<p>Metaphysical rationalism involves a commitment to a version of the Principle of Sufficient Reason that says, ‘every fact as a metaphysical explanation’. While the PSR has been the subject of renewed enthusiasm recently among certain analytic metaphysicians, I argue it faces a significant challenge stemming from an important connection between explanation and understanding. Many thinkers who hold competing accounts of what explanation is have argued that understanding is a constraint on explanation. I argue that the PSR is committed to an ontic or objective account of explanation that is inconsistent with this constraint. So long as this constraint on explanation holds, it follows the PSR is false. I argue further that even if understanding is not a constraint on explanation, the PSR holds no meaningful epistemic value because it plays no direct role in facilitating or guiding our inquiry and explanatory practices.</p>