After giving up his practice around 1784, Hahnemann made his living chiefly as a writer and translator, while resolving also to investigate the causes of medicine's alleged errors. While translating William Cullen's "A Treatise on the Materia Medica", Hahnemann encountered the claim that cinchona, the bark of a Peruvian tree, was effective in treating malaria because of its astringency. Hahnemann believed that other astringent substances are not effective against malaria and began to research cinchona's effect on the human body by self-application. Noting that the drug induced malaria-like symptoms in himself, he concluded that it would do so in any healthy individual. This led him to postulate a healing principle: "that which can produce a set of symptoms in a healthy individual, can treat a sick individual who is manifesting a similar set of symptoms." This principle, "like cures like", became the basis for an approach to medicine which he gave the name homeopathy. He first used the term homeopathy in his essay "Indications of the Homeopathic Employment of Medicines in Ordinary Practice", published in Hufeland's "Journal" in 1807.