Theory change in immunology. Part I: Extended theories and scientific progress.

This two-part article examines the competition between the clonal selection theory and the instructive theory of the immune response from 1957-1967. In Part I the concept of a temporally 'extended theory' is introduced, which requires attention to the hitherto largely ignored issue of theory individuation. Factors which influence the acceptability of such an extended theory at different temporal points are also embedded in a Bayesian framework, which is shown to provide a rational account of belief change in science. In Part II these factors, as elaborated in the Bayesian framework, are applied to the case of the success of the clonal selection theory and the failure of the instructive theory.