第三节 科学解释的历程

亨普尔（Carl Gustav Hempel，1905～1997）是逻辑经验主义后期的主要代表、柏林学派成员。他对逻辑经验主义的贡献体现在许多方面，如意义标准的重新阐述，理论术语与观察术语及其关系问题，科学理论的检验问题等[1]，但最有特色的还是亨普尔对于科学说明的机制进行了严格的分析，并提出了所谓的“演绎模型”（又称覆盖率模型）：1.被解释项（Explanandum）必须是解释项（Explanans）逻辑演绎的结果。2.解释项必须包含导出被解释项所不可缺少的普遍律。3.解释项必须至少在原则上能被实验或观察所证实。4.组成解释项的句子必须是真的。前三点为逻辑条件，代表了亨普尔解释理论的根本主张，这就是：（a）解释就是论证；（b）解释中的论证即意味着被解释项可由普遍律导出；（c）由此决定了解释与预见在逻辑上同构，区别仅在于其实用目的不同，或实际推演与被解释事件发生的时间顺序不同。[2]

逻辑经验主义认为，“科学解释即逻辑论证”。1948年，亨普尔依据逻辑经验主义的基本原则，提出了著名的科学解释的覆盖模式或D-N模式（Deductive-Nomological modal）：被解释项必须是解释项逻辑演绎的结果；解释项必须包含导出被解释项所不可缺少的普遍律；解释项必须具有经验内容；组成解释项的句子必须是真的。[3]“这类解释性说明，可成为通过演绎包容在一般规律内的解释或演绎-定律论解释。在科学解释中援引的规律，也可称之为被解释现象的覆盖律，而解释性论证可说是将被解释项包容于这种覆盖律内。”[4]自然科学解释是如此，人文科学的解释也是如此。“与经验科学的其他任何领域一样，在历史学中，对一个现象的解释在于把现象纳入普遍经验规律之下；解释的可靠性的标准不在于它是否诉诸我们的想象，并不在于它是根据有启发的类比提出来的，或是使它显得似乎真实的其他方法——这一切在假解释中也可以存在——而唯一地在于它是否依赖于有关初始条件和普遍规律的被经验完全证实的假设。”[5]

科学主义的解释学遭到了历史主义的批判。S．图尔敏在《科学哲学导论》（1953）和《预见与理解》等著述中，首先将科学区分为“描述的科学”和“解释的科学”，而且科学的目标是多向度的，科学的目标不仅在于解释，还包括科学预见和科学的推广等。N.R.汉森进一步指出，科学解释的真正本质在于发现。库恩则认为，科学解释总是受制于“流行的解释准则对他所能得到的解答的类型”[6]。

在科学哲学阵营，解释与理解之间的思想矛盾经历了从科学主义向历史主义的转折。对科学解释的总结性的著述当属Wesley C. Salmon撰写的《科学解释40年》（Four Decades of Scientific Explanation，University of Pittsburgh Press，2006）。该著作主要回顾了科学解释研究不同历史发展时期，其中包括：1948～1957年演绎律模型（The Deductive Nomological Model）的提出和完善；1958～1967年从演绎律模型到概率模型的转换（The DeductiveStatistical Model和The InductiveStatistical Model）；1968～1977年的概率相关模型（The Statistical Relevance Model）及其对因果性的质疑；1978～1989年的概率模型与因果模型冲突及其在经验论、实在论和还原主义等哲学理念的重新思考。[7]

从科学主义与历史主义在科学解释问题的争论中，我们不难看到：科学主义过分强调逻辑论证，而历史主义则过分强调对文化的依赖性，前者往往陷入基础主义的死胡同（如维也纳学派的“原子事实”），后者常常陷入独断的整体主义（如库恩的“范式”）。科学主义过分强调科学解释的经验性，历史主义过分强调科学解释的社会历史性，前者往往走向狭隘的经验主义（如维特根斯坦的“基本命题”），后者往往陷入无所适从的相对主义（如费耶阿本德的“方法论的无政府主义”）。科学主义过分强调科学解释的“语言学问题”（如卡尔纳普的“逻辑句法”），而历史主义往往过分强调科学解释的“语用学维度”（如蒯因的“本体论承诺”），前者往往陷入形式主义，后者往往陷入“语境主义”。

[1] 亨普尔重要著述如下：C.G.Hempel:Aspects of Scientific Explanation and other Essays in the Philosophy of Science ,New York,Free Press,1965；C.G.Hempel:Philosophy of Natural Science .Englewood Cliffs，N.J.,Prentice-Hall，1966.

