I'm thinking about relative position (left, right, up, down, ahead, behind). My general question is whether you think that these three oppositions (left/right, up/down, ahead/behind) have the same "status". For instance, for every point moving on a straight line, there is a meaningful and precise difference between ahead and behind, but not necessarily between left and right or up and down. Another example: for any (physical) object on the surface of a planet, the difference between up and down is clear, but not the remaining two oppositions. Another one: if it is settled, in a given 3D situation, what is left and right, then it is also necessarily settled what is ahead and behind, and what is up and down, but this does not (always) work the other ways around. What do you say? And do you think that the opposition between inside and outside has some relation to the other ones?