One popular criterion, associated with Plato, Descartes and a number of
world religions, is that persons are immaterial souls or pure egos. On this
view, persons have bodies only contingently, not necessarily; so they can live
after bodily death. Even though this so-called Simple View satisfies certain
religious or spiritual predilections, it faces metaphysical and epistemological
obstacles, as we shall see.
Another intuitively appealing view, championed by John Locke, holds that
personal identity is a matter of psychological continuity. According to this
view, in order for a person X to survive a particular adventure, it is
necessary and sufficient that there exists, at a time after the adventure, a
person Y who psychologically evolved out of X. This idea is typically cashed
out in terms of overlapping chains of direct psychological connections, as
those causal and cognitive connections between beliefs, desires, intentions,
experiential memories, character traits, and so forth. This Lockean view is
well suited for thought experiments conducted from first-person points of view,
such as body swaps or tele-transportation, but it, too, faces obstacles. For
example, on this view, it appears to be possible for two future persons to be
psychologically continuous with a presently existing person. Can one really
become two? In response to this problem, some commentators have suggested that,
although our beliefs, memories, and intentions are of utmost importance to us,
they are not necessary for our identity, our persistence through time.
A third criterion of personal identity is that we are our bodies, that
is to say, that personal identity is constituted by some brute physical
relation between, for example, different bodies or different life-sustaining
systems at different times. Although this view is still somewhat unpopular,
developments about personal identity theory in the 1990s promise an ideological
change, as versions of the so-called somatic criterion, associated with Eric
Olson and Paul Snowdon, attract a continuously growing number of adherents.
The aim of this article is to (1) add precision to the problem of
personal identity, (2) state a number of theories of personal identity and give
arguments for and against them, (3) formulate “the paradox of identity,” which proposes
to show that posing the persistence question, in conjunction with a number of
plausible assumptions, leads to a contradiction, and (4) explain how Derek
Parfit’s theory of persons attempts to answer this paradox.
Understanding the Problem of
Personal Identity
The persistence question, the question of what personal identity over
time consists in, is literally a question of life and death: answers to it
determine, insofar as that is possible, the conditions under which we survive,
or cease to exist in the course of, certain adventures. These adventures do not
have to be theoretically as fancy as the cases, to be discussed later, of human
fission or brain swaps: a theory of personal identity tells us whether we can live
through the acquisition of complex cognitive capacities in our development from
fetus to person, or whether we have survived car accidents if we find ourselves
in a persistent vegetative state. Furthermore, theories of personal identity
have ethical and metaphysical implications of considerable magnitude: in
conjunction with certain normative premises they may support the justification
or condemnation of infanticide or euthanasia, or they could prove or falsify
certain aspects of our religious outlook, in deciding the questions of how and
whether we can be resurrected and whether we are possessors of souls whose
existence conditions are identical with ours. It is not surprising, therefore,
that most great philosophers have attempted to solve the problem of personal
identity, or have committed themselves to metaphysical systems that have
substantial implications with regards to the problem, and that most religious
belief systems give explicit answers to the persistence question. Neither is it
surprising that virtually everybody holds a pre-theoretical theory of personal
identity, if only in the sense of having beliefs about afterlives and the
meaning of death. The task of solving the metaphysical problem of personal
identity essentially involves answering the question of how the phenomenon or
principle in virtue of which “entities like us” persist through time is to be
specified, under the widely but not universally accepted premises that there is
such a phenomenon or principle and that it can be specified. We are concerned,
in other words, with the truth-makers of personal identity statements: what
makes it true that our statement that an entity X at time t1 and an entity Y at
time t2 are identical, if X and Y are entities like us?
a. Criteria and the Identity
Relation
Answers to the persistence question often provide a criterion of
personal identity. A criterion is a set of non-trivial necessary and sufficient
conditions that determines, insofar as that is possible, whether distinct
temporally indexed person-stages are stages of one and the same continuant
person. (A temporally indexed person-stage is a slice of a continuant person
that extends in three spatial dimensions but has no temporal extension.) To say
that C is a necessary condition for E is to say that if E is the case, then C
is the case as well, and to say that C is a sufficient condition for E is to
say that if C is the case, then E is the case as well. Consequently, to specify
such a criterion is to give an account of what personal identity necessarily
consists in.
Let us distinguish between numerical identity and qualitative identity
(exact similarity): X and Y are numerically identical iff X and Y are one thing
rather than two, while X and Y are qualitatively identical iff, for the set of
non-relational properties F1…Fn of X, Y only possesses F1…Fn. (A property may
be called “non-relational” if its being borne by a substance is independent of
the relations in which property or substance stand to other properties or
substances.) Personal identity is an instance of the relation of numerical
identity; investigations into the nature of the former, therefore, must respect
the formal properties that govern the latter. The concept of identity is
uniquely defined by (a) the logical laws of congruence: if X is identical with
Y, then all non-relational properties borne by X are borne by Y, or formally “∀(x, y)[(x = y) → (Fx = Fy)]; and (b) reflexivity:
every X is identical with itself, or formally “∀x(x = x). (Note that congruence and reflexivity entail
that identity is symmetric, “∀(x,
y)[(x = y) → (y = x)], and transitive, “∀(x, y, z)[((x = y) & (y = z)) → (x = z)]). [Note: ∀(x, y) is an abbreviation of (∀x)(∀y).]
