The principle of least action : history and physics The Principle of Least Action originates in the idea that if Nature has a purpose it should follow a minimum or critical path. This simple principle (and its variants and generalizations) applies to optics mechanics electromagnetism relativity and quantum mechanics and provides an essential guide to understanding the beauty of physics. This unique text provides an accessible introduction to the action principle across these various fields of physics and examines its history and fundamental role in science