Waking up and smelling coffee, hearing music, seeing a rose on the windowsill… These are the simple experiences of one’s day. At its heart is consciousness, a rich, subjective evaluation of the sights, sounds and textures. There is nothing more intimately known than consciousness, yet there is nothing more mysterious. <br> <br>The mystery has its modern origins in René Descartes’s (1596–1650) idea that humans possess both a physical body and an immaterial mind. This view, which became known as <em>mind-body dualism</em>, however, left open the problem of interaction. In a correspondence with Descartes<sup>1</sup>, Princess Elisabeth of Bohemia, asks how can two fundamentally different substances causally interact? How, for example, does a conscious decision to raise one’s arm cause it to go up? And, why does a stubbed toe cause the feeling of pain? Descartes’s solution — the interaction takes place in the pineal gland — is perhaps the weakest link in his idea.<br> <br>Over the years, there has been no satisfactory account of the relationship between the mind and the body, leading many philosophers to adopt <em>materialism</em>: the view that conscious states of the mind are simply brain states. Materialism has raised many objections, amongst them is Thomas Nagel’s <em>What is it like to be a bat?</em> (1974)<sup>2</sup>. No matter how much is known about a bat’s ability to use echolocation, one cannot know what it is like to be a bat flying in the dark, navigating with reverberating sound waves.<br><br>Similarly, a scientific understanding of what happens inside the brain when a person undergoes a conscious experience says nothing about the subjective character of that experience: what it is like to see a rose.<br>