Grasp of the notion of numerical identity, to be sure, is essential to
our ability to distinguish between the events of picking out one thing more
often than once and picking out more than one thing. Although exact similarity
is, by congruence, a necessary condition for synchronic personal identity, it
is neither necessary nor sufficient for diachronic personal identity, that is
to say, the persistence of a person over time: two person-slices at different
times could be qualitatively identical slices of different people or
qualitatively distinct slices of the same person. This is not to say, however,
that it is ruled out that lack of similarity over time may obliterate numerical
personal identity: depending on what personal identity consists in, certain
qualitative changes in a person’s psychology or physiology may kill the person.
The question a criterion of personal identity answers is: what kind of changes
does a person survive?
This gives a distinctive sense to the claim that a criterion of personal
identity is to be constitutive, not merely evidential: in order for a relation
R to be constitutive for personal identity, it must be the case that,
necessarily, if some past or future Y stands in an R-relation to X, then X is
identical with Y. Hence, many elements of our successful everyday
reidentification practices, such as physical appearance, fingerprints, or
signatures, are inadequate if considered as constituting ingredients of
personal identity relations: for example, if the man in the crowd is wearing a
Yankees jacket, this might be sufficient evidence for you to conclude that he
is your friend Larry. However, wearing a Yankees jacket is not what it is for
Larry to persist through time: neither did Larry come into existence when he
wore the jacket for the first time nor does he die when he takes it off.
Does the logic of the concept of identity impose further restraints on
the concept of personal identity? Some commentators believe that identity is an
intrinsic relation, that is, that if two person-stages at different times are
stages of one and the same person, that will be true only in virtue of the
intrinsic relation between these two stages (cf. Noonan 1989; Wiggins 2001).
Others hold identity to be necessarily determinate, that is, that it is
necessarily false that sometimes there is no answer to the question of whether
X is identical with Y. These commentators typically reason as follows: suppose
that it is indeterminate that X is identical with Y. Since it is determinate
that X is identical with X, under the assumption that congruence and predicate
logic apply, X must be determinately identical with Y. Therefore, by modus
tollens, if X is not determinately identical with Y, X is not identical with Y
(cf. Evans 1985; Wiggins 2001). Consequently, the question does in fact have an
answer, and the claim that identity is indeterminate is self-contradictory.
This conclusion is strengthened, in the case of personal identity, by the
widely shared intuition that even if the identity of some objects might be
indeterminate, this could not be true of the identity of persons: one cannot,
it seems, be a bit dead and a bit alive in the same way in which one cannot be
a bit pregnant. As it turns out, however, there may be good reasons to deny
both the intrinsicness and the determinacy of personal identity (cf. 3.a.;
3.b.).
b. Personhood
While the formal properties of the concept of identity are necessary
constraints on our discussion, the truth of our identity judgments is subject
to material conditions of correctness, which these formal properties cannot
provide. These material conditions must be supplied by the nature of the relata
judged to stand in an identity relation. The obvious suggestion is that, given
that we are dealing with personal identity, these relata are person-stages
located at different times. This proposal, however, violates the requirement
that the persistence question ought to specify its relata without presupposing
an answer: should we choose to accept a definition in the vicinity of Locke’s
characterization of a person as a “thinking, intelligent being, that has reason
and reflection, and can consider itself as itself, the same thinking thing in
different times and places” (1689, II.xxvii.9), then those criteria of personal
identity that sanction the identity of a person at one time with a non-person
at another time are categorically ruled out. Fetuses, infants, or human beings
in a persistent vegetative state, for example, plainly do not fulfill the
criteria envisaged by Locke. As a result, since these beings do not possess
cognitive capacities, if they do at all, that qualitatively attain those of
thinking beings, couching the persistence question in terms of persons entails
that none of us has ever been a fetus or infant or ever will be a human
vegetable (Olson 1997a; Mackie 1999). To be sure, these initially baffling
claims could be true. However, since these are clearly substantial questions
about our persistence, we should not consider ourselves justified to settle the
matter by definition. Consequently, we should prefer vagueness over chauvinism
and pose the persistence question in terms of the wider notion of human being,
postponing the question of whether and in what sense the notions of person and
human being ought to be distinguished: for any person X and any human being Y
at different times t1 and t2, if X at t1 is numerically identical with Y at t2,
what makes this claim necessarily true?
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