"It should be quite clear, then, that there are no criteria to be laid down in general for distinguishing the real from the not real."

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The physicist may be satisfied when he has the mathematical scheme and knows how to use for the interpretation of the experiments. But he has to speak about his results also to non-physicists who will not be satisfied unless some explanation is given in plain language. Even for the physicist the description in plain language will be the criterion of the degree of understanding that has been reached.

— Werner Heisenberg