國王的新衣裳與舊衣裳

Ramsey(1931)對條件句所作的評論，啟發了至少三種不同的、有關虛擬條件句的語意論途徑。它們分別是：由Chisholm(1946)和Goodman(1947)所提出的可推導性途徑、由Stalnaker(1968)與Lewis(1973)所倡導的世界選擇途徑以及由Alchourron、Gardenfors和Makinson(1985)所共同奠基的信念修正途徑（簡稱AGM理論）。這三種途徑雖然在內容上很不一樣，但Hansson(1995)論證說它們在形式上其實是相同的。不僅如此，Hansson還進一步論證說，這些「國王的新舊衣裳」間其實有著三個共同的限制，Hansson分別稱它們為「非演繹性限制」、「循環性限制」以及「可移轉性限制」。本文的目的在探討Hansson的這些論證，而我的結論是：上述這三種語意論途徑並沒有、或不必然有Hansson所宣稱的限制。為說明上方便起見，我將以AGM理論作為主要的例子，但我也會約略地提及其他途徑中的一些理論。

關鍵字

信念變更；信念修正；雷姆濟測驗； AGM理論；條件句邏輯；違反事實條件句；虛擬條件句；共同支撐性；可能世界語意論

並列摘要

The Ramey test has inspired many research programs on the semantics of subjunctive conditionals. According to this test, whether one should accept a subjunctive conditional ”If P then Q” in an epistemic state depends on how the result of adding P to, and revising accordingly, that epistemic state turns out to be. If we accept the Ramsey test as a reasonable principle for determining the acceptability of a subjunctive conditional, then studying how our belief states change rationally when new information is added to them will provide the key to an adequate understanding of conditional sentences. In fact, scholars have launched researches aiming at the right account of conditionals via studying principles of rational belief changes since 1970s, and the most impressive achievement among them is the so-called AGM (Alchourron, Gardenfors and Makinson) theory. S.O. Hansson, however, argues that the belief change approach to subjunctive conditionals has three important limitations: non-deductiveness, circularity, and shiftability. He further argues that other approaches to conditionals, such as the derivability approach espoused by R.M. Chisholm and N. Goodman, and the world-selection approach espoused by D. Lewis and R. Stalnaker, are also affected by the same limitations. The purpose of this paper is to scrutinize and evaluate Hansson's sweeping arguments against these salient approaches to subjunctive conditionals, and my conclusion is that none of them suffers or must suffer from any of the limitations alleged by Hansson.

並列關鍵字

belief change ； belief revision ； the Ramsey test ； AGM theory ； conditional logic ； counterfactual ； subjunctive conditionals ； cotenability ； possible-world semantics

參考文獻

Adams, E. W.(1965).A Logic of Conditionals.Inquiry.8,166-197.

Google Scholar

Alchourron, C. E.,P. Gardenfors,D. Makinson(1985).On the Logic of Theory Change: Partial Meet Contraction and Revision Functions.The Journal of Symbolic Loigc.2(50),510-530.