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What is it like to be you right now? You’re seeing this text on the screen, you smell the coffee next to you, feel the warmth of the cup, and hear your housemates arguing about whether Home Alone was better than Home Alone 2: Lost in New York. There’s a lot going on in your head — your conscious experiences.

Now imagine beings that are identical to humans, except for one thing: they lack conscious experience. If you spill that coffee on them, they’ll jump like anyone else, but inside they’ll feel no pain and have no thoughts: the lights are off.

The concept of these so-called ‘philosophical zombies’ was popularised by today’s guest — celebrated philosophy professor David Chalmers — in order to explore the nature of consciousness. In a forthcoming book he poses a classic ‘trolley problem’:

Suppose you have a conscious human on one train track, and five non-conscious humanoid zombies on another. If you do nothing, a trolley will hit and kill the conscious human. If you flip a switch to redirect the trolley, you can save the conscious human, but in so doing kill the five non-conscious humanoid zombies. What should you do?

Many people think you should divert the trolley, precisely because the lack of conscious experience means the moral status of the zombies is greatly reduced, or absent entirely.

So, which features of consciousness qualify someone for moral consideration? One view is that the only conscious states that matter are those that have a positive or negative quality, like pleasure and suffering. But Dave’s intuitions are quite different.

He asks us to consider the ‘Vulcans’. If you’ve never seen Star Trek, Vulcans experience rich forms of cognitive and sensory consciousness; they see and hear and reflect on the world around them. But they’re incapable of experiencing pleasure or pain.

Does such a being lack moral status?

To answer this Dave invites us to imagine a further trolley problem: suppose you have a conscious human on one track, and five Vulcans on the other. Should you divert the trolley to kill the five Vulcans in order to save the human?

Dave firmly believes the answer is no, and if he’s right, pleasure and suffering can’t be the only things required for moral status. The fact that Vulcans are conscious in other ways must matter in itself.

Dave is one of the world’s top experts on the philosophy of consciousness. He helped return the question ‘what is consciousness?’ to the centre stage of philosophy with his 1996 book ‘The Conscious Mind’, which argued against then-dominant materialist theories of consciousness.

This comprehensive interview, at over four and a half hours long, outlines each contemporary answer to the mystery of consciousness, what it has going for it, and its likely ethical implications. Those theories span the full range from illusionism, the idea that consciousness is in some sense an ‘illusion’, to panpsychism, according to which it’s a fundamental physical property present in all matter.

These questions are absolutely central for anyone who wants to build a positive future. If insects were conscious our treatment of them could already be an atrocity. If accurate computer simulations of people will one day be conscious, how will we know, and how should we treat them? And what is it about consciousness that matters, if anything?

Dave Chalmers is probably the best person on the planet to interview about these questions, and Rob & Arden cover this and much more over the course of what is both our longest ever episode and our personal favourite so far.

They discuss:

• Why is there so little consensus among philosophers about so many key questions?
• Can free will exist, even in a deterministic universe?
• Might we be living in a simulation? Why is this worth talking about?
• The hard problem of consciousness
• Materialism, functionalism, idealism, illusionism, panpsychism, and other views about the nature of consciousness
• The story of ‘integrated information theory’
• What philosophers think of eating meat
• Should we worry about AI becoming conscious, and therefore worthy of moral concern?
• Should we expect to get to conscious AI well before we get human-level artificial general intelligence?
• Could minds uploaded to a computer be conscious?
• If you uploaded your mind, would that mind be ‘you’?
• Why did Dave start thinking about the ‘singularity’?
• Careers in academia
• And whether a sense of humour is useful for research.

Get this episode by subscribing to our podcast on the world’s most pressing problems and how to solve them: type 80,000 Hours into your podcasting app. Or read the transcript below.

Producer: Keiran Harris.
Audio mastering: Ben Cordell.
Transcriptions: Zakee Ulhaq.