Heo Mok was also well traveled and a fairly prolific writer. Among his works were the Cheokjuji (陟州誌) and Gi-eon (記言). The Gi-eon was a 67-volume set of writings on his studies of famous scholars and philosophers. One of the scholars written about in the Gi-eon was Kim Si-seup (金時習), who was a famous Korean scholar and government official who lived from 1435 to 1493.
According to the Gi-eon, Kim Si-seup could read by the time he was eight months old and could read and understand the Taehak (大學) and the Junyong (中庸) by the time he was five, which earned him the nickname "Ose-dongja" (五歲童子 - "the 5-year-old child"). He was so famous, in fact, that he got the attention of King Sejong (世宗), who sent a messenger with royal gifts and the message, "Raise him well because he will be of great service someday" (善養之 當大用也). By the time of King Munjong (文宗 : 1450 - 1452), Si-seup had already grown into a well-respected scholar with vast knowledge and extraordinary talent, but with the abdication of King Danjong (端宗) in 1455, Si-seup was forced to flee and begin a life as a wanderer. He took advantage of the situation, however, to visit many temples and famous places around the country. Among the famous places he visited was Weolsong Pavilion (越松亭), on the east coast of Korea, from which he viewed Ulleung/Usan. Here is the relevant passage:
登臨楓嶽五臺 窮海堧The above sentence is quite important in the Dokdo-Takeshima debate because, as you can see from the above quote, both Ulleungdo (鬱陵島) and Usando (于山島) were described as being visible from Weolsong (越松), which is on the east coast of Korea near Uljin. This quote is important because it is more evidence that the Usando in old Korean documents was not referring to Liancourt Rocks (Dokdo/Takeshima), which is much too far away to be viewed from the east coast of Korea. It is also evidence that the phrase "Ulleung/Usan" (鬱陵于山) was used to refer to the two islands as an "island group." In other words, the passage did not mean that both islands of Ulleungdo and Usando were seen from Weolsong, but that the Ulleung-Usan island group was seen.
He climbed (登臨) Mount Pung-ak (楓嶽) and Mount Odae (五臺), and the vast sea (窮海) was empty (堧).
He traveled (遊) to Weolsong (越松) and viewed (望) Ulleung/Usan (鬱陵于山).
The Ulleung-Usan island group is also what was being referred to in the following passage from King Sejong's 1454 geography text, which was describing the county of Uljin (蔚珍) and its surrounding area:
于山武陵二島在縣正東海中 二島相去不遠 風日淸明 則可望見Korean historians try to claim that the distance mentioned in the above passage was referring to the distance between the two islands, not to the distance between Uljin and the two-island group, but since the passage was among others giving distances and bearings from Uljin to places in the surrounding area, we know that the distance referred to was the distance from Uljin to the Ulleungdo-Usando island group. In the case of Ulleungdo and Usando, the bearing was given as "in the sea due east of Uljin," and the distance was given as "the distance to the two islands is close enough that they are visible on clear days when the wind in blowing" (二島相去不遠 風日淸明 則可望見).
The two islands of Usan and Muleung (于山武陵二島) are in the sea due east of this village (在縣正東海中). The distance to the two islands is close enough (二島相去不遠) that they are visible on clear days when the wind is blowing (風日淸明 則可望見).
Korean maps even showed the two islands as an island group. For example, the following Korean map of Gangwon Province from the mid-1700s showed Ulleung/Usan (鬱陵于山) as one island off the east coast of Uljin (蔚珍), but it was labeled as 鬱陵于山兩島, which means "the two islands of Ulleung and Usan."
As you can see from the above, the "Ulleung/Usan" reference in Heo Mok's story about Kim Si-seup was consistent with other Korean documents and maps, which means that the Usando (于山島) mentioned in old Korean documents and maps was not a reference to Liancourt Rocks (Dokdo/Takeshima), but rather was just a reference to a neighboring island of Ulleungdo. The neighboring island was almost certainly Ulleungdo's neighboring island of Jukdo (竹島), which is about 2.2 kilometers off Ulleungdo's east shore. By the way, in the above map, you can see Port Weolsong (越松浦) just to the lower right of Pyeonghae (平海), which is the lower green circle. Weolsong (越松) was mentioned in Heo Mok's story as the place from which Kim Si-seup saw Ulleung/Usan (鬱陵于山) .
Below is a copy Heo Mok's story of Kim Si-seup, under the title of "Cheongsa Yeoljeon" (淸士列傳), from the 11th volume of the Gi-eon (記言). The passage that mentions Ulleungdo and Usando is outlined in red. (Click on the images to enlarge.)