At the heart of Kant's critical idealism lies the notion that human cognition is discursive, precisely because it involves the process of thought. Experience arises not simply from the reception of data, but also requires the spontaneous activity of judgment, whereby these data are "taken up" in thought. According to this "discursivity thesis,"¹ cognition requires two separate elements: a receptive component provided by sensibility, and a conceptual component provided by the understanding. Moreover, the faculties of sensibility and of the understanding are both necessary for cognition and yet mutually irreducible. The two powers, Kant writes in a famous passage, "cannot exchange their functions. The understanding can intuit nothing, the senses can think nothing" (CpR, A 51/B 75).