In the spring of 1697, wonder-workers cured many citizens of Rotterdam by what was called "piss-work". Instead of attending to the patient, healers used a secret powder to treat the patient's fresh morning urine, and through a sympathetic interaction, the patient would be cured. The charlatans did not only draw censure from established physicians but some physicians supported the new cure bringing the charlatans into the fold. I will use this case study to discuss the epistemology of the charlatan. Charlatans have caused a historiographical headache for the history of medicine. The very category of "the charlatan" has been questioned again and again because of the intrinsic dangers of projection, anachronism and inappropriate judgment that seem to be embedded in the word itself. Instead of repeating the rhetoric of imposture and credulity, historians of medicine have recently tried to discover the “real” historical charlatan behind the polemics. In order to understand the charlatan we cannot ignore the divisive rhetoric, however. Key notions such as imposture, credulity, imagination and deception have to be historicized.