Language and cognition have been explained as the products of a homogeneous associative memory structure or alternatively, of a set of genetically determined computational modules in which rules manipulate symbolic representations. Intensive study of one phenomenon of English grammar and how it is processed and acquired suggests that both theories are partly right. Regular verbs (walk-walked) are computed by a suffixation rule in a neural system for grammatical processing; irregular verbs (run-ran) are retrieved from an associative memory.