As summarized in Part 1, folk Cartesianism consists of three strong intuitions:

1. There is a unified self that floats above mental processes and observes them

2. We know our own internal experience with complete certainty

3. The basis of all knowledge lies in our first-person subjective experience

In this post I’ll give a lot of arguments and intuition pumps for why each of these is probably incorrect, or at least isn’t “obvious.” This post is very long. My goal is that at the end you have a grasp of many of the most important questions in philosophy of mind, why so many philosophers reject the popular everyday understanding of mind, and that you hopefully come away with a bag of new mental tools for thinking about why machines might be able to replicate what happens in human minds. I’m writing this partly to be the guide I wish I had when I was first learning about this.

“1. There is a unified self that floats above mental processes and observes them”

How can a Cartesian ego fit within our scientific theories?

Cartesian egos are strange. They seem to behave like magical floating eyeballs, peering into physical reality from outside. No other physical object does this. No matter how many grains of sand I pile up, there will never be “something it is like to be the sand. The sand does not have an internal movie theater playing experiences for it to observe. Being sand feels identical to being dead. We would expect all physical objects to behave similarly, because all physical objects are ultimately just elementary particles that don’t have experiences, similar to the grains of sand. No physical object ever becomes an observer in an inner movie theater. So how do Cartesian egos fit within our understanding of science?

There are only two options: Cartesian egos are fundamentally physical, or they’re not. Both options have some weird implications that make them look implausible.

Cartesian egos are not fundamentally physical

If Cartesian egos are not fundamentally physical, they threaten to clash with physicalism, which says that the only things that have causal power over physical objects are other physical objects. If they have any kind of causal power, this breaks physicalism. We have two options:

Cartesian egos do not have causal power

Maybe Cartesian egos are silent observers, taking everything in but never actually having any causal effect on the world. ...