Starting from the stake of the Kantian philosophy to enclose everything within the firm limits of the system of reason and to exclude the external, threatening rest that is outside these limits, Jacques Derrida tried to deconstruct the effort of metaphysics to build artificial bridges over the ruptures of the philosophical systems, usually caused by the intrusion of this uncontrollable external remainders that does not submit to the authority of the metaphysical logos. Given that in the Kantian system this external (...) remainders received the name parergon, we followed this paradoxical structure of parergon, first on the ground of mystical-religious experience, and then on the ground of aesthetic-artistic experience. We point out that Derrida reverses the metaphysical perspective and argues that the parergon is rather a reliable, permeable, undecidable border structure, irreducibly involved between everything that is enclosed within formal limits and the a-formal rest that escapes outside. The original Derridean argumentation deconstructs the ergon / parergon distinction and the entire suite of metaphysical oppositions and rather leads us to an undecidable parergonal border. (shrink)