I was once asked the following question by a student I was tutoring; and I was stumped by it:
When one throws a stone why does it take the same amount of time for a stone to rise to its peak and then down to the ground?
One could say that this is an experimental observation; after one could envisage, hypothetically, where this is not the case.
One could say that the curve that the stone describes is a parabola; and the two halves are symmetric around the perpendicular line through its apex. But surely the description of the motion of a projectile as a parabola was the outcome of observation; and even if it moves along a parabola, it may (putting observation aside) move along it with its descent speed different from its ascent; or varying; and this, in part, leads to the observation or is justified by the Newtons description of time - it flows equably everywhere.
It's because of the nature of the force. It's independent of the motion of the stone.
I prefer the last explanation - but is it true? And is this the best explanation?