休謨關於因果關係和歸納推理的懷疑論究竟是激進的還是溫和的？本文引用許多文獻證據，重構了休謨的論證，試圖證明：休謨並未斷言在原因和結果之間沒有必然聯繫，他只是說我們不可能知道這樣一種可能存在的必然性。休謨不是極端的懷疑論者，更不是不可知論者，他在宣導理性的自律、節制、謹慎和謙卑。休謨對可設想性原則（CP論題）的表述以及對可設想性論證的使用是否存在問題？本文通過文本分析和論證重構表明，休謨清晰表述了CP論題，對它做了四種不同的使用，其中兩種使用是合法且有效的：一是用它去證明因果推理不具有邏輯必然性，另一是用它去證明事實命題的真實性和虛假性不能僅憑理性或邏輯來證明。不過，休謨用CP論題和可設想性論證去反駁因果必然性和自然齊一律是不成立的。他所表述的CP論題的一個形而上學模態版本是很有爭議的。

This paper discusses two questions about Hume’s philosophy. Question 1: whether Hume’s skepticism about causation and induction is radical or moderate? By analyzing the quotations from Hume and reconstructing his arguments, this paper tries to show that Hume does not assert that there is no necessary connection between cause and effect, he just wants to say that we cannot know whether or not there is such kind of connection; similarly, he does not and even cannot prove that objective causation does exist. So, Hume is not a realist about causation, not an extreme skeptic, let alone an agnostic; actually he advocates the self-discipline, prudence and humility of human reason. Question 2: whether Hume’s formulation and application of conceivability principle (CP) is problematic or not? Similarly, by analyzing textual evidence and reconstructing Hume’s arguments, this paper tries to show that Hume does articulate CP, and applies it to four different cases, of which two are legitimate and effective: to apply it to prove that causal reasoning is not logically necessary, and to apply it to prove that it is not the case that propositions about matters of fact is true or false only based on logic and reason. However, in using CP and conceivability argument to refute the necessity of causation and the uniform law of nature, Hume fails because he does not distinguish different kinds of necessity. The metaphysical version of CP is highly controversial.