Abstract…observations of skylfull pylotts is the onlye waye to bring it in rule; for it passeth the reach of naturall philosophy. – Michael Gabriel, 1576 (Collinson, 1867, p. 30)Abstract The tension between empirical data and formal theory pervades the entire history of geomagnetism, from the Middle Ages up to the present day. This paper explores its early-modern history (1500–1800), using a hybrid approach: it applies a methodological framework used in modern geophysics to interpret early-modern developments, exploring to what extent formal (...) conjectures shaped observation and vice versa. A range of pertinent case studies supports classification of this entire period as proto-scientific, characterised by the initial formation of theories being largely disconnected from observational constraints, and their subsequent evolution being advanced primarily by their empirical falsification, and not necessarily associated with the introduction of an alternative. The few exceptional instances of purely data-driven discovery were essentially due to an improved signal-to-noise ratio. (shrink)