Dualism

A 17th century philosopher named René Descartes struggled to make sense of the mind-body problem.

He understood that thoughts originate in the brain, but he observed that mental activity is ephemeral, without physical substance. How can this be? he wondered.

Ever since, philosophers have called Descartes’s philosophy “dualism.” They concur that what occurs within the mind exists in a separate reality from what occurs in the physical world.

The 16th century writer Michel de Montaigne tried to describe the way a human mind works. “Men do not know the natural disease of the mind; it does nothing...