You can doubt just about anything. But there’s one thing you can know for sure: you are having thoughts right now.

This idea came to characterise the philosophical thinking of 17th century philosopher René Descartes. For Descartes, that we have thoughts may be the only thing we can be certain about.

But what exactly are thoughts? This is a mystery that has long troubled philosophers such as Descartes – and which has been given new life by the rise of artificial intelligence, as experts try to figure out whether machines can genuinely think.

There are two main answers to the philosophical question of what thoughts are.

The first is that thoughts might be material things. Thoughts are just like atoms, particles, cats, clouds and raindrops: part and parcel of the physical universe. This position is known as physicalism or materialism.

The second view is that thoughts might stand apart from the physical world. They are not like atoms, but are an entirely distinct type of thing. This view is called dualism, because it takes the world to have a dual nature: mental and physical.