Einstein established that the resulting changes in the momentum and energy transformed as a 4-vector. He went on to claim, without proof, that if these changes constituted a 4-vector, the quantities themselves formed a 4-vector. Although his intuition was correct, that unsubstantiated assumption constituted a fatal flaw in the argument. Einstein, aware that he was on thin ice, commented in a footnote: “To be sure this is not rigorous, because additive constants might be present that do not have the character of a vector; but this seems so artificial that we will not dwell on this possibility at all.”