 What may come as some surprise is that Hempel exposed yet another significant problem confronting the theory of scientific explanation. One of the most remarkable features of his career is that he continued to publish original and innovative studies well into his eightieth decade. Rather surprisingly, he authored a series of articles that moved away from the influential conception of scientific theories as formal calculi combined with empirical interpretations that had been characteristic of logical empiricism. In Hempel (1966a), at the time, the most widely adopted introduction to the philosophy of science (which has been translated into ten other languages), he advanced the novel explication of scientific theories as consisting of internal principles and bridge principles, where the lawlike hypotheses that distinguish theories are linked to observation, measurement, and experiment by principles expressed in various mixtures of ordinary and of technical language. Now antecedent understanding replaces explicit definability, which seems to have been, in part, a response to the demise of the observational/theoretical distinction.