SummaryThe paper undertakes to disentangle the problem facing Kant's Third Antinomy from the problems confronting the transcendental reality of time and the distinction between things–in–themselves and appearances. Three different resolutions of the Third Antinomy are distinguished. It is shown that neither the appeal to the transcendental ideality of time nor to the distinction between things‐in‐themselves and appearances in the firstKritikresolution succeeds in resolving the antinomy as Kant states it. The appeal in theGroundworkto the distinction between heteronomy and autonomy shifts the problem. And the reliance in the secondKritikon our ability to knowa priorithat there are necessary connections in our experience fails in yet another way to forestall the antinomy. The common core of all of these difficulties lies in what is called here the Duplication Problem: The tendency of the distinctions introduced to resolve the problem to duplicate the very antinomy they are supposed to resolve. Finally, it is shown how isolating these issues helps to strengthen Kant's argument for transcendental ideality and to clarify the conditions under which the antinomy can he r