Summary The genealogy of observation as a philosophical term goes back to the ancient Greek astronomical and medical traditions, and the revival of the concept in the Renaissance also happened in the astronomical and medical context. This essay focuses primarily on the medical genealogy of the concept of observation. In ancient Greek culture, an elaboration of the concept of observation (tērēsis) first emerged in the Hellenistic age with the medical sect of the Empirics, to be further developed by the ancient (...) Sceptics. Basically unknown in the Middle Ages, the Empirics’ conceptualisation of tērēsis trickled back into Western medicine in the fourteenth century, but its meaning seems to have been fully recovered by European scholars only in the 1560s, concomitantly with the first Latin translation of the works of Sextus Empiricus. As a category originally associated with medical Scepticism, observatio was a new entry in early modern philosophy. Although the term gained wide currency in general scholarly usage in the seventeenth century, its assimilation into standard philosophical language was very slow. In fact, observatio does not even appear as an entry in the philosophical dictionaries until the eighteenth century—with one significant exception, the medical lexica, which featured the lemma, reporting its ancient Empiric definition, as early as 1564. (shrink)