In spite of these early hints, it was only in the twenty-first century that US physicist Christopher Fuchs and British–German physicist Rüdiger Schack4, 5, 6 put forth an understanding of quantum mechanics that restored the balance between subject and object. They call their new point of view 'QBism': Q is for quantum and B is for Bayesian — a view of probability that includes an agent who makes bets and updates odds. QBism attributes the muddle at the foundations of quantum mechanics to our unacknowledged removal of the scientist from the science. Fuchs and Schack adopt the latter view. They take a wavefunction to be associated with a physical system by an agent — me, for example, based on my past experience. I use the wavefunction, following rules laid down by quantum mechanics, to calculate the likelihood of what I might experience next, should I choose to probe further. Depending on what I then perceive, I can update the wavefunction on the basis of that experience, allowing me to better assess my subsequent expectations.
People who believe wavefunctions to be as real as stones have invested much effort in searching for objective physical mechanisms responsible for such changes in the wavefunction: a novel manifestation of gravity, for example, or a new kind of fundamental all-pervasive friction. But according to QBism, the change is only in my personal expectations, which I revise to accommodate my new experience.