How did minds first evolve out of matter? Could consciousness have evolved more than once? How do we tell which living things have minds? Is there something it’s like to be a crab and live a crab's life? This week we’re thinking about “Minds and Matter.”

What is a mental image? You might say it’s like a picture that belongs to the “mind’s eye.” Or you might say it's like a visual experience, only less vivid. But what, exactly, does it mean for mental images to be less vivid than genuine visual experiences of the world?

We tend to think that you can only have one thought at a time. You can switch between different kinds of thoughts quite quickly, or you can think many thoughts one after the other, but you can’t think more than one thought at the same time. That’s a mistake. There’s a way to think two thoughts at once.

Are humans limited to the senses we’re born with? Or is it possible to hack the brain and create new senses? Even if we could, would we want more senses than we already have? This week we’re thinking about hacking the brain: perception beyond the five senses.

In this installment of my series on Freud as a philosopher, I explain how Freud arrived at the view that mental states are brain states, that mental processes are unconscious, that we have only indirect access to our own minds, and that introspection is an inadequate tool for exploring the mind.