts now have to answer to their own objection to
our once having been a pre-embryo that can potentially give rise to two human beings.
This is, of course, the “subsumption problem” to which I refer above. Because of the
possibility of the “subsumption problem” even on a Biological account every single birth
has a tragic aspect since it came to be at the cost of a genetically identical sibling that
would have developed as a conjoined twin.
In order to maintain that the Biological Account=s claim that we came into
existence with the organism, I suggest a two-part solution. The first part is to deny that
any entity that divides into two of its kind naturally can also be divided into two of its
kind artificially. The reason why I suggest this is that although biology offers us cases in
which some human pre-embryos can naturally twin, it does not offer evidence that human
5
One could also claim that the single organism did not in fact twin, but grew extra organs, each cerebrum
belonging to a different person but both belonging to a single organism. This, though, is incompatible with
a Biological Account’s claim that the person is identical to the organism, and so might be an unwelcome
18
pre-embryos can be artificially twinned.6 All of the evidence for totipotency and forced
twinning has been found in animal studies and it is inferred from these that the human
pre-embryo has this capacity. And so, I suggest that a Biological Account make use of
this data (or lack thereof) on the human pre-embryo. They can claim that although the
organism can naturally twin it does not necessarily follow that it can be induced to twin
(a problem with this, though, is that it weakens a Biological Accounts own arguments
against our early pre-16 day origins). In doing so, a Biological Account can avoid the
“subsumption problem”. The problems of “fissioning out of existence”, “varying origins”
and “the tragedy of the multiples” can be avoided with the second part of my solution.
In order to avoid these (somewhat interrelated) problems, I suggest that a
Biological Account argue that in cases of pygopagus, thoracopagus and janiceps twins,
these twins were co-located at 16 or 21 days. It is not until a later point in development
that they manifest organic distinctness: with Olson’s empirically identifiable separate
“lives” or Smith and Brogaard’s “relatively isolated causal systems”.7 On a Biological
Account of Personal Identity, co-located twins which eventually separate would appear as
one organism, but two organisms (i.e., two human beings) would actually be present.8 At
16 or 21 days, each atom or cell of the twins would be spatially co-located; at a later
point in development, these two organisms would separate into two distinct but conjoined
organisms. This would allow for both the appearance of one organism and, when the
twins separate, the plausibility of the claim that there were two there all the while there
compromise.
6
The only published attempt at forced twinning of the human ‘pre-embryo’ was unsuccessful (See Hall et
al. 1993
7
Smith and Brogaard invite problems even in the case of developed conjoined twins. This is beyond the
scope of this paper.
8
For arguments against the Lockean claim that co-located entities cannot be of the same kind see
Hershenov, David, 2003; Fine, Kit, 2000; and Hughes, Christopher, 1997.
19
appeared to be but one. In this way, the concessions offered above that would entail
“varying origins”, “fissioning out of existence”, and “the tragedy of the multiples” need
not be admitted. And, on this account, even if there appears to be one organism, two
organisms are present. In the cases of conjoined twins mentioned above, both twins will
be present at the first sign of a single organism and yet each will be identical to a separate
but co-located organism.
This solution, though, implies that the physical characteristics argued to be
sufficient for indicating that the human organism is present are actually relatively
insignificant. This is because two organism (or even two organs sufficient for the
existence of an organism, such as the heart), could be present despite the appearance of
being one. And so, if conjoined twins are co-located, then the alleged individuating
physical traits, such as vital organs (on the 21 day account), or the primitive streak that on
the 16 day account lays out the body plan of the individual, are not sufficient to indicate
individuality; something else is. Since physical or biological characteristics of the
organism—which themselves should be evident to the biologist, and not requiring
metaphysical assistance in order to be identified—are not sufficient for determining when
the human individual(s), identical to the human being and organism, comes into
existence, a Biological Account should offer an account of how to determine when we
come into existence. This may be dismissed as an epistemic problem, rather than an
ontological one. However, a hallmark of a Biological Account is the simplicity with
which we can determine the origin of the human beings, and arguments made for
colocation may alienate it from a true biological account. Again, Smith and Brogaard
claim that “through a combination of biological fact and philosophical analysis” it is
20
possible to provide a definitive answer to the question “when does a human being begin
to exist” (2003: 45). And Olson=s account of this “well individuated event that is a life”
is that “lives are easy to count: in most cases there is a clear difference between a
situation that contains one life and a situation that contains two...a particle cannot
participate in two lives any more than one can serve in two armies at once; and two lives
cannot overlap” (1997: 137).
A second problem with a co-location account is that a much lauded advantage of
a BAPI is that it does not involve spatially coincident objects. One of the major criticisms
of non-Biological accounts (e.g., the Psychological Account) is that the relationship
between us and our bodies is not identity; rather, we are spatially coincident with our
bodies, i.e. we are human persons that share matter with an animal. However, in the case
of a vegetative coma, this animal could survive our demise. This relationship of nonidentity invites, it is argued, serious metaphysical challenges. (Olson 1997: especially 94123; Olson 1996; Olson 2001; Corcoran (ed.) 2001: 73-88). Because on a Biological
Account the human being is identical to the human organism, it avoids the problems that
accompany a theory of spatially coincident objects. In comparison to other theories of
personal identity, it is considered a significant advantage of a BAPI that it avoids those
problems that accompany the spatial coincidence of entities. 9 So, if a Biological Account
adopts a theory of co-located twins so as to avoid other problems, it loses one of its major
advantages over other theories of personal identity. Now it too has problems which come
with materially and spatially co-located entities.
