First up: dualism. In short, dualism is the idea that conscious experiences and physical phenomena are completely different entities from each other. Somebody's mental state is not the same thing as their brain at work — say, for example, that you had superpowered vision and could gaze right through someone's skull to their brain. You might see every single one of their neurons firing, but it's hardly fair to say that you saw their consciousness. Clearly, consciousness is a different thing from the physical brain, even if the two are closely related. Most modern flavors of dualism deal with different answers to the question of how, exactly, the physical body and the non-physical mind interact.

• Substance Dualism: René Descartes, called the "father of modern philosophy," is also the father of dualism. Substance dualism, specifically: the idea that the mind, soul, or consciousness (whatever you want to call it) is literally a separate substance that interacts with the physical world only in the brain.
• Property Dualism: Unlike substance dualism, property dualism doesn't claim that there are some physical phenomena in the world that we can't measure or interact with. Instead, it says that some types of matter (like a brain, for example) have two types of properties that can be ascribed to them: a physical property and a non-physical property. Most modern dualists are property dualists.

• Epiphenomenalism: You can be a property dualist or a substance dualist and believe in epiphenomenalism. Basically, it's the idea that only physical material can interact with physical material, so the non-physical mind can't actually cause anything to happen in the brain. Only neurons firing do that, and they do it because they're bound by physical laws. However, one of the side effects of a brain at work is that non-physical substances or materials arise out of them — even though they can't have an actual effect on the world.
• Parallelism: Parallelism (or psychophysical parallelism) is one of the weirdest forms of dualism. It's the belief that the mental and the physical worlds do not interact, but run parallel to each other such that any event in the former corresponds exactly with an event in the latter. It generally requires an outside force to actually "set up" these parallel paths, so it's not an especially popular opinion these days.

Monoism is the other main contender to dualism, and depending on your disposition, it might strike you as more natural. Basically, instead of saying that the world is made up of two types of things like dualism does, monism says that there's really only one type of substance in the universe and it explains both your consciousness and your brain. For most people who hold this belief (which is most philosophers and scientists), that one substance is physical matter, but it's not the only possible solution on the table.

• Materialism: The original monism. Materialism (aka physicalism) is the belief that's there's only one type of stuff in the world, and it's made up of atoms and subatomic particles. While it might seem like our hopes and dreams and emotions are made up of something else, that's really just the illusion caused by a system with sensory inputs.
• Idealism: A lot of materialists look down on dualists for having some vaguely spiritual, allegedly unscientific views, but idealism really takes the cake in that regard. Idealists like George Berkeley and David Hume believed that there was no such thing as physical material and that only conscious minds exist. It sounds a little out there, but consider this: Of all the other options on this list, idealism is the only one that accounts for all of the evidence we have, and doesn't posit phenomena we don't have evidence for.

• Neutral Monism: What if there was only one type of substance in the world, and it was neither physical nor mental? Or more accurately, it was somehow both? You might feel like neutral monism is dualism sneaking back into your monism, but try thinking of the physical and mental aspects of an object as being opposite sides of the same coin — the fact that it doesn't look the same depending on if you're on one side or on the other doesn't mean there's more than one coin.
• Reflexive Monism: Here's where it really feels like dualism has found a back door. Reflexive monism is the idea that, sure, there's only one type of substance, and sure, it's not really physical and it's not really mental. However, depending on context, it can become physical or become mental. Fascinatingly, this belief is one of the oldest on the list, and references to it can be found in the Upanishads (written circa 800–200 B.C.E.).

The fact is, neither dualism nor monism are sufficient to explain the world as we experience it, but maybe that's not the point. Maybe the point is to expand our minds to accept many possible solutions to one of the most persistent questions of the human experience. However true it is, it's certainly a lot of fun.

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