the Indiscernibility of Identicals,
and the Identity of Indiscernibles
Leibniz's Law is a
bi-conditional that claims the following: Necessarily, for anything, x, and anything, y, x is identical to y if
and only if for any
has, y has, and for any property y has, x has. Because this is a bi-conditional, it is comprised of
two conditional statements (i) and (ii):
If x is identical to y, then
for any property x has, y has and for any property y has, x has.
(ii) If for any property x has, y has, and for any property y has, x has, then x is identical to y.
The Indiscernibility of
(i) is called the Indiscernibility of Identicals because it
claims that self-identical object(s) must be indiscernible from
themselves. It is a fairly uncontroversial thesis. I say "fairly"
because there are philosophers
who deny this claim. And,
time permitting, we will be discussing them at some point in the
Almost everyone else, however,
will grant that if
something, x, is identical
with something, y, then x and y have all of the same properties; x and y are just one thing, after all, merely
called by two different names "x"
and "y." If Superman is
identical to Clark Kent, then Superman wears glasses and
Clark Kent has x-ray vision (because Clark Kent wears glasses and
Superman has x-ray vision). Since Superman is identical to Clark Kent,
there is no property that Superman has that Clark Kent does not have,
and there is no property that Clark Kent has that Superman doesn't;
"they" are just one guy, after all, not two.
Notice that another way to put the
Indiscernibility of Identicals is in
terms of qualitative and numerical (or
(which will be
discussed in class and on this handout here). We could state the Indiscernibility
Identicals as: If x and y are numerically identical, then x and y are qualitatively identical.
Identity of Indiscernibles
(ii) is called the Identity of Indiscernibles because it
claims that indiscernible objects must be identical. This thesis has
raised quite a bit of debate among metaphysicians. You might think it
is intuitive because of its practical applications: it seems that we do in fact use this principle when
we are trying to determine whether we've got one thing in front of us
or two. Suppose we are trying to figure out whether
Superman is identical to Clark Kent. We begin: "Well, Superman is 6ft
tall, and Clark Kent is 6 ft tall; Superman has dark hair and dark
eyes, and Clark Kent has dark hair and dark eyes; Superman can't get
injured by fire and--look!--Clark Kent can't get injured by fire,
either!", etc. If a certain identity claim such as 'Clark Kent =
not known, then tallying all of the properties that "each" has and
seeing if there is a property that one has that "the other" doesn't,
will aid us in determining whether Clark Kent is in fact idenitcal to
or distinct from Superman.
We might also think that
Identity of Indiscernibles is true because if there are in fact two,
things, then there must be something--some
property or feature or other--that makes these two things different. It is
explanatorily useful, in other words, to claim that two distinct
are two distinct objects because they
have at least one property that makes
them distinct. To put it
another way: if the Identity of
Indiscernibles were false, then that would mean that we could have
numerous qualitatively identical
no way to determine whether such 'things' are one or many. Indeed, if
distinct objects can have every property in common, then there would be
no way to determine whether we have 100 qualitatively identical things
in front of us, or ten qualitatively identical things in front of us,
just two, or one, or what. We would just have to stipulate that there
as opposed to ten or a hundred qualitatively identical things, since,
there is no quality or
feature that these things have that would explain the difference
between two or ten or a hundred qualitatively identical things.
Similar to the Indiscernibility of
Identicals, notice that another way
to put the Identity of Indiscernibles is in terms of qualitative and
numerical identity: If x and y
qualitatively identical, then x
for the Identity of
As intuitive as the Identity of
Indiscernibles may seem, there are a host of purported counterexamples.
Most famously is Max Black's "The
Identity of Indiscernibles"
[note: this is a link through jstor, so you may need to be on
campus or using a university proxy server to follow it].
Roughly, the idea is this: Imagine
a possible world that contains just
two perfectly round symmetrical spheres. The spheres have the same
diameter, they are an equal distance from each other, they are the only
inhabitants of a completely symmetrical universe, they are both made of
solid iron, etc. Intuitively, such spheres in such a world would have
all and only the same properties; they are qualitatively identical.
