Imagine that somewhere, very, very far away, there’s a planet that looks just like Earth. It looks so much like Earth that the trees, mountains, rivers, cities, buildings, and so on, are in exactly the same place as they are on Earth. There is even a country that the people there call ‘England’, and there are even people identical in all physical respects to the people here. Let’s call this twin of our own planet ‘Twin Earth’.
Twin Earth looks so much like Earth that if you and your twin on Twin Earth were switched in your sleep, neither of you would be able to tell the difference. But there is one fundamental difference. Here on Earth, the watery stuff that runs in the rivers, evaporates on warm days, and falls down as rain in England rather too often, is water (H2O). But on Twin Earth, the watery stuff that runs in the rivers, evaporates on warm days, and falls down as rain in Twin England rather too often, is not water. It’s not water, because it’s not H2O. The watery stuff on Twin Earth looks and behaves just like water (except in extreme conditions that only occur in science laboratories), but it has a different chemical composition – it’s a water look-a-like. Let’s call the watery stuff on Twin Earth ‘t-water’.
This difference between Earth and Twin Earth, says the externalist, makes a difference to the thoughts of you and your twin. No doubt sometimes you think that you’d like a drink of water. But your twin couldn’t possibly think that. Your twin would instead think that she or he would like a drink of t-water. This is because your thoughts are about England and Earth and water, while your twins thoughts are about Twin England, Twin Earth and t-water. This shows that the environment you live in makes a difference to what you can think.
This is a famous philosophical thought experiment based on the work of Hilary Putnam and Tyler Burge. Click on ‘Further Resources’ to find out about the original papers.