 Dualism is an attractive philosophy with an Achilles heel. Mind and body seem to be fundamentally separate things, yet dualists since Descartes have never been able to solve the famous problem of interaction. If mind and body are in different ontological categories, then how could they possibly interact with each other, even in principle? Descartes didn't give a good answer, nor has any dualists that I've ever encountered. They tend to respond, well, we don't know how mind and body interact, but that doesn't mean it's impossible. This is not a satisfying answer, and even though I agree with them, lots of philosophers don't find it compelling at all. In fact, many have claimed that the problem of interaction is so severe that it's a refutation of dualism. They think the problem can't even be solved in theory. I disagree. I've been trying to figure out a plausible mechanism of interaction for many years, and finally I've got one. I have a working resolution to the mind-body problem that solves the problem of interaction. It not only supports substance dualism, but substance pluralism, which doesn't restrict the amount of ontological categories to only two. If you're sympathetic to dualism or are familiar with its history, this is the type of theory we wanted Descartes to figure out a few centuries ago. I call it a theory of indirect interaction. Mind and body do not directly interact, but they effectively interact. In other words, the state of one affects the state of the other. This theory has many favorable properties. Number one, it gives a plausible mechanism of four interaction between objects in any ontological category, not just mental and physical, even if there are a hundred more categories, the mechanism could still work. Number two, it allows for two-way causality. Physical states can affect mental states and mental states can affect physical states. Three, it's free will agnostic. There's a clear opening here for the role of free will, but the system works perfectly fine without it. And number four, it doesn't break the laws of physics. Perhaps more precisely, it doesn't break the laws of causality. It might simply broaden the scope of the laws of physics. So the purpose of this article is not to claim that this is the way that things actually work in the world. Rather, it's to demonstrate that in principle, there could be a mechanism for things in different ontological categories to effectively interact with each other. Whether or not I've discovered the real mechanism is a separate question. The best way to illustrate this theory is by first taking the mind out of the equation and analyzing purely physical phenomena. By breaking down physical phenomena into their most fundamental form, it will elicit the concepts necessary to understand indirect interaction. My favorite example of purely physical phenomena is the motion of balls on a billiard table. Let's take a simple example. Imagine that there are only two balls left on the table, the white cue ball and the black eight ball. Imagine the cue stick strikes the white ball and the white ball rolls forward, hits the eight ball and the eight ball rolls into a pocket. So let's break down the scenario as thoroughly as possible. What are we really talking about when we reference pool balls? What exactly are such objects? What are we describing when we say the eight ball rolled into a pocket? Rather than give an extended analysis of the metaphysical status of pool balls, which you can read about in my article No Chairs Do Not Exist, let me tell you one plausible position. What a pool ball actually is, is fundamental units of matter arranged in a particular way in a particular part of space. Let's call those units atoms. Some philosophers might put it a pool ball is really just atoms arranged ball wise. Now consider the phrase, the eight ball rolled into a pocket. What exactly are we describing? What is this rolling phenomenon? If the pool ball is just atoms and we could rephrase it as, some particular atoms changed their positions in space. Another way to understand it would be to say, at time one atoms were in position one and at time two atoms were in position two. In fact, that's a pretty good description of motion in general. This is an abstract way to understand pool balls rolling on a table. So now let's ask two interesting yet difficult questions. When the white ball hits the eight ball, why does the eight ball start moving? And number two, why does the eight ball move on its particular path rather than some other path? Notice that when we reduced the phenomenon of motion to atoms changing position, it doesn't actually communicate an extremely important piece of information. Those changes in the position of atoms are not random. We didn't say at time one the white ball struck the eight ball and at time two the eight ball started orbiting Jupiter. No, there's a pattern to the motion, a predictable observable pattern. Why? Why isn't the motion of the eight ball completely random? Why should the motion be predictable at all? Hell, why doesn't the universe just spontaneously fall apart when the balls collide? What holds all of these objects into the same coherent and predictable system? One very plausible answer is this. There are laws of the universe that keep the whole thing together. They make motion predictable. The reason that the eight ball rolls into its pocket instead of orbiting Jupiter is because there are laws of physics which govern the behavior of objects. And these laws have a real existence. Now that's a nice sounding answer and physicists might like it, but it provokes many more questions. For example, are the laws of physics physical things themselves? Do the laws of physics take up space or weigh anything? And what is this relationship between the laws of physics and the