Mental
Substance
A mental substance is a substance
that exemplifies mental properties, like the
property of feeling sad or the property of being aware of a proposition. A simple mental substance would be a
mental substance that lacks parts.
It’s an open question whether there could be a mental substance that lacks parts. Some philosophers
think that no mental substance could have parts—that is, being mental entails
lacking parts. But they are in the minority. Most philosophers these days
believe that mental substances have parts and perhaps must have parts working together in the right way.
I find myself somewhat skeptical of the very possibility of complex substances (that isn’t to say that I’m
skeptical of the existence of the kind,
Complex Substance), or at least of the possibility of complex substances that
have separable parts (potentially
existing apart from the whole). My worries have to do with ontological
indeterminism. (See How Not to Build a Person.)
As a result of those worries, I seriously wonder whether mental substances
could really be built from material
parts, given that material things, like atoms and protons, would be separable parts if they were parts at
all.
The idea that I’m not built from material parts actually
doesn’t seem that implausible when I reflect on my experiences. For example,
when I introspect on what it’s like to have feelings or to grasp propositions,
the complexity I see is in the things I’m aware of, not in the me who is aware
of them. (Yes, there is a sense of
me. I’m not sure why Hume had trouble identifying a sense impression of himself. Maybe Hume was merely a machine, whereas I’m genuinely a person! But that’s gravely unlikely. Far more likely to me
is that Hume was looking for a sense impression that was comparable in
character to sense impressions gained by way of the five senses. He didn’t
recognize his sense of himself not
because he didn’t have one, but because his concept of sense was too restricted. At any rate, to me, it is just plain and obvious that I’m aware of me. I’m also
aware of my being aware of me! At least I just was a moment ago.) And the thing I’m aware of when I’m aware of me appears to be a simple thing. Or at
least, if there are parts in me, I’m
not aware of them when I’m aware of me. Perhaps the parts are hidden from my
awareness. That’s very possible. But as I said, I already find myself
moderately skeptical that a mental substance could have material parts. So, I
find myself leaning on the side of the minority of philosophers who think that
mental substances are simple beings—simple by virtue of their lack of separable
parts.
Of course, there is much complexity that surrounds me. There
are countless arrangements of atoms that are in intimate causal connection with
my mental experiences. Still, why should I think that these atoms are literally
parts of me? One reason is the Pairing Argument, which points out a difficulty
with accounting for how or why a non-spatial soul would be systematically
connected with a material body. But who says that a simple mental substance
would have to be a non-spatial soul? Perhaps, I am causally related to certain
material arrangements by being located near them, as Chisholm apparently
thought, or perhaps even by overlapping them. I find the concept of a spatially
extended mental substance that lacks material parts difficult to rule out.
Still, even if I’m made of material parts, it remains a
conceptual possibility open for debate that there could be mental substances
that lack parts.
The study of mental substances—be
they simple or complex—is the domain of philosophy of science, neuro and cognitive science, psychology, and philosophy of
mind.