474 *Received September 2001 revised March 2002. ���Send requests for reprints to the author, Philosophy Department and Center for As- trobiology, 169 Hellems, Campus Box 232, University of Colorado, Boulder, CO 80309 e-mail: cleland@spot.colorado.edu. ���I am indebted to Alan Franklin, Marc Lang, and Stephen Leeds for comments and helpful suggestions on an earlier draft of this paper. This research was supported in part by a grant from the National Aeronautics and Space Administration to the Uni- versity of Colorado���s Astrobiology Institute. Philosophy of Science, 69 (September 2002) pp. 474���496. 0031-8248/2002/6903-0000$10.00 Copyright 2002 by the Philosophy of Science Association. All rights reserved. Methodological and Epistemic Differences between Historical Science and Experimental Science* Carol E. Cleland������ University of Colorado, Boulder Experimental research is commonly held up as the paradigm of ���good��� science. Al- though experiment plays many roles in science, its classical role is testing hypotheses in controlled laboratory settings. Historical science (which includes work in geology, biology, and astronomy, as well as paleontology and archaeology) is sometimes held to be inferior on the grounds that its hypothesis cannot be tested by controlled labo- ratory experiments. Using contemporary examples from diverse scientific disciplines, this paper explores differences in practice between historical and experimental research vis-a `-vis the testing of hypotheses. It rejects the claim that historical research is episte- mically inferior. For as I argue, scientists engage in two very different patterns of evi- dential reasoning and, although there is overlap, one pattern predominates in historical research and the other pattern predominates in classical experimental research. I show that these different patterns of reasoning are grounded in an objective and remarkably pervasive time asymmetry of nature. 1. Introduction. Experimental research is commonly held up as the para- digm of successful (a.k.a. good) science. The role classically attributed to experiment is that of testing hypotheses in controlled laboratory settings. Not all scientific hypotheses can be tested in this manner, however. His- torical hypotheses about the remote past provide good examples. Al-

��������������� ���. ��������������������� 475 1. See, for instance, ���A Scientific Dissent from Darwinism��� (2001, 23), a statement that recently appeared in the New York Review of Books and was signed by approximately 100 (mostly physical) scientists they listed their fields after their names. though fields such as paleontology and archaeology provide the familiar examples, historical hypotheses are also common in geology, biology, planetary science, astronomy, and astrophysics. The focus of historical research is on explaining existing natural phenomena in terms of long past causes. Two salient examples are the asteroid-impact hypothesis for the extinction of the dinosaurs, which explains the fossil record of the dino- saurs in terms of the impact of a large asteroid, and the ���big-bang��� theory of the origin of the universe, which explains the puzzling isotropic three- degree background radiation in terms of a primordial explosion. Such work is significantly different from making a prediction and then artifi- cially creating a phenomenon in a laboratory. Scientists are well aware of the differences between experimental and historical science vis-a `-vis the testing of hypotheses. Indeed, it is some- times a source of friction. Experimentalists have a tendency to disparage the claims of their historical colleagues, contending that the support of- fered by their evidence is too weak to count as ���good��� science. A telling example is the startling number of physicists and chemists who attack neo-Darwinian evolution on the grounds that it hasn���t been adequately ���tested.���1 The most sweeping condemnation of historical science, how- ever, comes from Henry Gee, an editor of the prestigious science journal Nature. In Gee���s words ���they [historical hypotheses] can never be tested by experiment, and so they are unscientific. . . . No science can ever be historical��� (2000, 5���8). In other words, for Gee, a genuine test of a hy- pothesis requires classical experimentation. Philosophers of science have pretty much ignored the simmering con- troversy among scientists over the epistemic status of historical claims. Aware that experiment plays different roles in science, skeptical about the existence of a single method for all of science, and unable to provide an epistemically satisfying account of the rationality and objectivity of any scientific practice, philosophers have been reluctant to generalize, let alone make normative judgments, across different disciplines. Moreover, those philosophers who have cast an eye on historical research have spent most of their time on evolutionary biology and archaeology, where issues about teleology or human agency predominate. As a consequence, historical methodology is often characterized (e.g., Goode 1977 Kitcher 1993, 18��� 34) in terms of narrative histories. Analysis in terms of narrative histories does not, however, do justice to historical work in disciplines such as astronomy and geology that, like experimental physical science, do not involve even a prima facie appeal to purposes or ends. A more general understanding of the methodology of historical science and its differences from classical experimental science is badly needed.

������������������������������ ��� ������������������������������������ ������������������������ 476 2. These regularities may be statistical, as in quantum mechanics, and they may or may not be causal experimental scientists are interested in testing functional regularities (e.g., the Boyle-Charles��� law for ideal gases), as well as causal regularities. 3. I am using the term ���auxiliary assumption��� very broadly to include any assumption whose falsity could be used to salvage a hypothesis in the face of a failed prediction. Thus on my usage, auxiliary assumptions include assumptions about the particulars of One of the purposes of this paper is to sketch such an account. As we shall see, scientists engage in two very different patterns of evidential rea- soning, and one of these patterns predominates in historical research and the other in classical experimental research. These differences in evidential reasoning lie at the heart of the charge that historical science is inferior to experimental science vis-a `-vis the testing of hypotheses. But, as I shall also show, it is not an accident that historical research emphasizes one pattern and experimental research the other, nor is it an accident that some in- vestigations utilize both. Using examples from a wide variety of scientific disciplines, I show that these differences in evidential reasoning are un- derwritten by an objective and pervasive feature of nature, namely, a time asymmetry of causation between present and past events, on the one hand, and present and future events, on the other. Because each practice is tailored to exploit the information that nature puts at its disposal, and the character of that information differs, neither practice may be held up as more objec- tive or rational than the other. 2. ���Classical��� Experimental Science. As Ian Hacking (1983, 149���166) has emphasized, experiment plays a variety of roles in science besides the test- ing of hypotheses. Nevertheless, there is little doubt that a significant por- tion of experimental work is devoted to testing hypotheses in controlled laboratory settings. My central concern in this paper is with this (what I call the ���classical���) role of experiment since it provides the paradigm to which historical research is often compared unfavorably. Due to space limitations, I shall focus on simple, idealized experiments, ignoring the complications (see Franklin 1999, 13���38) that occur when one is working with fancy equipment, dealing with experiments that cannot (for practical or theoretical reasons) be ���repeated��� under controlled conditions, etc. The hypotheses investigated in classical experimental science postulate regularities among event-types.2 A test condition C is inferred from the hypothesis and a prediction is made about what should happen if C is realized (and the hypothesis is true). This forms the basis for a series of controlled experiments. In many of these experiments, C is held constant (repeated) while other experimental conditions are varied. When this activity is preceded by a failed prediction in an earlier experiment, it resembles the activity fa- mously condemned by Popper (1963), namely, an ad hoc attempt to save a hypothesis from refutation by denying an auxiliary assumption.3 But