  We reflect on the nature, role and limits of non-empirical theory assessment
in fundamental physics, focusing in particular on quantum gravity. We argue for
the usefulness and, to some extent, necessity of non-empirical theory
assessment, but also examine critically its dangers. We conclude that the
principle of proliferation of theories is not only at the very root of theory
assessment but all the more necessary when experimental tests are scarce, and
also that, in the same situation, it represents the only medicine against the
degeneration of scientific research programmes.
