You have a mind and you have a body.  What’s the connection between the two? All of us are aware of our physical being—our bodies—and we also have an immediate experience of our mental states—our thoughts, emotions, and sensations—but figuring out the relation between these has not been easy.  

The oldest and by far the most popular explanation of the relationship between mind and body is called substance dualism. Let’s unpack this term, starting with the notion of “substance.” Philosophers use the term “substance” differently than it’s used in ordinary speech. In the vernacular, substances are stuff like toothpaste or olive oil, but in philosophical jargon, substances are just things. Your left foot, the planet Neptune, and Donald Trump‘s hair are substances in this technical, philosophical sense.

The term “dualism” refers to kinds of substances.  If you’re a substance dualist you think that there are two kinds of things. There are physical things and there non-physical things. Imagine that you are taking an inventory of everything in the entire universe.  You are holding a clipboard with a piece of paper that’s divided into two columns. Column One is labeled “physical” and Column Two is labeled “non-physical.”  If you are a substance dualist, you think that some things belong in Column One (for example, your body, Neptune, and Trump’s hair) while other things belong in Column Two (for example, God, souls, and minds).  Alternatively, if you are a physicalist—someone who believes that everything is physical—then you believe that everything belongs in Column One and nothing belongs in Column Two (we will be exploring physicalism in a future installment of this blog).

Of course, the part of your body that’s most relevant to this discussion is the squishy piece of matter that’s lodged between your ears—your brain. If you’re a substance dualist you believe that your mind and your brain are not the same thing.  Your brain is a physical thing—to put it crudely, it’s a piece of meat—while your mind, the locus of your conscious experiences, isn’t a physical thing at all.

There are powerful reasons why substance dualism has been so attractive to so many people for so long. To many (probably, most) people, this view of things just seems obviously true. Think about it. Your body has characteristics that your mind doesn’t seem to have.  It’s palpable.  It’s extended in space.  It has surfaces, mass, and volume.  You can touch it, weigh it, and measure it.  But the mind that you are using right now to think about your body seems like a totally different kind of thing.

Wouldn’t it be bizarre to ask someone to weigh your mind or to measure the size of one of your thoughts?  Making such a request would be weird for a couple reasons.  One is that mental things don’t seem to have properties like weight and size. And by the same token, mental things seem to have characteristics that don’t belong to objects that inhabit the physical world. You might have an irrational thought, but it would seem extremely strange to say that you have an irrational electrochemical impulse coursing through your prefrontal cortex.  

Another reason why such questions seem odd has to do with access. Your body and everything that goes on in it are (at least in principle) knowable by others. In fact, sometimes other people have far better access to what’s happening inside of you than you do.  For example, a physician might be able to tell you what’s going on in your pancreas right now, even though you don’t have a clue what’s happening there.

But we seem to be hooked up to our own minds in a special way.  We seem to have direct and privileged access to our own mental life—our thoughts, fantasies, feelings and memories. And this realm is radically private and inaccessible to others, unless we chose to communicate our mental states to them.

These points about the difference between mental and physical states easily lead to dualistic conclusions. Here’s how the reasoning goes. If your mind has characteristics that your body doesn’t have, and vice versa, then your mind must be something separate and distinct from your body—hence, substance dualism must be true.  

This pattern of reasoning can be formulated more sharply by drawing on a handy philosophical principle that’s known as “Leibniz’ Law,” which is named after the 17th to 18th century philosopher-mathematician-physicist and all-round genius Gottfried Wilhelm Leibniz who first formulated it.  Leibniz Law is all about the conditions under which we can say two or more things (or, more accurately, what seems like two or more things) are identical.  

When philosophers talk about two things being identical, they don’t usually mean “identical” in the sense that identical twins are identical. Identical twins are qualitatively identical, which means that they are so similar to each other that it’s hard to tell them apart. But philosophers are usually more concerned with what’s called numerical identity.  To say that two things are numerically identical is to say that they are really the very same thing.  Bruce Wayne and Batman are numerically identical because they’re the very same guy.  

According to Leibniz’ Law, two things are numerically identical only if they have exactly the same properties. So the reason that Bruce Wayne is identical to Batman is that Bruce and Batman have exactly the same features. Everything that’s true of one is true of the other, and vice versa, because Batman is Bruce Wayne.

My students are often confused by the standard descriptions of Leibniz’ Law, because the idea that two things are really one thing doesn’t make sense. They’re right.  It is nonsensical to say that two things are really one thing. So let’s express Leibniz’ Law in a different way—one that focuses on names for things rather than the things themselves.  Two names (for example, “Batman” and “Bruce Wayne”) are names for the very same thing only if the things that they name have exactly the same properties.  Batman equals Bruce Wayne because the name “Batman” names a person who has precisely the same characteristics as the person named “Bruce Wayne” has.  If Bruce is in Gotham City then it would be impossible for Batman to be anywhere else, and if Batman is fighting crime at 4:30 in the afternoon on Tuesday, then it would be impossible for Bruce not to be fighting crime at that moment.

Now we can bring this discussion back to the issue of substance dualism.  If it’s true that mental states have different properties than brain states, and we have privileged access to our minds but not to our bodies, then it looks like Leibniz’ Law tells us that our minds are not numerically identical with our bodies.  It seems to follow that we consist of a compound of two different kinds of substances. We are physical bodies yoked to non-physical minds.

This seems like a satisfying conclusion, but is it true?  Anyone that advocates substance dualism owes us an explanation of how it works.  As we’ll see in the next installment, that’s where the trouble with dualism begins.