本文依智者所講述的，灌頂所筆錄的《摩訶止觀．正修止觀》來討論「不可思議境」。智者在該論中直接指出「心」就是「不可思議境」，且道出此「不可思議境界」在信解與觀行上是不能離開「思議境」。此指示可說是讀誦乃至理解《摩訶止觀》中之「不可思議境」不可忽視的一項指導原則。這項指導原則正是本論文敘述的次第與論述的主軸，智者為了闡示「不可思議境」的甚深義，釐清「思議境」即成了本文第二部分所論及的次要重點，而在第三部分回歸到本文所論述的主題「不可思議境」之界定與觀行，在界定上，智者提出「一心」是「法性」，是「非識所識，非語所語的心行處滅、言語道斷的境界」，是「不可思議境」，以此為修學基礎上，所體悟的世俗諦之萬 法世象，亦是「不可思議境」；在觀行上，其中敘述重點顯示智者以「中道不二」的指導原則來切入「觀不可思議境」，且站在以「思議境」為世俗諦，及「不可思議境」為言語道斷，心行處滅之第一義諦為見地，這是佛教二諦的理論基礎下來觀行，相應於龍樹菩薩的二諦立場，無有離開般若實相之法理，亦無有離開大乘中道實相的宗義。因此本文談及「不可思議境」的觀行成就上，不落入於假觀，亦不落入於空觀，是離兩邊的中道實相觀行，主要遍及三諦、三智、三教、三語的成就。這些成就的真正重點，溯源歸於「不可思議境」這個關鍵主題，如果修學者在《摩訶止觀》中不明白「不可思議境」的甚深法義，將無法展開此論典所提出的宗義與觀行成就。因是之故，本文為何要提出「不可思議境」這個關鍵詞來討論其定義與觀行，如是顯出此「不可思議境」之重大意義。

This paper intends to discuss “unthinkable circumstances”, based upon the statement from Zhi Zhe, taken down in writing by Guan Ding in “Mo He Chih Kuan; Cheng Hsiu Chih Kuan”, notes taken from the religious authorization process. Zhi Zhe directly point out “heart” is exactly “unthinkable circumstances” and explain that the interpretation and observation of “unthinkable circumstances” cannot be separated from “thinkable circumstance”, which is also a major guideline of reading and comprehending “unthinkable circumstances” in “Mo He Chih Kuan”. This principle is the main concept stated and discussed by this thesis. In order to interpret “unthinkable circumstances”, the clarification of “thinkable circumstances” has become the second focus discussed in the second chapter of this paper, while returning to the definition and observation of “unthinkable circumstances”, major theme of this paper, in the third chapter. In terms of definition, Zhi Zhe propose “wholehearted” which is “temperament”. It is a “realm having destruction where heart goes and breakage where words describe without recognizing what is seen and talking what is spoken”, an “unthinkable circumstance”. Based upon this principle to study scriptures, the world image and various laws of true meaning of common customs comprehended are also “unthinkable circumstances”. In terms of observation, focus of the discussion indicates the guideline of “one and only middle course” is used to interpret “observe the unthinkable circumstances” by Zhi Zhe. “Thinkable circumstances” is true meaning of common custom, “unthinkable circumstances” is the primary meaning for breakage where words describe and destruction where heart goes, which are the observation based upon theoretical basis of Buddhism two true meanings, corresponding with the standpoint of two true meanings from Lung Shu Bodhisattva without deviating from principle of highest wisdom reality and meaning of the Mahayana doctrine middle course reality. Thus, the accomplishment of “unthinkable circumstances” observation mentioned in this paper does not fall into either false observation or empty observation. It is an observation between either one and positioned in the middle course reality observation that among the achievement of three true meanings, three wisdoms, three teachings, and three sentences. The real emphasis of these achievements is from the key subject “unthinkable circumstance”. If the people who study “Mo He Chih Kuan” do not comprehend the profound logic of “unthinkable circumstance”, the definition and observation proposed from this perspective cannot be presented. Hence, the purpose of this paper intends to discuss the definition and observation of the key word “unthinkable circumstance” to show the significant meaning of “unthinkable circumstance”.