Van Inwagen thinks that the very same particles cannot have their
activities constitute two life events. I am not so sure.
Pretend for the sake of simplicity that our world is made of
fundamental particles with well-defined positions in three-dimensional
space, with n basic quantities
Q1, ..., Qn
(mass, charge, etc.), otherwise interchangeable, and with no particles
coming in and out of existence. And suppose we have dogs and cats (or something like them).
Suppose, further, that our world contains Rover the dog and Felix the
cat. No particle that is ever a part of one of them is a part of the
other. Finally, we have a very odd coincidence. There is a one-to-one
correspondence ϕ between the
particles that are ever a part of Rover and those that are ever a part
of Felix such that particle c
is a part of Rover at time t
if and only if ϕ(c)
is a part of Felix at that time. This last condition can be assured if
we assume that Rover and Felix don’t change in what particles they have
and they have the same number, but we don’t need such a strong
assumption.
Now imagine a six-dimensional world made of fundamental particles,
with 2n basic quantities U1, ..., Un, V1, ..., Vn.
Now suppose:
- The first three coordinates of the particle positions and the
quantities U1, ..., Un
over time are physically related by laws exactly like the ones that
govern the three coordinates of the particle positions and
(respectively) the quantities Q1, ..., Qn
in our world are.
Thus, if we project the six-dimensional space of this world to our
three-dimensional space by ignoring coordinates 4–6, and relabel Ui as Qi, the behavior
of the particles will look just like the familiar behavior of particles
in our world.
Then add:
- The last three coordinates of the particle positions and the
quantities V1, ..., Vn
over time are physically related by laws exactly like the ones that
govern the three coordinates of the particle positions and
(respectively) the quantities Q1, ..., Qn
in our world are.
In other words, likewise if we ignore coordinates 1–3, and relabel
Vi as
Qi, the
behavior of the particles will be like that of our world’s
particles.
Note that the Ui and Qj quantities
don’t interact, and the particle positions with respect to coordinates
1–3 and coordinates 4–6 don’t interact. This is like two non-interacting
universes in one, except that the particles are shared between them.
Now suppose that there is a one-to-one correspondence f between the particles of our world
and those of the six-dimensional world such that:
- If at time t, particle
c is at (x,y,z) and has
basic quantity values Q1 = a1, ..., Qn = an,
then at t in the
six-dimensional world, particle f(c) is at (x,y,z,u,v,w)
for some u, v and w, and has basic quantity values
U1 = a1, ..., Un = an.
And there is also also another one-to-one correspondence g such that:
- If at time t, particle
c is at (x,y,z) and has
basic quantity values Q1 = a1, ..., Qn = an,
then at t in the
six-dimensional world, particle g(c) is at (u,v,w,x,y,z)
for some u, v and w, and has basic quantity values
V1 = a1, ..., Vn = an.
In theory, these correspondences f and g could be the same. But they are
not. There is a twist:
- If c is ever a particle of
Rover, then g(c) = f(ϕ(c))
and if c is ever a particle of
Felix, then g(c) = f(ϕ−1(c))
where ϕ is our
correspondence between Rover and Felix’s particles. This twist implies
that g maps Felix’s and
Rover’s particles to particles that f respectively maps Rover’s and
Felix’s to.
We can mathematically arrange all this.
Now, fix a time t. If the
cs are Rover’s particles at
t, let the es consist of the particles
corresponding to them under the mapping f. Note that the es are also the particles
corresponding to Felix’s particles under the mapping g.
Here’s the fun thing. With respect to their first three coordinates
and the quantities U1, ..., Un,
the es behave just like
Rover’s particles. With respect to their last three coordinates and the
quantities V1, ..., Vn,
the es behave just like
Felix’s particles.
It seems right to say this: the es in our six-dimensional world have
two lives, a canine-style life just like Rover’s and a feline-style life
just like Felix’s. There is no interaction or unity between these two
lives other than due to their being realized in the very same
particles.
Suppose someone insists that there must be only one canine-feline
hybrid life here because the particles are the same. Modify the mappings
slightly, so that the Rover-like life is realized by particles e1, ..., eN
while the Felix-like life is realized by particles e2, ..., eN + 1.
Now the particles are not the same, but they are mostly the same. Now we
have the same life event but no longer the same particles (just an
overlap, like in the case of conjoint twins). But two particles should
make little difference here. The slight change in mapping shouldn’t make
a difference between have a canine-type and a feline-type life on the
one hand, and having a hybrid canine-feline life.
Note that I am not doing Aristotelian metaphysics here. By “life
events”, I mean the kind of empirical event a biologist might talk
about, not a form or anything like that. I doubt that the same matter
could have two different substantial forms.