 Imagine a planet, a twin earth. There are a few things you should know about this planet. First, it duplicates earth in almost every respect. On twin earth, at this very moment, a twin version of you is reading a twin version of this Stanford Encyclopedia entry, which has been written by a twin version of me. The twin version of you is your exact, molecule-for-molecule, duplicate. Second, the only difference between earth and twin earth is this: on twin earth, the substance that sits in oceans, flows in rivers, and comes out of faucets is not water. It is not water, because it is not made up of two parts of hydrogen to one part of oxygen. In fact, it is not made of hydrogen and oxygen at all but, rather, some other elements entirely absent on our earth. Nevertheless, third, this substance looks, tastes, and feels exactly like water does to people on our earth, and is used in the same way. Finally, this thought experiment is set in 1750 (and, a corresponding twin 1750), before people had any idea of the underlying molecular structure of substances such as water.