Jelke van der Pal presents in this PhD thesis a situated cognition theory in which situated learning is viewed from a Vygotskian perspective: the developmental situativity theory. It is assumed that learning is basically a non-symbolic inductive process that is greatly enhanced by formal systems which act as tools. Instruction should provide students with an environment containing elements to allow for situated activity as well as with appropriate formal tools. Tarski's World, which is an instructional Interactive Graphical Representation System in formal logic is an example of suchlike instruction. The developmental situativity theory was supported empirically: Tarski’s World was found to facilitate conditional reasoning, in contrast to instruction based on only-situated or only-formal tasks. The time frame in the developmental process in which the instructional measure is effective appeared relatively short.