This third volume of Boston Studies in the Philosophy of Science contains papers which are based upon Colloquia from 1964 to 1966. In most cases, they have been substantially modified subsequent to presentation and discussion. Once again we publish work which goes beyond technical analysis of scientific theories and explanations in order to include philo- sophical reflections upon the history of science and also upon the still problematic interactions between metaphysics and science. The philo- sophical history of scientific ideas has increasingly been recognized as part of the philosophy of...