A further problem with the colocation solution stems, again, from charges
9
For example, the problem of too many thinkers (both the body and the person are thinking) and a
contingent relationship between the organism (or body) and the person, since neither are identical to the
other). These specific puzzles would most likely not accompany spatially coincident 16 or 21 day
21
which advocates of a Biological Account have made against our origins at fertilization. If
it is granted that two individuals– in this case, organisms– of the same kind can be
spatially coincident at 16 days, then the same can be said about the unicellular zygote
stage: since the zygote is an organism, there could be more than one individual organism
that is co-located but will eventually separate (either successfully or unsuccessfully) into
monozygotic multiples. This would be useful as a solution to the twinning puzzles prior
to 16 days. If this is not possible, a Biological Account should offer an account of why
human organisms can be co-located only at 16 or 21 days. If this is granted, though, a
difficulty that arises is that a Biological Account would now have to present an argument
for why mitotic division of the unicellular zygote and its daughter cells does not pose
problems to the persistence of the individual(s). (This is, again, an objection typically
made on a Biological Account against our origins at fertilization (Olson 1997: 90-93; van
Inwagen 1990: 152-153; Smith and Brogaard 2003: 59)). It is also problematic that if
there are two organisms present at fertilization, the two human beings that would
eventually separate into conjoined or separated twins would go through a phase of not
being an organism. This is because biological considerations would deny that the
‘cluster’ of cells which exists from the first mitotic division to the ‘resurfacing’ of the
organism is an organism. Our having existed as non-organisms would be contrary to the
central claim of a BAPI, which is that we are essentially organisms.
VIII.) Conclusion
Anomalous cases of conjoined twinning, I have argued, present to a Biological
Account the puzzles that are usually considered sufficient reason to deny our early
origins. The solutions that are offered may lessen the extent of the problems posed, but
organisms.
22
there are nonetheless puzzles that arise in these cases of conjoined twins. These puzzles
are typically considered to be sufficient grounds for denying, on a Biological Account of
Personal Identity, that the zygote or early embryo is a human being. The solutions offered
may lessen the difficulties; however, a Biological Account of our origins may be more
complicated than is typically presented.
References
1.) Biswas A., Chia D.,and Wong, Y.C., 2001, “Letters to the Editor: Three-dimensional
Sonographic Diagnosis of Cephalothoracopagus Janiceps Twins at 13 Weeks”,
Ultrasound in Obstetrics and Gynecology vol. 13 pp. 289-291.
2.) Corcoran Kevin (ed.) 2002, Soul Body and Survival: Essays on the Metaphysics of
Human Persons, Ithaca, Cornell University Press.
3.) Fine, Kit 2000, “A Counterexample to Locke’s Thesis”, Monist vol. 83 no.3 pp. 357361.
4.) Ford, Norman 1988, When Did I Begin?, Cambridge, Cambridge University Press.
5.) Gilbert- Barness, Enid; Debich-Spencer, Diane; Opitz, John M. 2003, “Conjoined
Twins: Morphogenesis of the Heart and a Review”, American Journal of Medical
Genetics 120 A: 568-582.
6.) Grobstein, Clifford 1964, The Strategy of Life, San Francisco, Freeman.
7.) Hall, J.L. et al. 1993, “Experimental Cloning of Human Polyploid Embryos Using an
Artificial Zona Pellucida”, The American Fertility Society conjointly with the Canadian
Fertility Society and Andrology Society, Program Supplement, Abstracts of the Scientific
Oral and Poster Sessions, Abstract 0-001 S1.
8.) Hershenov, David 2002, “Olson’s Embryo Problem”, Australasian Journal of
Philosophy vol. 80 (4) pp. 502-511.
23
9.) Hershenov David 2004, “Countering the Appeal of the Psychological Approach to
Personal Identity” Philosophy 79, pp. 445-472.
10.) Hershenov, David, 2003, “Can There Be Spatially Coincident Entities of the Same
Kind?” Canadian Journal of Philosophy vol. 31 no. 1.
11.) Hughes, Christopher, 1997 “Same-Kind Coincidence and the Ship of Theseus”
Mind, vol. 106 no. 421. pp. 53-67.
12.) McDowell, Brad et al., 2003, “Case Report: Separation of Conjoined Pygopagus
Twins”, Plastic and Reconstructive Surgery vol. 111 no. 6 pp. 1998-2002.
13.) McMahan, Jefferson, 2002, The Ethics of Killing: Problems at the Margins of Life,
Oxford, Oxford University Press
14.) Moore and Persuad, 2003, Before We are Born: Essentials of Embryology and Birth
Defects, (6th edition), Philadelphia, Saunders.
15.) Olson, Eric,1997, The Human Animal: Identity without Psychology, Oxford, Oxford
University Press.
16.) Olson, Eric, 1996, “Composition and Coincidence”, Pacific Philosophical Quarterly
vol.77 no. 4 pp. 374-403.
17.) Olson, Eric 2001, “Material Coincidence and the Indiscernibility Problem”, The
Philosophical Quarterly vol. 51, no. 204.
18.) Smith, B. and Brogaard. B. 2003, “Sixteen Days” Journal of Medicine and
Philosophy, vol. 28 no. 1 pp. 45-78.
19.) Strachan T. and Read, A. 2004, Human Molecular Genetics (3rd ed.), New York,
Garland.
20.) van Inwagen, Peter, 1990, Material Beings, Ithaca, Cornell University Press.
24
21.) Wirt, Shelia W. RN; Algren, Chris L. RN; Wallace, Valerie R. RN; Glass, Nancy
MD, 1995, “Separation of Conjoined Twins (Clinical)”AORN online: The Association of
Perioperative Nurses vol 62(4) pp. 527-545.
25