objects governed by them? What is the mechanism, if you will, for the laws of physics? In other words, how do laws keep objects in order? If our explanation for physical phenomena appeals to laws, then we've posited the existence of two radically different types of things, physical phenomena and the laws which govern them. Atoms in space by themselves are not actually sufficient to explain why they move in predictable ways. There must be underlying principles or laws which determine their behavior. By thinking about purely physical phenomena in this way, it gets us one step closer to solving the mind-body problem. We can break down our billiard example even further. Instead of only identifying what we see, we need to identify exactly what we don't see. Let's treat the table and the balls as a whole system. We see changes in the position of balls, which means we see the system in a different state at different times. But we don't see the glue between those states. We don't actually see the laws that we're appealing to in order to explain the underlying phenomenon. We're simply inferring the existence of laws and causality to explain the patterns in our observation. But we don't actually see the laws themselves. It's helpful to keep rephrasing this and condensing our language. So instead of saying at time 1 the object was in position 1 and time 2 the object was in position 2, we can simply talk about states of the system. We can say state 1 was followed by state 2. So, another very abstract way to understand physical phenomena is to say there is a series of states and each state contains a particular arrangement of atoms in space. The changes between those states are non-random and they happen in accordance with laws. This allows us to re-ask the previous questions. Number one, why is state 2 the way that it is in particular and not some other way? And question number two, why doesn't state 2 include the 8-ball orbiting Jupiter? And the answer? Any given state is determined by its previous state. Since state 1 was in a particular way, the laws of physics determined that state 2 must follow state 1. Or to put it even more succinctly, preceding states determine future states. Let's consider these states of the universe from another perspective. Let's call them inputs and outputs. Inputs yield outputs. So if we call state 2 an output, we could say that state 1 was its input. But what determines that state 2 will be the output of state 1 in particular? It's the laws of physics. If we think about states of the universe as being inputs and outputs, then we can understand the laws of physics as a kind of mathematical function. They take inputs and turn them into specific outputs. This is a theoretical picture in which the universe is like a gigantic computer that keeps turning out new output states. These outputs are determined by their inputs. Then those outputs are themselves used as inputs for the next state. The laws of physics are the specific code that determines exactly how inputs relate to outputs. So we could reduce the physical universe to a very abstract formula. Input state plus the laws of physics yields the output state. Then that output state is treated as the next input state and the universe turns out a new state. Alright, so we've posited the existence of two radically different types of things to explain physical phenomena. Namely, spatially extended atoms in space and non-spatially extended laws of physics which govern their behavior. Whether or not it's necessary to give the laws of physics a real existence is an interesting question. And it turns out it's awfully hard to explain the regularity of physical phenomena without laws. Regardless, this metaphysical picture allows us to understand how objects in different ontological categories might be able to interact with each other. However, we must go deeper. Let's reduce the physical universe to atoms in their particular position in space at any given time. Those atoms themselves are not enough to determine the future state of the universe. There must also be laws. But this brings up several more very difficult questions. What is it that connects the physical states to the laws? Why aren't the physical states completely separated from the laws? What's the glue between the laws and the states? And number two, how do the laws of physics know what the present state of the universe is? Why doesn't the universe get it wrong, if you will, when determining future states? Number three, how are states treated as inputs? What's the format? All of these questions can be answered by the final piece of the puzzle. Information. The universal mathematical function that takes inputs and turns them into outputs has information about the present physical state. This information itself is non-physical. The information is the glue between the laws of physics and the physical states themselves. So what actually gets used as the input is the information about the physical state rather than the physical state itself. The subsequent output is another purely physical state. Then information about that output is used as the new input state. This is an abstract way to understand the mechanics of a physical system. Crucially, it allows for real ontological differences between the physical state, the information about the physical state, and the laws of physics which take that information and generate new output states. Think about the relation between ordinary objects and your knowledge about them. Take your information about chairs. There's a categorical difference between physical chairs and your information about physical chairs. Chairs take up space while your knowledge about chairs does not take up space. The concept of a chair is not somehow embedded inside of chairs. Information is not the same thing as what the