Berkeley subscribed to the principle of heterogeneity, that what we see is qualitatively and numerically different from what we touch. He says of this principle that it is “the main part and pillar of [his] theory.” The argument I present here is that the theory to which Berkeley refers is not just his theory of vision, but what that theory was the preparation for, which is nothing less than his idealism. The argument turns on the passivity of perception, which is what is at stake in the principle of heterogeneity. The author targeted by Berkeley's theory is Descartes, who explicitly denies heterogeneity.