 (Two points should be made parenthetically about this example. (1) The observable properties of XYZ do not need to be identical to those of water; all that is needed is that Oscar and Twin Oscar have not observed the differences. (2) In some ways it is unfortunate that the water/XYZ example has become Putnam’s best known example, because it has a failing that many of his other examples lack, namely that most of the human body consists of water. This has the consequence that Twin Oscar cannot be an exact duplicate of Oscar unless Twin Oscar also consists largely of water. Other examples Putnam considers involve switching aluminum and molybdenum, beeches and elms, diseases, and so on, and these examples do not suffer from the same problem.)