Dissociatives offer a lot of insight on this subject. If you ever try DXM, for example, you can experience very odd sensations. The neat thing is that these sensations change based on the dose and on the individual time you are taking the drug, sometimes you just get different results (sometimes very different). At lower levels, to me, my arms and hands feel like they exist in different parts of space than they visually correspond to. Often it feels like they are emerging from the middle of my chest or, while maintaining which direction they are going, they switch sides. At higher doses you can lose all sense of your body altogether, and if you watch yourself move or do anything, it is like watching a totally separate person or entity perform the action--both in look and in feeling. It probably has to do with part of your brain that is responsible for orientation in space and letting you know where you stop and the physical world begins having its signals reduced or not even being communicated to the rest of the brain, as a result your conscious self loses these feelings of the self in a physical sense or where you should be in physical space, or whether or not you are indeed moving or sitting still.