Complex sentence: Kant believed that he had discovered another attribute of propositions, which allowed him to frame the problem of "a priori" knowledge in a new way: the distinction between "analytic" and "synthetic" judgments.
Split into simpler sentences: Kant believed that he had discovered another attribute of propositions, which allowed him to frame the problem of "a priori" knowledge in a new way. This was the distinction between "analytic" and "synthetic" judgments.