 Arguably any interpretation of the correspondence principle faces the following four challenges: First, determining which (or which combination) of the various analogies or relations between classical and quantum mechanics Bohr intended to designate by the correspondence principle; second, determining the scope of the correspondence principle (i.e., does it apply only to large quantum numbers or all quantum numbers); third, offering an explanation for why Bohr thought that the correspondence principle should be thought of as a law of quantum theory; and finally, offering an explanation for why Bohr thought that the correspondence principle had been formalized, and hence preserved, in modern (matrix) quantum mechanics.