Humboldt’s theory of language reflects Kant’s influence to a far greater extent than is generally realized. In fact, Kantian cognitive and perceptual universals play a crucial role in the later formulations of Humboldt’s linguistic thinking. Indeed, not only did Humboldt derive the universals of grammatical case from the Kantian categories of relation in his mature work, but he also applied Kant’s schema concept in a systematic way to the fundamental problems of language during his last period of scholarly activity. Moreover, it can be shown that the Kantian aspects of his theory do not conflict with his widely quoted and misinterpreted formulations concerning the nature of linguistic diversity. It is suggested that much of the confusion in regard to just what Humboldt actually meant would have been dispelled long ago had researchers made adequate use of two neglected, but extremely important, major manuscripts from his mature period. More importantly still, one can plausibly argue that Humboldt was correct in assuming that Kant, not Descartes, provided linguistics with the key for dealing adequately with the central problems of language and mind.