An ancient cosmological debate lies behind the spatial part of the first antinomy in Kant’s Critique of Pure Reason. Against the Aristotelian conception of a finite universe, a thought experiment proposed we imagine ourselves situated on the boundary of the world: what happens if we stretch a hand beyond the boundary? This article first shows that aspects of this debate persist in the cosmological claims of Huygens, Wolff, and Crusius. With his presentation of opposing arguments in the first antinomy, Kant (...) famously rejected both sides of the debate and asserted that we cannot meaningfully inquire into the spatial boundary of the world. I then argue that the critical-period Kant nevertheless does not simply dismiss the issue of an outer boundary of the world: he rather reconceives it as the question of the boundary of the legitimate use of our cognitive faculties. The cosmological question of the boundary of the world is transformed—from Kant’s Inaugural Dissertation through his Reflexionen of the 1770s to the first Critique—into the critical question of the boundary of reason. -/- . (shrink)