You unlock a door with a key of imagination. Elsewhere in the universe, there is a Twin Earth. Identical in almost every single way. Even the surroundings, such as the Sun, are exactly as they are here, on our Earth. On our Twin Earth, there is a twin equivalent of every person and every thing. They are the same, a perfect copy, except for one, small thing. They have a substance similar to water, but chemically different. A long, complicated formula that we shall abbreviate as XYZ, not composed of H2O like our water. In an ancient time, two residents of each planet meet. Long before the chemical compositions of these two liquids, identical to these two laymen, were known. For them, meeting for the first time, the liquid we know at H2O and the liquid they know at XYZ, are identical. They look and feel the same. But when the Twin Earth resident says ‘water’, he’s referring to XYZ, but we think he’s referring to H2O. And when we say ‘water’, we’re referring to H2O but he thinks we’re referring to XYZ. You’ve just crossed over into The Twilight Zone. Do-dee-do-do, do-dee-do-do…
Of course, if you’re very young you won’t at all know what The Twilight Zone is, for which I say, for shame. In fact, go and watch it right now and then come back. You’ll then realise how similar this paradox is to the intro of said show. This paradox comes from the crazy age known as 1973. From the mind of philosopher Hilary Putnam. A man who is famously contradictory. He said himself this paradox is nonsense. But it is very intriguing.
They speak English on Twin Earth. Experiences with the liquid in question are identical for them and for us. But when they say ‘water’ do they mean the same thing? Their brains are identical. Molecule-for-molecule. They believe they mean the same thing, but we advanced onlookers know they are wrong. Or are they wrong? Is either of them correct? How do we know what we know is actually what we know? Is the sky blue or is it grey? Our two residents grew up in an environment where they referred to water by different names. So what is it about this paradox that is so nonsensical when actually, it sounds quite brilliant?
Philosophers argue that the two are actually referring to something that is ‘water-like’, not ‘water’ but what they believe to be ‘water’. ‘Water’ isn’t an absolute. When we think of ‘ocean’, we think of a lush blue body of water, but actually, on other worlds, there are oceans of methane. Among many other examples. When we say ‘ocean’, we mean ‘ocean-like’. The same could be applied to ‘water’. Those two residents actually mean ‘water-like’. Wetness, transparency, colour, are all the same. ‘Water’ is a general term. Yes, the paradox does make us question our thoughts, somewhat, but we’ve always been like that. We investigate to learn, we don’t just ‘know’ things. And, of course, this is a science-fiction paradox. This scenario is nonsense, it will never happen. The state of affairs is radically different from the actual one, therefore our intuitions are unreliable and we cannot draw any conclusions from them. This befalls all science-fiction paradoxes, sadly, as wonderful as they sound.
So, to solve this paradox one must realise that actually, it isn’t one at all.
What do you think of this thought experiment, readers?
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