He's talking about phenomenal consciousness, of course; qualia, the real redness of red or blueness of blue. We often do things best, he points out, when we do them automatically: when we stop to become really conscious of how things look or sound, we make mistakes. Perhaps when Mary finally emerged from her black-and-white room, seeing colour for the first time would merely make her trip over the carpet.

But if that's true, then zombies (hypothetical people who function normally but have no conscious experience, just colourless registration of data) would have a consistent edge over the rest of us. They would be bound to take over the world. Just a minute, now - you don't think that they could already...

First, everything requires energy, however small an amount. Qualia have been likened to the humming noise that the computer makes when you switch it on, or the whistle on a steam engine. Neither, it is claimed, affect the operation of the machine. In the case of the computer, it looks as if this might be true, but if you try to make one which is otherwise identical but doesn't make the noise, you'll soon find out it isn't so. The whistle is an even worse example, because it does have a small direct effect on the operation of the engine. If you blow the whistle long enough and hard enough, the locomotive will actually begin to lose power. It follows that zombies couldn't be exactly like normal people. 

Second, everything has consequences. If there could be people with qualia and people without qualia, one sort would have an advantage. Either the qualia would help you spot food and predators, or, as Marks suggests, they would be an unwelcome distraction. Either way, one sort of person would have died out long ago.