Yet, by stipulation, there is supposedly two of these spheres, not one. But
if "they" are qualitatively identical, then by the Identity of
Indiscernibles, "they" are numerically identical; "they" are one,
not two. Thus, either (i)
there cannot be a world where there are two
qualitatively identical objects or else (ii) the Identity of
is false. Since it seems that we can
imagine a world with two
numerically distinct yet qualitatively
identical objects, then (by disjunctive elimination) the Identity of
Indiscernibles must be false.
Perhaps, you think, there is a way to
distinguish between the two spheres. One of the spheres, for example,
might have the
property "being identical to sphere a" or "being
distinct from sphere b" or
and not that one," etc. The
idea would be that there is something special about individual
objects--something that makes one object distinct from another,
independent of any quality, property, or feature. Such a move would
appeal to individual
essences or "thisnesses" or
"haecceities". And such a move would block
the counterexample, but at the cost of inviting even more problems.
Problem 1: Suppose you are
a materialist--you believe that there is only physical stuff in the
universe. Or, even weaker, suppose that you are a materialist only when
it comes to material objects such as rocks and trees and spheres and
the like (you are agnostic or a dualist when it comes to people, say).
What are you to make of haecceities? If we take away material bits of
a (merely) material object like a sphere--if we remove one
metallic bit of it one by one, and one at a time--when do we get to the
haeccity? Where is the
haecceity? Is it one of the material bits we remove? It couldn't be,
because no one material bit of an object seems essential to the object
(we could remove any one material bit from a material object, and that
object will still exist). In this way, a commitment to haecceities is
skirting dangerously close to
a sort of dualism for objects, which might rub more conservative
ontologists the wrong way.
Problem 2: If you endorse
individual essences or haecceities then you seemingly save the Identity
of Indiscernibles, but you have to ultimately bite the bullet and admit
that it is impossible for there to be a world with two qualitatively
identical spheres. Becuase, ultimately, two spheres that are seemingly qualitatively identical
are not entirely qualitatively
identical--they differ in their haecceitties. You might want to insist
that haeccaties are not strictly speaking qualities or properties,
since they can only be had by one individual, and are not repeatable,
like redness, and roundness, and bumpiness are. But, intuitively, if
you posit individual haecceties, then there is something that makes the
difference bewteen two otherwise qualitatively identical objects. If
haecceitties are (by stipulation) a difference-making feature, then
they are a feature, period. And so you do not
think that there can be a world with two completely qualitatively
identicial spheres (haecceities included). So you will have to explain
why it is that it
certainly seems as if we can
imagine such a world.
Problem 3: Imagine a world
with just one sphere. According to the haecceitist, there is just one
haecceity or thisness or individual essence. Now imagine that the
sphere morphs like an amoeba and splits itself in half. Each half now
morphs into a small sphere, each seemingly qualitatively like the
other. No new material is made; no old material disappears. The
original sphere is simply changing
shape and reforming itself out of the material that was already in the
universe. Now we have a world that seemingly has two qualitatively
identical spheres. According to the haecceitist, there must now be two
haecceities (since distinct individuals are (at the very least) made distinct by their distinct
haecceities). But where did the second haecceity come from? By
stipulation, the world
began with just one! A haecceitist may bite the bullet and say that
there was always two, but this seems unmotivated. Consider the
counterfactual case: if the sphere had
never morphed, then there would not be a need to posit two haecceities.
Does the world just 'know' ahead of time whether objects in it will
morph or not, and then supplies the appropriate number of haecceities? That would be wildly ad hoc and
implausible. Moreover, anything we can do with a case of fission, we
can work in reverse to cases of fusion. If we begin with two distinct
shperes that fuse into one, then does the resulting, singular object
have one haecceity or two? If one, then where did the other one go? If
then how can we say there is only one
object as a result of the fusion?
