16th-century Swiss physician Paracelsus was frustrated with established medical practice and academia and he was sometimes on the lam because of his beliefs. He wrote at length about the idea that items in the natural world carried “signatures” in their appearance that could tell you visually how they could be used medicinally.

Research:

• Bennett, B.C. Doctrine of Signatures: An explanation of medicinal plant discovery or Dissemination of knowledge?. Econ Bot 61, 246–255 (2007). https://doi.org/10.1663/0013-0001(2007)61[246:DOSAEO]2.0.CO;2
• Dafni, Amots, and E. Lev. “The Doctrine of Signatures in Present-Day Israel.” Economic Botany, vol. 56, no. 4, 2002, pp. 328–34. JSTOR, http://www.jstor.org/stable/4256605
• “The Doctrine Of Signatures.” The British Medical Journal, vol. 1, no. 627, 1873, pp. 19–19. JSTOR, http://www.jstor.org/stable/25233757
• “The Doctrine of Signatures.” John Moore Museum. May 11, 2021. https://www.johnmooremuseum.org/the-doctrine-of-signatures/
• The Editors of Encyclopaedia Britannica. "laudanum". Encyclopedia Britannica, 19 Jul. 2025, https://www.britannica.com/science/laudanum
• The Editors of Encyclopaedia Britannica. "Peasants’ War". Encyclopedia Britannica, 20 Aug. 2020, https://www.britannica.com/event/Peasants-War
• Grzybowski, Andrzej and Katarzyna Pawlikowska-Łagód. “Some lesser-known facts on the early history of syphilis in Europe.” Clinics in Dermatology. Volume 42, Issue 2. 2024. Pages 128-133. https://doi.org/10.1016/j.clindermatol.2023.12.003.
• Hargrave, John G. "Paracelsus". Encyclopedia Britannica, 3 Jul. 2025, https://www.britannica.com/biography/Paracelsus
• “The history of syphilis part two: Treatments, cures and legislation.” Science Museum UK.  Nov. 8, 2023. https://www.sciencemuseum.org.uk/objects-and-stories/history-syphilis-part-two-treatments-cures-and-legislation
• Kikuchihara, Y., Hirai, H. (2015). Signatura Rerum Theory. In: Sgarbi, M. (eds) Encyclopedia of Renaissance Philosophy. Springer, Cham. https://doi.org/10.1007/978-3-319-02848-4_405-1
• Lund, F B. “PARACELSUS.” Annals of surgery vol. 94,4 (1931): 548-61. doi:10.1097/00000658-193110000-00009
• Michaleas, Spyros N et al. “Theophrastus Bombastus Von Hohenheim (Paracelsus) (1493-1541): The eminent physician and pioneer of toxicology.” Toxicology reports vol. 8 411-414. 23 Feb. 2021, doi:10.1016/j.toxrep.2021.02.012
• Paracelsus. “Of the supreme mysteries of nature. : Of the spirits of the planets. of occult philosophy. The magical, sympathetical, and antipathetical cure of wounds and diseases. The mysteries of the twelve signs of the zodiack.” London. 1656. Accessed online: https://archive.org/details/paracelsvsofsupr00para/page/n9/mode/2up
• Simon, Matt. “Fantastically Wrong: The Strange History of Using Organ-Shaped Plants to Treat Disease.” Wired. July 16, 2014. https://www.wired.com/2014/07/fantastically-wrong-doctrine-of-signatures/
• Tampa, M. et al. “Brief history of syphilis.” Journal of medicine and life vol. 7,1 (2014): 4-10.https://pmc.ncbi.nlm.nih.gov/articles/PMC3956094/#R6
• Waite, Arthur Edward. “Lives of alchemystical philosophers based on materials collected in 1815 : and supplemented by recent researches with a philosophical demonstration of the true principles of the magnum opus, or great work of alchemical re-construction, and some account of the spiritual chemistry.” London. G. Redway. 1888. Accessed online: https://archive.org/details/livesofalchemyst1888wait