I’ve long wondered what it means to say that a bunch of things are
“irreducibly jointly producing an effect.” One place this comes up in
contemporary metaphysics is strongly non-reductivist materialist
theories of mind on which the particles of our bodies working together
produce thought.
This irreducible production—if there is such a thing—is supposed to
be different from the reducible way of how, say, four rocks can depress
an analog spring-based scale. Yet I’ve always found it hard to grasp the
difference. After all, no single one kilogram rock has the power to
depress the scale to the four kilogram point. You need all four. Why is
that not an irreducibly joint production of an effect?
A paradigm example of irreducible production would be something like
this, however. Suppose the laws of nature ensure that when you arrange
four spherical objects of equal size in a tetrahedron, they begin to
whistle (I am adapting an example by Trenton Merricks). You arrange
three or five or any other number of marbles, nothing like that happens.
What’s the difference?
In the case of the rocks and the scale, it is tempting to say that
the outcome is just a matter of “summing forces”. But as we learn from
Nancy Cartwright and by reflecting on Hume’s insights, summing forces is
not as innocent as it sounds. The world could have been so that when
multiple forces impinge, they average, or the greatest force carries the
day, or the weakest does. Thus the laws of nature seem to have to
include some clause like
- When a number of objects act on an object with forces, the latter
accelerates in proportion to the vector sum of the forces.
But now consider.
- When a number of spherical objects are arranged, whistling results
with an intensity equal to one if the objects are four in number, equal
in size, and tetrahedrally arranged, and intensity zero otherwise.
The main difference seems to be that “vector sum” is a simpler and
more mathematically natural description than “one if the objects are
four in number, equal in size, and tetrahedrally arranged, and zero
otherwise”. And that’s just a difference of degree, while the difference
between reductively and non-reductively joint action is not a difference
of degree.
Here is a thought. Perhaps the worry arises out of a mistaken way to
think about the case of the scale. The force on the scale’s platform
(subsequently transmitted to the spring) is not the sum of the
pressures imposed by the four rocks. In the Newtonian setting, each
particle of the platform is affected by gravitational attraction to
every other particle with mass in the universe, as well as affected by
interaction via other forces (e.g., electromagnetic) with countless
particles. There are no small closed systems. Perhaps, then, we should
rethink the ordinary case: it isn’t a case of reducibility either. On
the contrary, it is a case of an irreducible interaction with,
basically, everything else—or, in a relativistic context, everything
else in its past light cone.
The difference between the whistling tetrahedron of balls and the
scale’s depression then isn’t that the whistling is irreducibly jointly
effected by multiple things and the scale’s depression isn’t. It is,
rather, that the scale’s depression is jointly effected by vastly more
things, most of which have only a slight effect, while the
magical-seeming whistling is produced only by the four
things.
On this picture, what would reducible joint influence be like? It
would have to be a case of independent production, where maybe object
x produces property P in object z while object y independently produces property
Q in z, so that x and z reducibly jointly produce P ∧ Q in z. But it’s not clear that this ever
actually happens in our world—all we have are better and worse
approximations to such causally independent production.
Some non-reductivist materialists will want to say that our
particles, through irreducibly joint activity, produce mental states.
Can we accommodate this in the above story? Yes, but only if we suppose
that the mental states are in no way directly affected by anything
outside our body.
Here is a worry about what I said. Suppose that the four rocks are
sufficient to push the scale to a position where the indicator needle is
right around the middle of the 4 kg dashmark, while the sum total of the
other forces in the universe is insufficient to shift the needle to the
outside of the 4 kg dashmark. Then while the exact position of
the needle isn’t effected by the four rocks alone, the fact that the
needle points somewhere within the 4 kg dashmark on the scale seems to
be accounted for by the four rocks alone. Thus, with respect to the
coarse grained event of the needle being within the 4 kg dashmark, the
four rocks are sufficient to produce the effect. And if that’s right,
then we’ve lost the distinction between the four rocks depressing the
scale and four marbles causing whistling.
Conjecturally, I answer as follows. The coarse-grained event of a
needle’s being in some range (given by the thickness of the 4 kg
dashmark) has an identity dependence on fine-grained event of the
needle’s being exactly where it is. Thus, had the fine-grained event
been different, the coarse-grained event would have been a numerically
different event. Hence the coarse-grained event does actually depend on
vast numbers of things.