"One serious mystery of decoherence is where the Born probabilities come from, or even what they are probabilities of. What does the integral over the squared modulus of the amplitude density have to do with anything? … So what could it mean, to associate a "subjective probability" with a component of one factor of a combined amplitude distribution that happens to factorize? … But what does the integral over squared moduli have to do with anything? On a straight reading of the data, you would always find yourself in both blobs, every time. How can you find yourself in one blob with greater probability? What are the Born probabilities, probabilities of? Here's the map—where's the territory? I don't know. It's an open problem. This problem is even worse than it looks, because the squared-modulus business is the only non-linear rule in all of quantum mechanics."