不是苦心人不知

“If you do not exert yourself, you cannot understand.”
From “Sayings of Zen Master Dahui Pujue” (大慧普觉禅师语录)
http://zenmonk.cn/czdj/dj05.htm

不 not
是 this/is
苦心 literally: “suffer mind”, to make an effort, to “take pains” (Sino-Korean: 고심 goshim)
人
知 understand

“Collected Works of Korean Buddhism, Volume 7-1, Gongan Collections 1”, pp. 573-574