«Koyo gunkan» («Military Mirror of Kai», 甲陽軍鑑) is an important and valuable historical source on Sengoku Japan and particularly on the history of Takeda domain. As follows from «Koyo gunkan», the action of the laws was mediated by the ties of the provincial lord and his vassals. The warriors believed that faithful service should be taken into account at the daimyo court, and reliance of the daimyo only on the law at the court is unacceptable. That is what justice is in their view. Pictured in «Koyo gunkan» Shingen fully shares these views.
According to «Koyo gunkan», excessive lawmaking is not only superfluous, it, according to the author of the «Military mirror of Kai», is simply harmful. Good and reliable laws cannot fail to rely on the personal virtues of the daimyo.