This paper is a first report from the project "Activity Learning and Cooperation," financed by the Swedish Board of Education. The aim of the project is to establish a theoretical basis for a field study of locally initiated experiments using various teaching strategies. More specifically, this paper is restricted to a discussion of the cognitive framework underlying the theoretical basis. A review of the psychological foundations of activity learning shows that theorists often postulate the efficiency of activity learning, or that the relation between recommendations for teaching strategies and psychological theory is often handled in a superficial way. An attempt is made to formulate characteristics of "Activity" with reference to Piaget's description of cognitive development. Activity is described as a mode of thinking about phenomena. This mode of thinking can be characterized by a willingness to try different ways of interpreting a phenomenon, a searching for intentions behind an act or a text, and a testing of knowledge already accessible in new situations. This is called the assimilative aspect of the acquisition of knowledge. "Activity" is also characterized by a willingness to change one's mind, and the efforts made to understand, which is called the accommodative aspect of the acquisition of knowledge. A note is also added about the field study, and about certain expected results. (Author/MP)