The problem of navigation, reduced to its very simplest terms, is how to get from X to Y. But this statement appears at first sight to require the proviso ‘given the means of transport available’, whether camel, ship, plane or rocket. Each sets up its special limitations to free movement, but in effect merely determines that XY must be broken up into segments, X Y1, Y1 Y2 and so on. Further reflection would probably add two additional considerations, the navigator must get safely and expeditiously from X to Y. But this again can only break down the route into fresh segments whereby dangers and difficulties are avoided. It is still for the navigator just the problem of getting from X, where he is, to a designated Y, where he wishes to be.