Kōsaku Yoshio was a surgeon and interpreter of Dutch of the middle Edo period.
Background
Kōsaku Yoshio was born in 1724. His personal name was Nagaaki, he also for a time went by the name Kosakuemon, and in his old age by the professional name Kogyu. His family had for generations been interpreters of Dutch in Nagasaki, and from the time of his grandfather Juzan they took an interest in medicine as well.
Career
In 1737 he became a keiko tsushi, apprentice interpreter, and advanced in rank until in 1748 he was promoted to the position of daitsushi, or senior interpreter. Thereafter he served nine times as nemban daitsushi, or senior interpreter for the year, and until 1790 held the post of tsushi metsuke (superintendent of interpreters).
In his later years he was accused of making an error in the translation of a letter sent from the head of the Dutch trading office in Nagasaki to the shogunate, and was condemned to punishment, but was pardoned shortly after.
He died in the eighth month of 1800.
Religion
He shaved his head and entered the Buddhist priesthood, taking the name Kogyu, and was ordered to act as Bangaku shinan, instructor in Western learning.
Personality
While fulfilling his duties as an interpreter, he applied himself to the study of astronomy, geography, botany, and particularly medicine. He read a work on medicine by the German doctor Joseph Jacob Plenck in Dutch translation, which greatly advanced his knowledge and allowed him to become something of an expert in surgery. He also studied under the Swedish doctor Carl Peter Thunberg, who visited Japan during the Temmei era (1781-89), and assisted Thunberg in his studies of Japan. Among his students in what came to be known as the Yoshio school of surgery were his younger brother Yujiro, Maeno Ryotaku, and Sugita Gempaku, as well as a large number of others, his students being said to have numbered over six hundred. When Maeno, Sugita, and others produced and published the Kaitai shinsho, a translation of a Dutch work on dissection, he was invited to write the introduction.