Jung's political or scholarly vision was to establish a newly reformed country run by a philosophy of common sense, rooted in grass people. He was very critical of Buddhism in those days especially the hypocritical elites who accumulated wealth and secured their future salvation in heaven. Temples and elites became richer, and people became poorer.
He rejected "reincarnation," a core doctrine of Buddhism according to which persons may continue to live a life after death. In exchange for a better place with "reincarnation," they were asked to donate lots of money to the temple. This is where corruption began in religion; elites were busy accumulating their wealth by connecting with this kind of elite religion.
Jung Do-Jeon challenged the absurdities of such an idea of reincarnation in his book Bulsee Japbyun. In this book, his observation goes like this: "We look at the beautiful flowers or leaves in the spring and see them fading and falling off to the ground. They return to the place where they were." Here he rejects the idea of rebirth, saying like this: "How is it possible that we expect to see the same fallen flowers or leaves coming back to life in the next spring?" Simply, an old life is gone and a new life is born or started! What comes out from the ground in the next spring is a new life, not a re-birth. Old leaves or flowers are rotten and become fertilizers for a new life. Simply, what he says is that reincarnation is an absurd idea. He also says: "How can we inhale the same breath that we exhale? Each time we exhale a breath inhale a new one, not the same breath." Likewise, the spring puts forth the ever-springing water, which is never the same. Original water evaporates and turns into clouds. Furthermore, in his view, "reincarnation" is a selfish desire that people want to prolong their lives after death. Death is the end, and it must be accepted as natural.
I think his critique makes very good sense for today when people are concerned too much about the next life while ignoring their responsibilities in the world now.
The following poem was written by Jung DoJeon when he first met Sunggae Lee.
蒼茫歲月一株松 / 아득한 세월에 한 그루 소나무
生長靑山幾萬重 / 푸른 산 몇 만겹 속에 자랐구나.
好在他年相見否 / 잘 있으시오. 훗날 서로 뵐 수 있으리까?
人間俯仰便陳蹤 / 인간 세상이란 잠깐 사이 묵은 자취인 것을.
— 정도전, 《제함영송수 (題咸營松樹)》
a pine tree standing alone for a long time;
It has grown so big now through millions of mountain trails;
Goodbye now to you; can we see each other in the future? The human world is passing like a fast-moving arrow.
[Trans. Yung Suk Kim]
The following poem was written by Jung DoJeon before he died (or was killed).
操存省察兩加功 / 조심하고 조심하여 공력을 다해 살면서
不負聖賢黃卷中 / 책 속에 담긴 성현의 말씀 저버리지 않았네.
三十年來勤苦業 / 삼십 년 긴 세월 고난 속에 쌓아 놓은 사업
松亭一醉竟成空 / 송현방 정자 한 잔 술에 그만 허사가 되었네.
— 정도전, 《자조》
With every effort and caution, I have lived a life very well;
I did not break the wisdom of the sages in their literature;
a 30-year achievement made through trials and errors
came to an end in vain because of a glass of wine at the Songhyunbang.
[a rough translation of mine]