One of the daily tragedies of being human is that notions in our heads of unaided flight, levitation - any thought of lift-off from our material horizon - lie in drastic disproportion to what flesh and muscle permit. As children, we dream of flying, or living, say, on ocean floors without gas-tanks. As adolescents, we dream of many things, most of them impossible. As adults, sportspeople and dancers strain to defy nature, but never do. Most of us go on to live resignedly alongside, or inside, nature, glum in the knowledge that our "machine", as Hamlet terms his mortal frame, will of course wholly fail.