A functional (as opposed to structural) theory of intellectual development is presented and used to generate specific performance models for Piagetian tasks involving the control of variables (cf. Inhelder and Piaget, 1958). On the basis of these models, it is concluded that intelligent, field independent 7- and 8-year-olds should be able to acquire the control of variables scheme, even though they have not yet acquired either conservation of weight or the combinatorial system. Preliminary data (n=52) are presented to support this conclusion. They are discussed with regard to Piaget's formal theory of intellectual development and the functional limitations of development of learning. (SDH)