section_path: Contents/Internalism and externalism
section_name: Internalism and externalism
paragraph_text: Internalism and externalism
  
  Internalism and externalism disagree about whether the contents of our beliefs are determined only by what's happening in our head or also by other factors. Internalists deny such a dependence on external factors. They hold that a person and a molecule-by-molecule copy would have exactly the same beliefs. Hilary Putnam objects to this position by way of his twin Earth thought experiment. He imagines a twin Earth in another part of the universe that is exactly like ours, except that their water has a different chemical composition despite behaving just like ours. According to Putnam, the reader's thought that water is wet is about our water while the reader's twin's thought on twin Earth that water is wet is about their water. This is the case despite the fact that the two readers have the same molecular composition. So it seems necessary to include external factors in order to explain the difference. One problem with this position is that this difference in content does not bring any causal difference with it: the two readers act in exactly the same way. This casts doubt on the thesis that there is any genuine difference in need of explanation between the contents of the two beliefs.