Besides, imagine that after the sphere fisses into two spheres, each
sphere fisses again into two, making four (seemingly) indiscernible
spheres. So now, according to the haecceitist, there are four
haecceities to account for the four distinct spheres. But where did
haecceities come from? If the haecceitist insists that there were
always four, then this seems as unmotivated as saying that there were
always two haecceities. And we can keep fissing the
spheres, ad infinitum, forcing the haecceitist to ultimately say that
there are infinitely many haecceities in just one sphere! If we thought
a commitment to just one haecceity for one sphere was ontologically
burdensome, positing infinitely many is surely worse!
Problem 4: Positing
haecceities seems ad hoc. Do we have any other reason to posit these
weird, suspicious "thisnesses" except to save the Identity of
Indiscernibles? If the haecceitist can provide no other reason for
positing individual essences other than to save the Identity of
Indiscernibles, we may wonder why the principle was worth saving in the
Problem 5: Positing
haecceities seems to make the Identity of Indiscernibles trivially
true. This is a problem if you thought that the principle was supposed
to be substantial, metaphysical thesis.
Another way we might be able
to distinguish the spheres by location. However, the world was
stipulated to be a symmetrical one, and the twin spheres were lonely
twins--they were the only inhabitants in the entire world. So this
means that we cannot specify their location relative to anything else
in the universe. So, someone might suggest, the spheres are located at
distinct absolute locations of space-time points.
Problem 1: our best
scientific theories suggest that that the universe cannot have anything
like absolute location.
Problem 2: even if such a
suggestion didn't contradict the best scientific theories of the day,
it would be odd to think that mere a
priori reflection on the identity relation would yield as
substantial a metaphysical (and physical!) thesis about whether space
was absolute or not.
Problem 3: this
response requires that we accept distinctness of space-time points as
primitive. Yet if we are going to do this, why not accept distinctness
of individuals as primitives as the haecceitist does? What's the real
difference, in other words, between individual essences of objects and
individual essences of space-time points? If we had problems with
haecceities, it seems these problems would repeat themselves at the
level of primitively distinct space-time points.
A third response is to claim
that the counterexample was set up incorrectly. The mistake, you might
claim, was in claiming that there are two
spheres in the possible world
under consideration. Claiming that there are two spheres already begs
the questions against someone who accepts the Identity of
Indiscernibles. Rather, there are not "two" qualitatively identical
spheres, but just one sphere that happens to be bi-located--i.e., one sphere in
two places at the same time.
Problem 1: This violates
the following intuitive metaphysical principle: no one material object
can be at two
distinct places at the same time. Let us call this principle P.
objects can be bi-located would violates P. So we might
ask ourselves: which do we think is more intuitive? The Identity of
Indiscernibles? Or principle P? Common sense seems to favor principle
P; so the Identity of Indiscernibles should be abandoned.
Problem 2: Suppose
objects can be bi-located,
and that Max Black's supposed counterexample is merely a description of
a world where one sphere is bi-located. Now imagine that someone came
along and spit in the direction of one of the locations of the
bi-located sphere. What happens to the sphere at the opposite location?
Does it get spit on, too? How would that be? The person just spit once,
in one direction. Yet if there really is just one sphere there (even if
it is bi-located) then, by the Indiscernibility of Identicals, it can't
have any properties dissimilar from itself. Yet now it does: it (in one location) was just spit
upon yet it (in the other
location) wasn't. Surely this just goes to show that there are two spheres in this world, not one!
Max Black, "Identity of
Indiscernibles," Mind, vol. LXI, no. 242, 1952.
Jason Bowers, personal notes and
Zimmerman, Dean, "Distinct Indiscernibles and the Bundle Theory" in Mind, New Series, vol. 106, no.
422, April 1997: 305-309
O'Leary-Hawthorne, John, "The Bundle Theory of Substance and the
Identity of Indiscernibles" in Analysis,
Vol. 55, No. 3, July 1995: 191-96.
Last Updated: Sept. 3, 2009