 As a characterization of philosophical reasoning, this prompts Kant to optimism, but it may puzzle his readers. Kant is optimistic, because what philosophy has to investigate is not the infinite scope of the empirical world, but rather “what reason brings forth entirely out of itself… as soon as [its] common principle has been discovered” (Axx). (One application of this idea is found in the Transcendental Dialectic of the first Critique, where Kant insists that there are only three transcendental ideas—the thinking subject, the world as a whole, and a being of all beings—so that it is possible to catalogue exhaustively the illusions to which reason is subject.) But there is also much room for puzzlement. Kant is suggesting that reason conduct an experiment upon itself—an idea that comes close to paradox. He also suggests that reason has a “common principle”—but nowhere in the first Critique does he explain what this consists in.