  To understand the Moral Twin Earth Objection, we need to first understand how causal regulation semantics are supposed to work. The following thought experiment, from Putnam (1975), has been highly influential: Imagine a world—Twin Earth—where there is no H\(_2\)O, but there is another substance called XYZ. This substance XYZ, while distinct from H\(_2\)O, fills the rivers and lakes, is clear and tasteless, etc. XYZ even has the property of being called ‘water’—when residents of Twin Earth fill up a glass with XYZ from the tap, they will say “I have a cup of water.” Yet for all this, we would not say that Twin Earth is a planet where water is XYZ. We would say that this is a planet where there is no water. There is, instead, another substance—XYZ—that plays the same functional role. Yet when the person on Twin Earth fills a cup from the tap and declares “I have a cup of water”, that assertion seems true. What could account for that? Putnam argues that the word ‘water’ just means something different on Twin Earth than it does in the actual world. In the actual world, ‘water’ means H\(_2\)O because, in the actual world, H\(_2\)O is that thing that is clear, tasteless, fills the rivers and lakes, and is referred to with the inscription ‘water.’ But on Twin Earth, ‘water’ means XYZ because, on Twin Earth, it is XYZ that is clear, tasteless, fills the rivers and lakes, and is referred to with the inscription ‘water.’ Natural kind terms like ‘water’ refer to those properties that causally regulate the use of those terms. That is why the word ‘water’ literally means something different on Twin Earth than it does in the real world.