[2] Hempel and Oppenheim’s essay “Studies in the Logic of Explanation，” published in volume 15 of the journal Philosophy of Science ，gave an account of the deductive-nomological explanation.A scientific explanation of a fact is a deduction of a statement （called the explanandum ）that describes the fact we want to explain;the premises （called the explanans ）are scientific laws and suitable initial conditions.For an explanation to be acceptable，the explanans must be true. According to the deductive-nomological model, the explanation of a fact is thus reduced to a logical relationship between statements: the explanandumis a consequence of the explanans . This is a common method in the philosophy of logical positivism. Pragmatic aspects of explanation are not taken into consideration. Another feature is that an explanation requires scientific laws; facts are explained when they are subsumed under laws. So the question arises about the nature of a scientific law. According to Hempel and Oppenheim, a fundamental theory is defined as a true statement whose quantifiers are not removable （that is, a fundamental theory is not equivalent to a statement without quantifiers）, and which do not contain individual constants. Every generalized statement which is a logical consequence of a fundamental theory is a derived theory. The underlying idea for this definition is that a scientific theory deals with general properties expressed by universal statements. References to specific space-time regions or to individual things are not allowed. For example, Newton’s laws are true for all bodies in every time and in every space. But there are laws （e.g., the original Kepler laws） that are valid under limited conditions and refer to specific objects, like the Sun and its planets. Therefore, there is a distinction between a fundamental theory, which is universal without restrictions, and a derived theory that can contain a reference to individual objects. Note that it is required that theories are true; implicitly, this means that scientific laws are not tools to make predictions, but they are genuine statements that describe the world—a realistic point of view. There is another intriguing characteristic of the Hempel-Oppenheim model, which is that explanation and prediction have exactly the same logical structure: an explanation can be used to forecast and a forecast is a valid explanation. Finally, the deductive-nomological model accounts also for the explanation of laws; in that case, the explanandumis a scientific law and can be proved with the help of other scientific laws. Aspects of Scientific Explanation , published in 1965, faces the problem of inductive explanation, in which the explanansinclude statistical laws. According to Hempel, in such kind of explanationthe explanans give only a high degree of probability to the explanandum, which is not a logical consequence of the premises. The following is a very simple example. The relative frequency of P with respect to Q is The object a belongs to P Thus, a belongs to Q The conclusion “a belongs to Q” is not certain, for it is not a logical consequence of the two premises. According to Hempel, this explanation gives a degree of probability r to the conclusion. Note that the inductive explanation requires a covering law: the fact is explained by means of scientific laws. But now the laws are not deterministic; statistical laws are admissible. However, in many respects the inductive explanation is similar to the deductive explanation. Both deductive and inductive explanation are nomological ones （that is, they require universal laws）. The relevant fact is the logical relation between explanansand explanandum : in deductive explanation, the latter is a logical consequence of the former, whereas in inductive explanation, the relationship is an inductive one. But in either model, only logical aspects are relevant; pragmatic features are not taken in account. The symmetry between explanation and prediction is preserved. The explanansmust be true.

[3] C.G.Hempel：Aspects of Scientific Explanation and other Essays in the Philosophy of Science ，New York，Free Press，1965:247-248.

[4] 〔美〕亨普尔：《自然科学的哲学》，张华夏等译，北京，生活·读书·新知三联书店，1987，第95页。

[5] 〔德〕亨普尔：《普遍规律在历史中的作用》，《哲学译丛》1987年第4期。

[6] 〔美〕T.S.库恩：《必要的张力》，纪树立等译，福州，福建人民出版社，1981，第30页。

[7] 该书主要内容包括：Introduction；0.1A Bit of Background；0.2 The Received View。1.The first decade （1948-1957）,Peace in the Valley but Some Trouble in the Foothills:1.1 The fountainhead: The DeductiveNomological Model; 1.2 Explanation in History and Prehistory; 1.3 Teleology and Functional Explanation. 2. The Second Decade （1958-1967）, Manifest Destiny Expansion and Conflict: 2.1 A Major Source of Conflict; 2.2 Deeper Linguistic Challenges; 2.3 Famous Counterexamples to the Deductive Nomological Model; 2.4 Statistical Explanation （2.41 The DeductiveStatistical Model; 2.42 The InductiveStatistical Model）; 2.5 Early Objections to the InductiveStatistical Model. 3. The Third Decade （1968-1977）, Deepening Differences: 3.1 The Statistical Relevance Model；3.2 Problems with Maximal Specificity; 3.3 Coffas Dispositional Theory of Inductive Explanation; 3.4 Explanation and Evidence; 3.5 Explanations of Laws; 3.6 Are Explanations Arguments？ 3.7 The Challenge of Causality; 3.8 Teleological and Functional Explanation; 3.9 The End of a DecadeThe End of an Era？ 4. The Fourth Decade （1978-1987）,A Time of Maturation: 4.2 Theoretical Explanation; 4.3 Descriptive vs Explanatory Knowledge; 4.4 The Pragmatics of Explanation; 4.5 Empiricism and Realism; 4.6 Railtons NomotheticMechanistic Account; 4.7 Statistical vs Causal Relevance; 4.8 Probabilistic Causality; 4.9 Deductivism; 4.10 Explanations of Laws Again; 4.11 A Fundamental Principle Challenged. Conclusion Peaceful Coexistence？ 5.1 Agenda for the Fifth Decade; 5.2 Chronological Bibliography.