If we want to compare the explanatory and descriptive adequacy of the MP and OT, the original definitions by Chomsky (1964) are or little direct use. However, a relativized version of both notions can be defined, which can be used to express a number of parallels between the study of individual I-languages and the language faculty. In any version of explanatory and descriptive adequacy, the two notions derive from the research programme and can only be achieved together. They can therefore not be used to characterize the difference in orientation between OT and the MP. Even if ‘OT’ is restricted to a particular theory in Chomskyan linguistics (to the exclusion of, for instance, its use in LFG), it cannot be said to be stronger in descriptive adequacy than in explanatory adequacy in the technical sense of these terms.

Chomsky’s Cartesian Linguistics frames into his linguistic work and the resulting debates between rationalists and empiricists. I focus on the following key aspects: (i) the historic connection of Cartesian Linguistics to previous linguistic theorizing, (ii) the development of Chomsky’s own theorizing, (iii) the empirical work addressing the problem of language acquisition and (iv) the problem of computational modeling of language learning. Chomsky claims that his view is situated within a rationalist Cartesian tradition and that only rationalists will be able to account fully for all aspects of human language. My thesis challenges both claims. I found only remote connections between Cartesian and Chomskyan commitments. Chomsky holds that (i) language is species-specific, (ii) language is domain-specific, and (iii) language acquisition depends on innate knowledge. Descartes accepted (i), but argued that language is an indicator of domain-general intelligence. Innate resources play a different role for language acquisition for Chomsky and for Descartes. Chomsky revived linguistics during the 1950s by promising to make it a rigorous part of the biological sciences. However, his work has not resulted in a better understanding of language acquisition and use. Key concepts like ‘innateness’, ‘Universal Grammar’ and ‘Language Acquisition Device’ remain in need of precise definition, and the Poverty of the Stimulus Argument does not rule out data-driven domain-general language acquisition. Empirical work in developmental psychology has demonstrated that children acquire and practice many language-related cognitive abilities long before they produce their first words. Chomsky’s dictum that language learning is uniform across the species and invariably follows genetically determined stages remains empirically unconfirmed. Computational modeling has accounted for some internal structure of language acquisition mechanisms and simulates the specific conditions under which children learn language. Contemporary models use samples of child-directed-speech as input and have replicated numerous aspects of human performance. Given my findings I suggest that Chomskyan linguistics is not Cartesian in substance or in spirit. Descartes was wary of those “who take no account of experience and think that truth will spring from their brains like Minerva from the head of Jupiter” (CSM I, p. 21). His science relied on sense experience (empiricism) and deduction (rationalism) and a truly Cartesian Linguistics will revive this part of the Cartesian tradition.