Physical – relating to the set of all things that can interact with one another Mental – relating to neurological processes that we’re consciously aware of

The categories of physical and mental are often used to split everything up into two entirely different worlds: one world that we’ve been able to increasingly understand through science and another world that we barely understand, that we only know exists because we each individually experience it. The way we’ve split things up, though, says more about the limitations of our knowledge and less about the truth of things. Consciousness is incredibly difficult to study, so it has been relegated to a different category of things—things which are different than the physical world, which is easy to interact with and learn about. But as we learn more about consciousness, there are only two real possibilities: either we’ll realize it obeys the same physical laws that we already understand, or we’ll discover some new laws to describe how it works and add them to the list of physical laws that govern the universe. Eventually mental and physical won’t be opposites; instead, mental things will be a subset of all physical things. To see why, let’s consider how we interact with the world.

Everything that we’re aware of clearly starts off in the physical world. Everything we see was once a photon hitting an eye. Every sound was a pressure wave in the molecules of the air. Those sights and sounds propagated through a series of purely physical interactions. Something interacted with our senses and sent an electrical pulse through our nerves,

Let’s leave that end of process for a moment and consider what happens when things run in the opposite direction. Imagine asking me if I am awake right now. In my brain, there would be some kind of check to see if I was awake and conscious and if I was, that would have triggered the whole series of physical events that followed. The end result would have been a bunch of muscles in my head moving to make sounds, and that would have been caused by neurons firing, sending electrical signals from my brain to those muscles.

We don’t know enough to precisely connect the two ends of this story. On one end, we have a series of physical interactions heading up into my brain, and on the other end, a series of physical events that start in my brain and then head out into the world. We can’t check to see exactly which neurons fired to connect one to the other, but whatever was happening, we’d want to call it physical. Even though one of the steps in the series was the creation of a conscious experience, and we can’t explain how that works, we can say that physical events contributed to that experience and the presence of that experience contributed to my response. We call consciousness a mental phenomenon because of a lack of knowledge, but it undoubtedly interacts with the physical world.

What would it mean for something to not be physical? It would mean that nothing physical could interact with it. It couldn’t be touched, seen, or heard. It wouldn’t react to magnetism, gravity, or other physical forces. It would be completely separate from our world, and there would be no way we could ever learn anything about it or even learn that it existed at all. That seems to be the fate of many non-physical and supernatural things—ghosts and spirits. If they do exist, they seem impossible to detect. But that’s not true of consciousness. It appears to be part of the set of all things that can interact with each other. Our conscious experiences are directed by what we observe in the world, and we seem to be able to make conscious choices or, at the very least, to be able to confirm that our consciousness exists and has certain qualities.

Mental events represent the neurological processes that we’re aware of, so it’s still worthwhile to distinguish the mental parts of the world. It’s just that these mental parts must still follow the same physical rules as everything else. They aren’t separate and distinct—they’re just a different kind of physical thing. When we experience thoughts, memories, or emotions, the physical events that are producing those experiences are happening somewhere in our physical brains, but we’re also consciously aware of them. That’s what we mean by mental—there’s lots of other stuff happening in our brains that we’re completely unconscious of: the tiny details of which neurons fire when moving a muscle, processing a visual image, or being fooled by an optical illusion. Even though these are all happening in our brain we don’t consider them mental because we don’t have any conscious experience of the internal processes behind them. It’s the difference between knowing that you recognize someone’s face and not knowing how that happens. The feeling of recognition when seeing an old friend is caused by physical changes in our physical brain, but that feeling is important to us, so it’s worth giving those kinds of physical changes a special name: they’re mental changes. In the next article, we’ll define consciousness and see why these kinds of changes feel so important to us.