we don't? probably because you got it backwards. The idea of attraction entails gravitation (and other forces that hold the ball up before the drop), which is t

we don't? probably because you got it backwards. The idea of attraction entails gravitation (and other forces that hold the ball up before the drop), which is the conclusion of a line of reasoning, that is a more detailed, generalized version of the observation "things fall when dropped". Sure, backtracking is a rather simple, if not the most simple approach, but for N=NP kind of problems, presumably, it is the only one and heuristics only change the order of the track, but never reduce complexity. That's my layman opinion, that seems to agree with the etymology of reason. Reason > ... > Ratio ... Reor. Reor is latin for to think, or calculate. Arithmetic in its simplest form, addition in the unary system ie. arranging pebbles (= lt. calculus), counting knots, simply counting. Now backtracking is just enumeration and elimination of possibilities. Ratio itself means measure, and a measurement always entails statistical error (does heisenbergs uncertainty principle prove that?).