What is the Relation between Prediction and Explanation?

Transcription

1 What is the Relation between Prediction and Explanation? Paul Hoyningen-Huene Leibniz Universität Hannover, Inst. of Philosophy Universität Zürich, Department of Economics

2 The answer (roughly) Predictions Explanations 2

3 Content A little history Conceptual and terminological remarks Where explanations and predictions overlap Explanations without predictive power Predictions without explanatory power Conclusion 3

4 A little history 50 years ago, this would have been a fairly short talk because of the thesis of the structural identity of prediction and explanation: Every adequate prediction is a potential explanation Every adequate explanation is a potential prediction 4

5 A little history (2) Background of this thesis: The Hempel-Oppenheim theory of explanation E, the covering law model : An explanation consists of a derivation of the explanandum from natural laws and antecedent conditions The thesis follows from the nature of scientific predictions: they have the same structure Nowadays, there is consensus that the Hempel-Oppenheim theory of explanation is inadequate on many different counts Thus, we have to start anew 5

6 Procedure We have to study predictions and explanations separately, without theoretical prejudices, and then try to compare the resulting accounts Furthermore, the historical account presupposes that the terms prediction and explanation are unambiguous which they are not I therefore start with 6

7 Conceptual and terminological remarks 1. Prediction in the literal (or narrow) and in the wider sense Literal sense: refers to something in the future, relative to the time of the statement. Example: weather forecast Wider sense: prediction of a theory : anything that is derived from the theory Given the program of the conference, I assume that we are interested in predictions in the literal sense, and I will treat only those 7

8 Conceptual and terminological remarks (2) 2. There is an ambiguity in the specificity of predictions in the narrow sense: prediction that vs. prediction when Prediction that The Big One will happen: major earthquake in the San Francisco area Total solar eclipse on Aug 21, 2017 in Kansas City: Start of total eclipse at 18:08:12.5 (UT); altitude 62.5, azimuth

9 Conceptual and terminological remarks (3) 3. Explanation Explanation is a highly pragmatic concept: it strongly depends on the respective audience: What is an explanation for a lay person may not be an explanation for an expert and vice versa What is the exact subject of an explanation of event E ( explanandum ): that E occurred? Properties of E? Which ones? Therefore, there may be legitimate disagreement about examples of explanations 9

10 Conceptual and terminological remarks (4) 4. Explanation vs. understanding There is a tradition that contrasts explanation as belonging to the natural sciences with understanding ( verstehen ) as belonging to the social sciences and humanities In this contrast, there are two heterogeneous elements involved A theoretical claim: a fundamental methodological difference between the natural sciences on the one hand and the social sciences and the humanities on the other A terminological decision: to call the respective (different) activities explanation and understanding 10

11 Conceptual and terminological remarks (5) The terminological decision is extremely unfortunate, even if one subscribes to the theoretical claim: In everyday parlance, explanation and understanding does not correlate with groups of academic disciplines Therefore, in the given context explanation and understanding must be seen as technical terms, but people tend to overlook this Roughly, there is an activity in research fields in which whyquestions are posed and answered, which goes beyond pure descriptions Despite all possible distinctions, I call this explanations 11

12 Overlap area 1. Explanation and prediction indeed overlap in some of those cases in which the old structural identity theses holds, i.e., where the prediction/explanation is directly based on natural laws or theories (in the sense of the natural sciences) Three essential constraints: The laws/theories must be causal and not indicator laws (otherwise no explanation see below) When a descriptive (or phenomenological ) law is used, it is sometimes dubious whether a real explanation is achieved Example: Derive the fall of a stone for a time of fall of two seconds from the law of free fall (result: 20 m). Is this an explanation? 12

13 Overlap area (2) The necessary factual information ( antecedent conditions ) must be accessible before the event (otherwise no prediction) Example: Explanation after the fact of some properties of the 7.8 magnitude earthquake in Nepal on April 25, 2015 by the theory of plate tectonics: collision of the Indo-Australian and Asian tectonic plates. Roughly 25% of built-up stress energy released; epicenter and hypocenter known Next earthquake in the region cannot be predicted in any detail: neither magnitude, nor location of epicenter, nor time. Reason: missing factual information about the detailed state of the plates 13

14 Overlap area (3) In addition, there is overlap in areas probably not covered by the covering law model 2. Social sciences and humanities: explanations of actions with recourse to intentions of actors and their beliefs about their situation Example: Why did President Truman order the dropping of atomic bombs over Hiroshima and Nagasaki on August 6 and 9, 1945? Standard explanation: Truman wanted to end the war with Japan as quickly is possible (intention) and thought that the unannounced dropping of atomic bombs would be the most effective means for this goal (belief about the situation) 14

