I recently started learning Yale’s Introduction to Psychology course, and the first module is really interesting. It’s called “The Brain”, but it kept circling back to something I hadn’t really thought to question before: What makes our mind?

I’ve learned a lot from it. Below is my note from this module:

Dualism

The idea that humans are more than just matter, that we have a body and something else, something non-physical. Most religions are built on some version of this.

One way to think about it:

You can doubt that your body exists, but you can’t doubt that your mind does, because the doubting itself is the mind.

The mind doesn’t take up space. It doesn’t need a body to exist, at least in theory.

Beliefs about personal identity, the afterlife, reincarnation, God, all of these only make sense if you already accept that the mind is separate from the brain.

Dualism is the background assumption behind a lot of what people believe.

Moves from the philosophical to the physical. If the mind is the brain, what is the brain actually made of?

Neurons: physical seeds of thoughts

There are about 100 billion neurons in the brain. They come in three types:

Sensory: pick up information from the outside world
Motor: carry signals from the brain to the muscles
Interneurons: connect other neurons together

One thing worth knowing:

Neurons don’t actually touch each other. There’s a tiny gap between them.

When a neuron fires, it releases chemicals across that gap, and those chemicals either activate or suppress the next neuron.

This is how drugs work on the brain: they interfere with those chemical signals, turning them up or down like a volume dial.

Alcohol suppresses, amphetamines amplify.

The “all or nothing” principle means a neuron either fires completely or doesn’t fire at all, there’s no halfway.

Parts of the brain

Some basic reflexes don’t involve the brain at all. The spinal cord handles them on its own, faster than consciousness.

The brain has two main layers:

Subcortical: medulla (heart rate, breathing), cerebellum (complex movement), hypothalamus (eating, sex, sleep)
Cerebral cortex: the most important part, “makes us human.” Organized like a map; the face and hands take up a disproportionately large area.

What I found most interesting were the disorders that come from cortical damage. Each one is weirdly specific:

A person with Apraxia can move their limbs but can’t coordinate them into a purposeful action.

Someone with Agnosia can see perfectly but can’t recognize what they’re looking at. They can describe every detail of a picture without knowing what the picture is.

Prosopagnosia means you can see faces but can’t identify them, even your own family’s.

Sensory Neglect causes a person to simply ignore one half of the world entirely, asked to draw a clock, they’ll cram all twelve numbers into the right side.

And damage to the frontal lobe can strip someone of their sense of right and wrong, their ability to hold back impulses, or their conscience altogether.

These cases suggest that a lot of what we think of as “character” or “self” is, at some level, just tissue.

Our two brains

Most right-handed people have language in the left hemisphere. Left-handed people are more varied.
Contralateral organization: the right brain controls the left side of the body, and vice versa.

The “left brain / right brain” thing you’ve probably heard, creative people are right-brained, logical people are left-brained, is mostly nonsense.

Almost every function exists in both hemispheres; one side just tends to dominate. The course was pretty direct about this.

The two hemispheres are connected by the corpus callosum, a bundle of fibers that lets them talk to each other constantly. That’s why we experience the world as one continuous thing, not two separate streams.

The split-brain experiments are where things get genuinely strange. Surgeons used to sever the corpus callosum to treat severe epilepsy. It worked, but the patients effectively became two people in one body. The left hemisphere could speak; the right couldn’t, but had its own awareness, its own reactions.

It raises a question I don’t have an answer to: if your brain were split in half, which half would be “you”?

A bit of humility

Modern science’s position is that dualism is wrong. The mind is the brain, one thing, not two. When the brain dies, you end. A useful way to picture it:

Brain = hardware
Thoughts = software.

It explains a lot.

But then there’s what David Chalmers called the “hard problem of consciousness.” Even if we accept that the brain produces experience, we still can’t explain why physical processes feel like anything at all. Why does pain hurt? Why does red look red?

Thomas Huxley called it a miracle.

We know the brain creates consciousness, we just have no idea how.

Thoughts after learning these things:

This module changed how I think about the brain, but it also changed how I see everything.

Now when I come across something about the soul, or anything spiritual, I can’t take it as the way I used to. There’s always a question sitting in the back of my head: Is this dualism, or is this materialism?

I didn’t have that lens before and I’m sure I can’t put it down now.

It’s a excited feeling, like a door opened onto something I didn’t know was there.

Thoughts

2026