 Einstein first derived mass-energy equivalence from the principles of special relativity in a small article titled “Does the Inertia of a Body Depend Upon Its Energy Content?” (1905b). This derivation, along with others that followed soon after (e.g., Planck (1906), Von Laue (1911)), uses Maxwell’s theory of electromagnetism. (See  Section 3.1.)  However, as Einstein later observed (1935), mass-energy equivalence is a result that should be independent of any theory that describes a specific physical interaction. This is the main reason that led physicists to search for “purely dynamical” derivations, i.e., derivations that invoke only mechanical concepts such as energy and momentum, and the conservation principles that govern them. (See  Section 3.2)