One of the reasons why it still lacks a precise, agreed definition is that the problem may take very different forms depending on the interpretation of QM embraced. I propose to identify the common root of the puzzle in an interpretation-independent way (i.e. as a property of the probabilities only) and derive its philosophical consequences. The key point is that the violation of the total probability formula in QM does not allow to construct an objective ontology, independent from epistemology. This enables to: (i) better compare the different interpretations of QM; (ii) propose a reason why QM and GR are so fundamentally incompatible, not relying on purely mathematical or technical arguments.