The equations of quantum mechanics and their consequences are used daily in widely varying fields... yet they remain mysterious. For they do not describe what happens to a physical system, but only how a physical system affects another physical system. What does this mean? That the essential reality of a system is indescribable? Does it mean that we only lack a piece of the puzzle? Or does it mean, as it seems to me, that we must accept the idea that reality is only interaction?
Carlo Rovelli: Seven Brief Lessons On Physics (Second Lesson: Quanta)