Principle of Least Action

This says, essentially, that things happen in the way that requires least effort.

So, a beam of light will travel in a straight line because that is the shortest path between two points. If you drop a ball, it will travel toward the center of the Earth. No one is quite sure who came up with the principle of least action, but your everyday experience would probably lead you to come up with it if you thought about it for a bit. In the 18th century, though, this was a big deal. Some of the greatest names in mathematics, such as Leonhard Euler, Pierre de Fermat, Gottfried Leibnitz, and Voltaire were involved in the argument over who came up with the idea first. It was important to make these kinds of statements at the time, because they led to the formation of the equations that describe how things move when acted on by forces. They also led to the concepts of potential and kinetic energy.

by Michael Brooks, 30 Second Theory