本文探索佛教系統理論的後設倫理學 (metaethics)，以佛法之倫理判斷與道德、原則本身為研究對象，探究道德語詞或道德語句是否能予以定義，並追問其道德判斷的客觀真理。 依休謨法則 (Hume’s law)，實然命題不必然能推出應然命題，事實判斷不等同於價值判斷。一般於此均須預設自明真理 (self-evident truth) 或第一因 (first principle)，但佛法卻依於「實然」之現象與法則，藉諸經驗檢證與理性分析，而直接推出「應然」之原理與原則。某種程度而言，佛法傾向於「自然主義」(naturalism)，直接由自然性質或具經驗意義的語詞，來定義倫理語詞。亦即：依現象以歸納出「緣起」與「我愛」的實然法則，依「緣起」與「我愛」復可證成「護生」之應然律令。「應」與「不應」的道德論述，恰恰是由生命之欲求 (而非上帝之欲求) 出發，以定義並簡別「善、惡、對、錯」之倫理語詞與價值判斷。 准此「護生」原則，以滿足生命「快樂」或「效益」之欲求，作為倫理實踐之目標的佛法，就帶有強烈的目的論 (teleological theory) 氣息。人必須依於「自通之法」的心理能力，以自他互替原則來平等照顧其他生命「快樂」或「效益」的需求，就此而言，佛法也同樣有著濃厚的義務論 (deontological theory) 傾向。 依此系統理論之脈絡，本文提出「實然如何證成應然」的四個問題，並一一作根源性的哲學分析以解明之。

This article explores the metaethics of Buddhist systematic theorization. First, can moral words and moral sentences be defined through Buddhist ethical judgment and moral principles? Second, what is the objective reality of Buddhist moral judgment? By Hume’s law, an "is" proposition does not necessarily imply an "ought" proposition; i.e., fact judgment differs from value judgment. Generally speaking, we need to suppose a self-evident truth or a first principle. Yet, in Buddhism, we directly derive the "ought" principle from the "is" phenomenon by experience verification and reason analysis. In some sense, Buddhism is inclined to naturalism, which directly defines an ethical word by individual nature or an experience-meaningful word. Thus, Buddhism induces the "is" principle of pratitya-sarnutpada and self-love by phenomenon. From pratitya-samutpada and self-love, Buddhism proves the "ought" principle of life-protection. The moral argument of "ought" or "ought not" originates from the desire of life, not the desire of God. This defines and distinguishes the ethical words and value judgments of good, evil, right and wrong. The ethical and practical goal of the Buddhist life-protection principle is to satisfy the desire of sentient beings’ happiness or utility. Therefore, Buddhism is strongly inclined to teleology. On the other hand, empathy must be given equally towards all sentient beings, and their need for happiness and uti1ity. In this sense, Buddhism is also strongly inclined towards deontology. From the thread of the above systematic theorization, this article asks four questions about how the "is" proposition proves the "ought" proposition. Throughout the discussion, each question is philosophically analyzed in a radical manner.