After leaving the UN he worked all over the world for the World Health Organization, as well as being a prolific author of Esperanto works.
Piron spoke Esperanto from childhood and used it in many countries, including Japan, the People's Republic of China, Uzbekistan, Kazakhstan, a few places in Africa and Latin America, and almost all European countries. For some of his books he used the pseudonym Johán Valano.
In a lecture on the current system of international communication Piron argued that "Esperanto relies entirely on innate reflexes" and "differs from all other languages in that you can always trust your natural tendency to generalize patterns... The same neuropsychological law...—called by Jean Piaget generalizing assimilation—applies to word formation as well as to grammar."
His diverse Esperanto writings include instructional books, books for beginners, novels, short stories, poems, articles and non-fiction books. His most famous works are Gerda malaperis! and La Bona Lingvo (The Good Language).
Gerda malaperis! is a novella which uses basic grammar and vocabulary in the first chapter and builds up to expert Esperanto by the end, including word lists so that beginners may easily follow along.
In La Bona Lingvo, Piron captures the basic linguistic and social aspects of Esperanto. He argues strongly for imaginative use of the basic Esperanto morpheme inventory and word-formation techniques, and against unnecessary importation of neologisms from European languages. He also presents the idea that, once one has learned enough vocabulary to express himself, it is easier to think clearly in Esperanto than in many other languages.
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