遊走於理論與實情之間:一項經濟模型操作的案例研究

Maneuvering between Theory and Fact A Case Study of Economic Modeling

經濟理論針對目標現象所作的一些斷言經常與發生在真實經濟世界中的目標現象不一致，這個情形時常困擾著經濟理論學者，並且也常常引發經濟理論學者與經濟方法論學者之間的爭辯。傳統解決這項爭論的方式是藉由將焦點放在討論經濟理論的不實在假設之關注上，以解決實在與理論之間的爭議。與以往方法論學者所認定的觀念相左，本論文以南施‧卡特萊特的起因說明模型為基礎，希望藉由回顧國際貿易理論之中的一項議題之一系列的理論構作，企圖來論述：就實證傳統的觀點而言，經濟理論學者在理論構作之中援引不實在的假設之作法，並非一項方法論上的瑕疵；而毋寧應將這項經濟理論建構之作法，視為是與物理學家在建構物理理論時，所使用的「對理想化條件進行部署與操控」一樣的作為。根據這項看法，經濟理論學者在理論模型之中引入不實在假設的主要目的是要作為一項控制裝置，以便將理論模型安全地隔絕於其他較不相關或較次要的干擾因素的影響之外，進而確保該已被保護了的理論模型能誘導出主要的目標現象。被保護的理論模型所推導出來的結論－－亦即，經濟理論學者的主要目標──即所謂的抽象的起因定律（或抽象的儲能宣稱）。若從這項觀點視之，「不準確性」（或「理論與實情之間常存在著不一致」）作為經濟理論的一項特徵也就不足為奇了。此外，經濟理論學者也執行另一種型式的模型操作──理論具體化。當理論遭遇了異常現象，被部署在理論之中的不真實假設就會漸次地被去理想化－－亦即，被修正或移除－－以便能推導出有關這項異例的解釋。就是在這兩項實作的意義下－－亦即，理論抽象化與理論具體化的意義下，經濟理論學者強烈地為我們展示著，他們的這些實作代表著欲對目標現象提供較完整的起因說明之一項企圖。

The discrepancy between what is asserted in economic theory and what really occurs in the economic world embarrasses economic theorists and prompts debates on economic theorizing between economic theorists and methodologists. Traditionally, economic theorists deal with the issue of realism versus theory by focusing on the concern of unrealistic assumptions in economic theories. This paper, based on Nancy Cartwright’s causal explanatory model, uses a case study from international trade theory to argue that, contrary to traditional wisdom, economic theorists’ application of unrealistic assumptions is not a vice with respect to the empirical tradition; rather, it figures in economic theory-building in the same way that physicists’ deployment and manipulation of idealized conditions does in the theory-building of physics. According to this view, unrealistic assumptions are introduced into theoretical models to act as controlling devices and thereby to safeguard such shielded models from disturbing influences produced by other, less relevant or less important causal factors and to ensure that these models elicit the main targeted phenomenon. The conclusions derived from these shielded theoretical models-i.e., the main targets of economic theorists-are the so-called abstract causal laws (or abstract capacity claims). From this perspective, inaccuracy as a characteristic of economic theories is not surprising. In addition, economic theorists practice theoryconcretization, another type of model-manipulation. When theories are faced with anomalous phenomena, the deployed assumptions are, one by one, de-idealized-i.e., revised or removed-in order to derive an explanation of the anomaly. In these two ways-i.e., theory-abstraction and theory-concretization, economic theorists make a strong case that these practices represent an attempt to provide more complete causal accounts of the economic phenomenon in question.