information is about. Information about physical states does not need to be embedded within the physical state. In this theory, the physical states are entirely concrete. They're not abstract. They are reducible to atoms in space or even atoms of space. Yet, there can be information about those systems which is abstract and not reducible to atoms in space. There's an interesting question here about the metaphysical status of information. We can say it's abstract, but what exactly are abstract things? Are they mental things? Are they platonic things? This theory doesn't actually require a particular answer, but it should be clarified it doesn't necessarily imply consciousness. Your knowledge of chairs is within your mind. You can have a kind of conscious experience of it, but the universe doesn't need to necessarily have an internal experience of knowing about physical states, just like your computer doesn't need to have an internal experience of reading and knowing the state of your hard drive. Information is processed in your CPU without consciousness. So, to revise our picture of a physical system one more time, we start with atoms in space. The universe then has information about the position of atoms in space. That information is used as an input into a function that we call the laws of physics. It then generates a new output state, in other words, atoms change position. The universe has information about this new state, which then gets put back into the function to generate subsequent output states, and the universe progresses. Now, if this theory works, then we've just solved the mind-body problem and the problem of interaction. All we've got to do now is add mind. The picture I've just painted includes effective interaction between at least two ontological categories, namely the laws of physics and the spatially extended objects that are governed by them. Now, it doesn't matter how many ontological categories we posit, the same mechanism can still work, so instead of restricting output states to only spatially extended physical stuff, we can expand the category of output states to include mental stuff as well, like feelings and experiences or qualia, as philosophers call them. For example, take the conscious experience of seeing red. It's a particular type of mental state. In this system, it's simply another output that will get generated with the correct input. Whenever the physical universe is arranged in a particular way, the output state of experiencing redness is generated. That output state does not need to be physical. It could be in an entirely different ontological category. This allows us to expand the laws of physics to include things like the laws of mental representation too. Just like particular physical inputs yield particular physical outputs according to laws, the universe can also generate mental outputs with the right input. In other words, the universal function includes the informational criteria for generating both physical outputs and mental outputs. This theory accords perfectly with the physical mechanics of sight. So when physicists talk about light rays entering the eye and stimulating particular nerves, etc., they're simply talking about changes in physical states. And as these physical states change, that means the information going into the universal function also changes. And at some point, when the correct physical state has been reached, mental states start getting generated. So note, it's not the physical state itself that's generating mental phenomenon. It's not some mechanism in the brain or the secretion of the brain. It's information about the physical state, which gets used as an input to generate a mental state in a different ontological realm. In this theory, brains are not some unique object that creates consciousness, as some philosophers have suggested. Consciousness is not to be found within the skull. There's nothing intrinsically special about the atoms that compose a brain. What's important is their arrangement and the corresponding information about them. If patterns and information about the brain are indeed what generates consciousness, then we also have no need to posit panpsychism, which suggests that each atom might be a little bit conscious itself. So the reason that the brain is so closely correlated with conscious states is because it's precisely the information about the atoms in space that we call a brain that yields consciousness. The brain state itself is not enough. It requires the brain state plus the laws of physics and mental representation. So it shouldn't be surprising that when people get brain damage, their conscious experience changes. This isn't because the brain loses an ability to secrete consciousness. It never had that ability. It's because when the brain state changes, information about the brain state changes. And when that changes, it changes the input going into the universal function and, subsequently, the output. This is why I call the theory a mechanism of indirect interaction. The brain isn't directly generating consciousness. Instead, it's the pattern of information corresponding to the physical state of the brain that generates consciousness. The effect is essentially the same. The state of the body affects the state of the mind, but it's via an abstract mechanism instead of a purely physical or purely mental one. Our experience of the world suggests that physical states can affect mental states and that mental states can affect physical states. For example, experienced meditators can regulate their body temperatures through deliberate mental focus. Even regular people can make their mouth water simply by envisioning a delicious, juicy steak when they're hungry. Or take one of the most significant examples of mental states appearing to affect physical states. The