如何克服公衛訴訟中因果推論的難題：法律系統面對風險社會的一個挑戰

Overcoming Difficulties in Causal Inference in Public Health Litigations: A Challenge that Legal System Faces in a Risk Society

人類雖因逐漸取得操弄自然的能力而降低人類生活的風險，卻也因為操弄自然而製造或促成人類社會生活中前所未見的不確定性與未知的新危害。當公衛與流行病學企圖在科學有限的認識能力下，放棄因果二分邏輯，務實地透過統計機率的歸納，呈現超越既有生活經驗的不確定性時，法律系統則面臨回應公衛與流行病學所標定之不確定性的挑戰，具體表現在新科技風險所引發的各類「公衛訴訟」案件中。本文指出，圍繞在公衛訴訟的因果課題至少牽涉證據資格、舉證責任與證明程度、因果關係理論等三面向。在面對複雜因果事件時，科學的有限認識能力雖提供各種可能但非確定的風險機率證據，但法律系統若為了降低此種證據本身的不確定性，在完美科學的想像下過度提高證據資格的門檻，將導致真正被害者無法獲償，也可能遲滯應有的管制作為。完美科學想像透過證據資格標準所產生的不利影響，雖在採取職權調查主義的制度下可稍微緩解，但仍不免因現行的舉證責任分配原則與證明程度標準，使原告被課予完成證明完美因果關係的不可能任務。然而調整舉證責任分配雖是克服公衛訴訟中原告舉證困境的常用途徑，但倘若沒有根本地改變建立在錯誤想像之上的因果概念觀，舉證責任的調整也僅是緣木求魚。正本之道則必須放棄將「事實上因果關係」認為僅具有單一本質的法學因果關係理論。任何因果概念觀都是特定社會、文化、制度與學科典範的價值判斷與取捨而然。法學的「事實上因果關係」作為伺服特定價值目的之工具必然具有實用主義性格，既然總是在避免第一型錯誤或避免第二型錯誤間做出價值決斷，沒有理由不能以更細緻的價值決斷而放棄全有或全無的因果二分模型，改採比例因果理論，重新依照「最有可能原因推論」中所使用的機率證據，決定複雜因果事件中的因果貢獻比例及應負的法律責任比例。

While human capacity to manipulate nature has contributed to the reduction of major risks in social life, it has also produced new uncertainty and unprecedented risks that elude current scientific knowledge. Abandoning the dichotomic approach to causal inference, epidemiology with its statistical techniques attempts to characterize and assess the new uncertainty and therefore poses critical challenges to the legal system, which has long relied upon perfect science to proffer a clear answer to dichotomic causation. This paper argues that although the challenges to the legal system are commonly analyzed in three different aspects of public health litigations, including evidence admissibility, burden of proof, and the theory of causation, only when we recognize that even the conception of 'causation in fact' inevitably involves normative decisions and value judgment can we determine properly when the use of a certain kind of causal conception to avoid 'type I error' is justified to serve certain functions of law and when it is indeed more justifiable to accept a conception of proportional causation to serve different legal functions.