15 Overlap area (4) Alternative explanations: Main motive was revenge for Pearl Harbor (including general prejudice against Japanese) In anticipation of the postwar situation, Truman wanted to impress the Soviets Difficult problem for historians to find the correct explanation In principle, this pattern of explanation of actions can also be used for predictions of actions Essential constraint: People may change intentions or beliefs or both unexpectedly 15

16 Overlap area (5) Example: President Truman, entry in his diary of July 25, 1945 only 12 days before the Hiroshima bomb: This weapon is to be used against Japan between now and August 10th. I have told the Sec. of War, Mr. Stimson to use it so that military objectives and soldiers and sailors are the target and not women and children. [ ] He and I are in accord. The target will be a purely military one and we will issue a warning statement asking the Japs to surrender and save lives. Apparently, he changed his mind Any prediction based on this entry would have gotten essential details of the upcoming event fundamentally wrong 16

17 Overlap area (6) 3. Predictions by Delphi methods Roughly: Define a group of experts Ask experts individually about their predictions concerning the subject matter in question Feed back results to the experts for the possibility of corrections Repeat procedures until answers converge If the prediction is appropriate, it may also be an explanation because the experts may have used causal knowledge and also intentions and beliefs of major players 17

18 Predictions without explanation Some predictions do not, even if proven true, provide explanations This may be puzzling because these predictions should be ultimately based on causal regularities However, for one reason or another, these regularities cannot be exploited for explanatory purposes 18

19 Predictions (2) 1. Predictions based on mere correlations between data sets Barometric chances b: b w weather changes Correlation may be due to an underlying causal mechanism correlating b and w (common cause) causal b mechanism causal w predict Clearly, b may be predictive for w without being explanatory 19

20 Predictions (2) 2. Predictions based on models Example: Weather forecast models Global model of the German Whether Service: Earth atmosphere is decomposed into 60 vertically ordered layers, each layer containing 1,474,550 grid points Horizontal distance between grid points: 20 km Total number of points: 88 Mio 20

21 Predictions (3) Variables at each point: air pressure, temperature, wind components, water vapor, clouds and precipitation, connected by a set of dynamic equations Embedded in the global model: regional model COSMO-EU of Europe: grid length 7 km, 40 layers, 17.5 Mio grid points Embedded in regional model: Local model COSMO-DE for Germany: grid length 2.8 km, 50 layers, 9.7 Mio grid points 18 hour forecast for Germany calculated every 3 hours 21

22 Predictions (4) Clearly, the dynamic equations in the model contain the causal mechanisms for the weather development that are explanatory in principle However, in the intended output of the model, the weather forecast, the causal mechanism is hidden In principle, it should be possible to extract from the calculations of the model the workings of the causal factors that are explanatory for the predicted weather state 22

23 Explanations without prediction Most important case of explanations without predictive power in the historical sciences: narrative explanation Narrative explanations tell a story about the coming about of the explanandum (event, process, thing, etc.) and this story is explanatory Example: In 18 th century France, many roads were built in the countryside Where topography allowed, these roads were straight connections between A and B 23

24 Explanations (2) Now imagine you encounter a road designed and built at that time in a flat area: What explains this surprising design? In the sweep, there used to be chateau built before the road, whose owner had excellent relations to the King of France The owner could persuade the King to realize a road design that saved his chateau from being flattened In the 19 th century, the chateau was dismantled 24

25 Explanations (3) Clearly, this explanation cannot be brought in the form of the covering law model There is no law of the form: Whenever the chateau is in the way of a road planned and built in the 18 th century in France and the chateau owner had excellent relations to the King of France, the road will be built around his chateau This becomes even more evident if one uses more complex examples like the outbreak of World War I 25

26 Explanations (4) A long story has to be told about the situation of various European countries and their mutual relations up to 1914 Then, on June 28, 1914, the Archduke of Austria was murdered This event triggered a series of events that finally led to the beginning of World War I There is certainly no law connecting all these events, nor is there an overarching intention that can serve as the basis of an explanation Rather, there was a complicated mishmash of heterogeneous elements that finally led to the outbreak of the war 26

27 Explanations (5) Thus, historical explanations of this kind cannot be used, in different circumstances, for predictions The reason is that in historiography, descriptions are typically so specified as to make the described event uniquely identifiable The specificity of the description is necessary in order to understand how the event originated and what its effects were This makes it impossible to subsume the event under general laws that would respect the specificity of the event Thus, narrative historical explanations cannot be transformed into predictions 27

28 Conclusion 1. In cases where events are subject to known dynamic causal regularities and where we have cognitive access to the relevant antecedent conditions, predictions and explanations may coincide. 2. In cases where we know peoples intentions and their beliefs, predictions and explanations of their behavior may coincide. 3. Models are often used in a way that they only provide predictions. 4. Historical explanations can typically not be transformed into predictions. 28