According to the unrestricted version of the Principle of Sufficient Reason (PSR), every truth has an explanation. I argue that there is defeasible methodological justification for belief in an unrestricted PSR. The argument is based on considerations about our cognitive limitations. It is possible that our cognitive limitations prevent us from even recognizing the explanatorily open character of some propositions we can now represent: the fact that these propositions are explicable in the first place. If this is the case, then (...) a proper recognition of our limitations and a standing goal of reasoning to attain complete explanatory knowledge justify the adoption of an unrestricted PSR. (shrink)