It is a fundamental quantum doctrine that a measurement does not, in general, reveal a pre-existing value of the measured property. On the contrary, the outcome of a measurement is brought into being by the act of measurement itself, a joint manifestation of the state of the probed system and the probing apparatus. Precisely how the particular result of an individual measurement is brought into being-Heisenberg's "transition from the possible to the actual"-is inherently unknowable. Only the statistical distribution of many such encounters is a proper matter for scientific inquiry.