Early childhood learning of language has led some to postulate innate knowledge of an abstract symbolic linguistic system. However, if the child's abstract understanding initially requires concrete support in the form of agreement of the message with his nonlinguistic experience, the indication would be that the development of syntactic comprehension does not derive from genetic prewiring. Rather it indicates that syntactic comprehension develops through concrete experiences from which abstractions are only gradually derived by the child. In order to test abstract knowledge, which requires the removal of concrete sources of support for comprehension, probable and improbable active and passive sentences were presented to 120 three- and four-year-olds. The results showed that age, syntactic voice, semantic probability, and the interaction of voice and probability had significant effects. Passive voice still required support from semantic features. Syntactic concepts apparently mature from concrete understanding to abstract knowledge, in the same manner as other aspects of cognitive development. (Author/LG)