placebo effect. How is that possible? Well, just like the outputs of the universal function can be mental or physical, so can the inputs. The universe can have information about physical and mental states. So information about mental states might be used as inputs to generate outputs. Let's take the placebo effect as an example. Simply taking a sugar pill is not enough to generate improvements in one's symptoms. It also requires belief that the pill will help you. So in order to generate the desired result, the universal function requires an informational input from both mental and physical states. Having only the correct physical state or only the correct mental state is not enough. Both must be in the correct state. This two-way causality accords with our experiences and contrary to the claims of many philosophers, it doesn't need to break the laws of physics. The laws of physics can simply be expanded to include mental states as well. Instead of calling them the laws of physics, it would probably be better to call them the laws of the universe to include governance over all types of phenomena. Yet another benefit of this theory is that it allows for the existence of free will in a rather straightforward way. If mental states are used as inputs into the universal function, then what if some mental states are volitional? If not all mental states are determined by their previous states at the universe, it could allow for volitionally determined mental states. And those volitionally determined states would then be used as an input to generate a particular output. So for example, whether or not you eat dinner tonight at 6 p.m. or 7 p.m. might not be a predetermined fact. The universe could require a volitional state in order to determine which output gets generated. In other words, information about your choice, whether 6 p.m. or 7 p.m., will determine what happens and without your choice your dinner might not have happened at all. Now, I don't currently have an answer to the question of free will, but I think it's a strong benefit of this theory that it can seamlessly allow for its existence. The mechanics of indirect interaction give us a concrete mechanism for minds to affect the world. Whether that mind is controlling its mental states or is merely a predetermined output of the universal function. The theory I've just explained is extremely flexible. It allows for the existence of arbitrarily many ontological categories. If you think the world is constituted by only physical and mental stuff, it can work. If you think platonic objects also exist, well that's fine too. If you think there are a hundred different ontological categories all of which interact with each other, well that's fine as well. Those ontological categories can be completely separated so long as there's a simple fact about them. The universal function has information about their state. This isn't that difficult to imagine since in this theory the universal function is the thing that's outputting the different states into their various categories in the first place. Indirect interaction also allows for a plausible story of emergence. It might be the case that the universe began with only physical phenomena and laws. Then over time, as matter rearranged itself, a pattern of information yielded the first conscious output. If this actually happened, then other types of emergence might also be waiting to come into existence with the correct informational input. The theory works whether the interaction is causally one directional or two directional and it allows for the existence of free will. It's also consistent with a modern conception of the relationship between body and mind that views the body or the brain as fundamental. I mean it might really be the case that the physical state of the brain entirely determines mental states. Mental phenomena could be purely epiphenomenal. Indirect interaction simply gives a causal mechanism for brain to affect mind. So if stimulating one area of the brain causes changes in mental experiences, it's not because some particular gland starts secreting consciousness a bit differently, because the underlying physical structure of the brain changes, which changes the information going into the universal function. In fact, this mechanism is even consistent with idealism. Even if one rejects the existence of physical stuff completely, the regularity of our mental phenomena still requires explanation. If the laws of the universe only govern mental phenomena, because that's all that exists, it might be the underlying mechanics are the exact same. Mental state plus information about them, plus the laws, yields the next output state. It also explains why the problem of interaction is lasted for so long, because people keep looking at the wrong places. While there's a tight correlation between brain states and mental states, you'll never find consciousness in the brain. You'll only find the correlating physical state. The mechanism is not within the skull, because consciousness is simply not a physical phenomenon. You can't see its generation from the outside. There are no levers, no pulleys, or glands, or fluids that contain it. That's because the relationship between brain and mind is abstract. The information about the physical state is not to be found within the physical state. Now, there are many parts of this theory that one can object to. Perhaps you think the laws of physics aren't real, for example, or you think that the continuity of time makes the story less plausible. The details don't matter. The point is to paint a picture of at least one conceivable mechanism in different ontological categories to effectively interact with each other. If such a picture exists, then the interaction problem is not a refutation of substance dualism or pluralism.