The law of cosmic censorship (cosmic censorship hypothesis) is that the inside of a black hole cannot be observed. More specifically, it is that the singularity which would logically be at the center of the black hole cannot be observed. And more specifically than that, it is conjectures about the mathematics of black holes (based on general relativity) that appear necessary to preserve causality and determinism, that observing a singularity (i.e., a naked singularity, one not surrounded by an event horizon) leads to contradictions of some generally-presumed basis of knowledge and science. Two variants exist, known as strong cosmic censorship and weak cosmic censorship.
Theories (e.g., of black holes) that appear to violate the law of cosmic censorship motivate considerable analysis to determine whether the violation is truly implied and whether the violation is plausible, i.e., whether the law is ultimately false.