 The same principle of reasoned unity also applies to judgments that are not readily decided by everyday experience. Why are we sure that the sun does not orbit the earth, despite all appearances? To answer such questions, we need to consider reason’s role in scientific knowledge. Kant claims that reason is “the origin of certain concepts and principles” (A299/B355) independent from those of sensibility and understanding.  Kant refers to these as “transcendental ideas” (A311/B368) or “ideas of [pure] reason” (A669/B697). And he now defines reason as a “faculty of principles” (A299/B356) or the “faculty of the unity of the rules of understanding under principles” (A303/B358). The problem is how to justify these concepts and principles. This problem is acute because Kant also argues that they often lead us into error and contradiction.