 One of the most interesting questions regarding the PSR is why accept it at all. Insofar as the PSR stipulates that all things must be explainable, it seems that the PSR itself demands an explanation. Several modern philosophers attempted to provide a proof for the PSR, though so far these attempts have been mostly unsuccessful. Another important question related to the PSR is the possibility of self-explanatory facts and self-caused entities; particularly, one might wonder how these are distinguished from unexplainable, brute facts and uncaused entities. One might also wonder whether the PSR allows for any primitive concepts that cannot be further explained.