## Abstract

There are two famous sayings that describe our knowledge of quantum theory. The first says that in order to learn quantum theory one has to solve 500 problems. The other says that nobody understands quantum theory. There is no contradiction. The point of the first saying is that quantum theory consists of some rules of calculation that one can learn with enough experience. The point of the second saying is that nobody can explain the rules in and of themselves, except that they give a correct description of physical systems.

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