Yoshio Kōsaku

Last page of 'Yoshio geka-kikigaki' , a medical manuscript copy with Western drugs and prescription. The text ends with an explanation: 'Translated into Japanese by Senior Dutch translator, teacher Yoshio Kōgyū. Copy made in Nagasaki, to be kept secret.'

(1724 – October 4, 1800), also known as was a Japanese physician and scholar of "Dutch studies" (Rangaku), and the chief Dutch translator in Nagasaki, often accompanying Dutch East India Company officials on missions to Edo and other official business.

As a member of one of the five samurai families supported by the Tokugawa shogunate as hereditary official Dutch translators, Kōsaku vetted imported documents (Christian materials were prohibited in Edo period Japan) and helped to keep the shogunate informed of global political matters. From roughly 1770 to 1800, he served as the chief mediator between the Dutch community on Dejima and the shogunate.

Kōsaku was quite prolific in his writings, and has been described as "perhaps ... the most knowledgeable person [in Japan] about the West in his day.

Japanese scholars, officials, and artists visiting Nagasaki frequently made a point of visiting Kōsaku; his lectures and Dutch-style home attracted great interest. Ranga painter Shiba Kōkan visited Kōsaku in 1788, stayed at Kōsaku's home for a few nights, and was given a tour of Dejima. A portrait he painted on that occasion depicting Kōsaku, partially in the style of Western oil paintings and partially in Japanese ukiyo-e style, survives today. The diaries of physician Tachibana Nankei, who also stayed at Kōsaku's Dutch-style home for a time, include descriptions of that home. The house featured a Dutch-style lie-down bathtub, a Western-style green-painted staircase rather than the unpainted, ladder-like stairs typical of Japanese homes, window drapes, and chairs which Nankei described as "grueling" to sit on, being more used to sitting on a tatami floor. and mirror paintings.