How Einstein confirmed $E0=mc2$

The equivalence of mass $m$ and rest-energy $E0$ is one of the great discoveries of all time. Despite the current wisdom, Einstein did not derive this relation from first principles. Having conceived the idea in the summer of 1905 he spent more than 40 years trying to prove it. We briefly examine all of Einstein’s conceptual demonstrations of $E0=mc2$⁠, focusing on their limitations and his awareness of their shortcomings. Although he repeatedly confirmed the efficacy of $E0=mc2$⁠, he never constructed a general proof. Leaving aside that it continues to be affirmed experimentally, a rigorous proof of the mass-energy equivalence is probably beyond the purview of the special theory.