These are deep questions, and sometimes we have to drag out some heavy machinery to answer them. However, since we often already have strong intuitions about these matters, we can accomplish a lot through some conceptual analysis. One of the most powerful tools in the philosopher's toolkit is the thought experiment, which is an imaginary scenario designed to elicit intuitions. A thought experiment might begin as follows: Suppose you are a brain-in-a-vat controlled by an evil scientist… Suppose that your experience of the color of grass is like my experience of the color of a ripe tomato… What if, while standing at the edge of a swamp, you are struck by a bolt of lightning that simultaneously reduces your body to its elements and causes the generation of a molecule-for-molecule duplicate of your body from the primordial goo…