This article unearths little-studied accounts of the status and role of hypotheses in late eighteenth- and early nineteenth-century Germany. German thinkers regarded hypotheses, including those about unobservable causes for visible effects, as legitimate and necessary ingredients of scientific inquiry. They debated the nature of probable hypotheses resulting from inductions, proposed heuristics for making causal hypotheses, and advanced criteria for assessing and testing them. My survey of these rich and multifaceted discussions shows that many themes and topics that we commonly associate (...) with modern philosophy of science were discussed decades earlier by authors of educational and practice-oriented books on logic: consequential testing, underdetermination, auxiliary hypotheses, the problem of unobservable entities, fallibility, and elaborate methodologies of observation and experimentation. It also illuminates the long-term history of present-day criteria for hypothesis evaluation. (shrink)