I n 1980, when I returned from a 2-year Fulbright fellowship in Poznań, Poland, Dan invited me to spend some time at Berkeley in the summer. At that time, Dan was editing the rst two volumes of the Crosslinguistic Study of Language Acquisition. After we became acquainted that summer at Berkeley, our paths crossed from time to time over a period of 25 years at IASCL conferences and at smaller meetings designed to brainstorm a functional theory of language acquisition. I was already immersed in crosslinguistic research at the time that we rst met, and some of my work on Polish was aimed at an evaluation of operating principles, e.g., Weist (1983) on operating principle A. In this chapter, I discuss the way in which Slobin’s (1973) Cognitive Prerequisites paper stimulated my research comparing Polish with Finnish in the acquisition of the spatial and temporal systems.