The document discusses Albert Einstein's Special Theory of Relativity, which resolved a conflict between Newtonian mechanics and Maxwell's electromagnetic theory of light. Newton's mechanics treated space, time, and mass as absolute, while Maxwell found light travels at a constant speed. Einstein resolved this by establishing that all motion is relative to the observer's frame of reference and the speed of light is constant for all observers, regardless of their motion. His theory reconciled the two apparently contradictory principles through his postulates of relativity and the constancy of the speed of light.