It is broadly agreed that consciousness poses a profound challenge to contemporary science. Will neuroscience one day crack it? The problem is that consciousness is unobservable – you can’t look inside someone’s head and see their feelings and experiences – and this severely constrains our capacity to investigate it experimentally.

Because we can’t observe consciousness, our only way of gathering data about it is by relying on people’s testimony regarding their private, inner feelings. If we scan their brains at the same time, we can map correlations between various kinds of brain activity and various experiences. This is important data but it’s not itself a theory of consciousness. What we ultimately want from a theory of consciousness is an explanation of those correlations. Why is it that certain kinds of brain activity are correlated with certain kinds of experience?

Materialists hope to account for these correlations by explaining experiences in terms of brain activity. The trouble is you can’t capture the qualitative character of an experience – what it’s like to see red, or to smell coffee – in the purely quantitative language of neuroscience. And hence, so as long as our theory of the brain is framed in the purely quantitative language of neuroscience, we’ll always leave out these qualities, and in doing so leave out consciousness itself.

Rather than trying to explain experiences in terms of brain states, panpsychists explain brain states in terms of experiences.