This article argues that the Edinburgh physician Archibald Pitcairne made a significant and original contribution to European religious heterodoxy around 1700. Though Pitcairne has been studied by historians of medicine and scholars of literary culture, his heterodox writings have not been analysed in any detail. This is partly because of their publication in Latin, their relative rarity, and their considerable obscurity. The article provides a full examination of two works by Pitcairne: his Solutio problematis de historicis; seu, inventoribus (‘Solution of the problem concerning historians or inventors’) (1688); and the Epistola Archimedis ad Regem Gelonem (‘Letter of Archimedes to King Gelo’) (1706). As well as untangling their bibliographical and textual difficulties, the article places these tracts in the context of Pitcairne's medical, mathematical, and religious interests. A range of readers deplored the sceptical implications of the pamphlets, but others, particularly in free-thinking circles in the Netherlands, admired Pitcairne's work. And yet Pitcairne himself was no atheist. He doubted a priori proofs of God's existence, but had been convinced by a version of the argument from ‘design’. The article concludes by relating Pitcairne's complex religious attitudes to his background in late seventeenth-century Scotland.