This very subtle and paradoxical sounding situation can perhaps be most easily understood in terms of the similar situation that exists with respect to Newton's second law of motion: In textbooks and in classrooms the law F = m a is attributed to Newton, but Newton in fact wrote his second law F = p ˙ ˙ is clearly visible in a glass case in the Wren Library of Trinity College, Cambridge, where Newton's manuscript is open to the relevant page. as F = p ˙ ˙ , where p ˙ ˙ is the time derivative of the momentum p . This seems a trivial enough fact until you realize that F = p ˙ ˙ remains true in special relativity, without modification.