Kant's third antinomy introduces freedom as the unconditioned cause that allows reason to form a synthesis of causal linkage. What different thinking conditions, I ask, does this antinomy open up for reason even to entertain any possibility of success? Reason's ability to form a synthesis depends on acknowledging the role in Kant's argument of a subject that does not need to be envisioned as a standpoint—whether intelligible or empirical, as Kant's explicit solution has it—but, rather, as the site of a relationship between the series and its outside. We may call it “unconditioned subjectivity,” clarifying from the outset that subjectivity in this sense names a structural position that plays an exceptional role in the series: it is the element that exceptionally introduces a boundary, a fleeting moment of closure. Unconditioned subjectivity would name the relationship and boundary that anchor the phenomenal series time and again. I argue, moreover, that the synthesis of causality forming here coheres as a dynamic system that is not stable or self-contained but, rather, contingent and “in progress.” Perhaps unexpectedly, the potential of this antinomy to form this dynamic system hinges on the fact that its antithesis denies freedom.