In natural law theory, legal scholars gained 'die Freiheit, de lege ferenda zu argumentieren und das positive Recht mit stärkeren Argumenten zu schlagen, als es antike Überlieferung und Gerichtsgebrauch sind' (H. Thieme, Das Naturrecht und die europäische Privatrechtsgeschichte, Basel: Helbing&Lichtenhahn 1954, p. 22). Secondary literature, however, has rarely paid attention to the influence of this newly found freedom on civil law doctrine. Discussing the genealogy of the liability of professional sellers for defects in the wares they sell in early-modern legal doctrine, the presenter aims to illustrate how natural law thinkers broke loose from the Roman law as ratio scripta and began using it as a means to illustrate new legal ideas, rather than seeing it as an end in itself. First, the professional seller's liability for defects in his wares will be discussed, such as it appears in various legal systems today. Secondly, the presenter will go back to the roots of this particular liability. Classical Roman, Justinianic and medieval ius commune law had always been reluctant in granting duped buyers a remedy against sellers in good faith and, if granted, it was rarely for more loss than the price paid. Thirdly, the presentation focusses on the changes brought about by natural law thinkers in this respect. A pivotal role in their efforts to make the ius commune receptive to a particular liability for professional sellers irrespective of their knowledge was played by a Digest text which states that the seller of a barrel has to compensate its buyer if the barrel turns out to be unsound, regardless of whether he was aware of the defect (D. 126.96.36.199). Though medieval scholars explained this text as a somewhat peculiar exception to the rule that unknowing sellers were not liable for more than the price paid, natural law scholars such as Grotius and Domat elaborated its latent assumptions into a general liability for every professional sellers for all loss. Lastly, the presenter will use these finds to speculate on the effects the often thought unwordly natural law theorizing nevertheless had on such a down-to-earth topic as the liability for defective things sold.