The Cartesian theatre is a term coined by the philosopher and cognitive scientist Daniel Dennett in his 1991 book Consciousness Explained. Echoing earlier work by the philosopher Gilbert Ryle, it is an attempt to expose weaknesses in the classical Cartesian idea of mind-body dualism, in which the brain receives signals from the sensory organs and relays them to the soul, which does the thinking and decision-making, before sending its commands back to the brain for execution.^[1] The place within the brain at which sensation occurs – the “inner theatre of the mind” – was known as the sensorium, a term particularly associated with Isaac Newton (1643–1727), who considered the whole world to be God’s sensorium.^[2][a]The terms “mind” and “soul” both derive from the ancient Greek word psychê.^[3] The French philosopher René Descartes considered the mind and soul to be equivalent.^[4]

In his Meditationes de prima philosophia (Meditations on First Philosophy), published in 1641, the French philosopher René Descartes introduced the notion of what would today be called indirect realism, the idea that we cannot be directly aware of the world around us, but only of how it is presented to us by the images of colours, shapes, sounds and so on created by our senses. Although he may have believed that he was sitting by the fire in his dressing gown, writing, how could Descartes be absolutely certain he was not dreaming, hallucinating, or even that there was a physical world to perceive at all? He concluded that the only thing he could say with any certainty was that he was thinking, that certain events were taking place in his mind.^[1]

Descartes claimed that consciousness requires an immaterial soul, which interacts with the body via the pineal gland of the brain.^[5] Very few modern philosophers believe in the idea of an immaterial soul, rather that minds are “functioning, embodied brains”.^[1] Dennett argues that without an immaterial soul, all that remains of Descartes’s dualistic model amounts to imagining a tiny theatre in the brain where a homunculus (small person), now physical, performs the task of observing all the sensory data projected on to a screen at a particular instant, making decisions and sending out commands.^[6]