In theOptics, Descartes claims that telescopes and microscopes lead to morally certain knowledge. It is unclear, however, that Descartes’s expressed confidence in these instruments is warranted. In this article, I show how a limited range of telescope and microscope observations could lead to morally certain knowledge for Descartes, and how observations beyond this range admit of enough reasonable doubt to undermine moral certainty. I also explain moral certainty as a form of knowledge in Descartes’s scientific practices, his epistemic commitment to optical instruments, and I offer an explanation for why Descartes never used optical instruments in his scientific endeavours.

Medical knowledge in eighteenth century England was characterized by the growth of numerous often contradictory theoretical systems, founded upon a common underlying analogy of bodily processes. This foundation of metaphor was derived from the ancient humoural model of classical medicine, attenuated by the introduction of concepts imported from contemporary mechanical philosophy. Medical theory may be viewed as a form of social interaction between physicians—the elite of the eighteenth century profession—and their aristocratic patients. The phenomenological conceptualization of disease, the speculative and systematic character of pathology, the psychosomatic interpretation of illness and the swingeing use of heroic therapies may be ascribed to the pre-dominance of the upper class patient in the consultative relationship. Furthermore the contemporary career system constrained physicians both to establish their credentials as members of the upper class and to advertise their services by individual display. Fashion provided a form of social behaviour which reconciled these demands, but also generated a profusion of medical theories. In addition the structure of relationships between patients and practitioners, and among medical men themselves, prohibited the emergence of a scientific community dedicated to the analysis of medical problems.

The year 2005 marks the 100th anniversary of Robert Koch's Nobel Prize. Here, we describe the scientific career of Robert Koch, the discoverer of the etiologic agent of tuberculosis but also of those of anthrax, cholera and wound infections. Equally important, Koch developed the methodologies and concepts that made medical microbiology a scientific discipline. Despite great efforts, however, Koch failed to conquer tuberculosis, which still causes enormous health problems worldwide 100 years after his Nobel award.