"What's in your head? In your head? Zombie! Zombie! Zombie!" - The Cranberries

Conscious mental phenomena seem really weird- there is a disconnect between what we see as "the physical world", namely all this stuff that's "outside" of us, and the mental world, which is all this stuff "inside" of us. However, people (like me) who buy into reductionist views of the world claim that the "inside" stuff can be explained in terms entirely of the "outside" stuff.

Philosophers have developed a thought experiment that is supposed to show that this cannot be the case. The thought experiment goes like this: imagine a world where everything looks exactly the same as our world, everything works exactly the same- in fact, every physical thing about the universe is exactly the same as ours, except nobody is conscious! In this world, when you have a conversation about consciousness with your friend Bob the philosopher, everything goes exactly the same except that Bob is really a zombie. (Yes, "zombie" is the actual term used in the philosophical literature.) All the lights are on, but nobody's home.

There is something appealing about this thought experiment- in ordinary situations, it's entirely possible for there to be something such that, due to your limited observation powers, you don't have any different experience whether it is or is not there. For instance, as I write this there is a cupboard across the room from me. If I just sit here (in my philosopher's armchair) and stare at the cupboard, I cannot tell whether or not there is anything inside it. I can pretty well infer that there is nothing in there that moves or makes sound, but I certainly couldn't distinguish between whether or not it held, say, a pile of magazines. From my vantage point, it is pretty straightforward for me to imagine a world in which there is nothing inside the cupboard or a world in which there is something inside the cupboard- furthermore, if I don't open it, nothing about my experience will change if there is or is not a pile of magazines. However, each of these imagined worlds makes different predictions on possible observations I could make (in this case, opening the cupboard doors).

However, the moment we allow an idea of something that "exists" and yet has no possible effect on any observation we could make (note the emphasis on the word "any"- the thought experiment hinges on this qualifier) then the possible things that could "exist" multiply like a cancer. Here are merely a few possibilities:
  • Everyone is conscious
  • Everyone but you is a zombie
  • Everyone is conscious, except your friend Bob the philosopher, who is a zombie
  • All women are conscious, all men are zombies (or vice versa)
  • Everyone who claims to be conscious is conscious, and everyone who claims to be a zombie is a zombie
  • Everyone who claims to be conscious is a zombie, and everyone who claims to be a zombie is conscious
Who's to say that consciousness or zombie-ness is static? Namely, if you can't tell whether something is conscious or a zombie, then there's no reason to believe that zombie-ness couldn't be conditional on some other arbitrary factor. Some more possibilities:
  • Everyone but you flickers between being conscious and being a zombie once every second
  • Other people are only conscious when they are within 15.691 meters of a manhole cover, otherwise they are zombies
  • There is only one other conscious person in the world at any given time, randomly chosen each day from a probability distribution weighted towards people whose last name contains the letter 'a'
Furthermore, if things can be conscious or unconscious with no possible observable differences, then what's to say that those are the only two options? Couldn't things have conscious experiences that had no relation to what their "merely" physical bodies were doing?
  • Everyone but you really only "sees" in shades of red
  • Everyone (but you) born before 1963 is a zombie, everyone born from 1963-1988 is conscious, and everyone born after 1988 is also conscious, but their conscious experience consists of only the song "Zombie" by The Cranberries playing on repeat
  • Everyone else experiences exactly your conscious experience
Note that if the presence or absence of consciousness has literally no effect whatsoever on any interaction we could possibly have, not only do we not know whether humans might be zombies, we don't know whether non-humans might be conscious!
  • You and all pigeons are conscious, but everything else is a zombie
  • You, igneous rocks, every third wave hitting the Cliffs of Dover, and phlegm are all conscious, but everything else is a zombie
  • Any object of volume greater than 30 cubic centimeters is conscious, everything smaller is a zombie.
  • The entire universe was conscious for a brief, shining moment when Albert Hoffman discovered LSD, but has unfortunately since reverted to zombie-dom

The possibilities that we open up by simply not requiring that our hypotheses have any observable effects are infinite! Note that all of these situations are perfectly "imaginable"- after all, I just imagined them when I made them up. (Or did I imagine them? Did I have any internal experience at all while writing this post?) Arguing in support of an idea on the basis that it's imaginable is a bit silly- obviously if you're talking about it it's "imaginable" in some sense!