Parhaego (Korean: 발해고; Hanja: 渤海考) or the Reflections on Balhae is a history book about the Balhae civilization, written by Yu Deuk-gong, who was a Silhak (practical studies) scholar during the Joseon Dynasty.
Parhaego is notable for referring to Balhae and Silla as the northern and southern kingdoms, within a unified account of Korean history.
He made it clear that Balhae was the heir of Goguryeo, and brought it into the category of Korean folk history, and argued that the period of war with Silla should be regarded as the era of the Republic of South Korea.
[1] Park Pak Chega, a practical scholar of Bukhakpa during the reign of King Jeongjo of the Joseon Dynasty, wrote about his friend, Yu Dŭkgong, saying, "This book is in line with his previous reviews.
What he is trying to say is that Wang, the founder of Goryeo Dynasty, lamented that he could not restore Goguryeo territory.