Arguing for Uniformity: Rethinking Lyell’s Principles of Geology Victor Joseph Di Fate Department of Philosophy, The Johns Hopkins University The standard interpretation of Lyell’s argument strategy in the Principles of Geology turns the actual strategy on its head. Lyell is not arguing for a pic- ture of the earth based upon a priori methodological assumptions, as is usu- ally thought. Rather, he is attempting to ground a geological method upon empirical conclusions he draws about the earth and its past. Lyell’s attempt to ground a methodology upon a picture of the earth, rather than the other way around, challenges important assumptions about method and its relation to science. 1. Introduction1 Charles Lyell’s Principles of Geology is widely regarded as one of the great works of 19th century science, and one of the most inºuential works in the entire history of the earth sciences. Yet the standard critical interpretation of the Principles makes such high regard and inºuence look puzzling at best. We are told, for instance, that Lyell’s argument rests on a contentious a priori methodological distinction between scientiªc and non-scientiªc explanations, the former featuring observed causes at their present intensi- ties, the latter featuring anything else (Whewell [1840] 1996; R. Laudan 1972, 1987; Ruse 1976; Gould 1987; Rudwick 1990).2 We are also told that Lyell simply assumes a priori that explanations featuring such causes 1. I would like to thank Peter Achinstein and two anonymous referees for helpful sug- gestions and criticisms of earlier drafts of this paper. 2. All of the commentators cited here take Lyell as being committed to the so-called vera causa principle, which demands that scientiªc explanations feature only independently known causes—that is, causes known to exist independently of their capacity to explain and even predict phenomena. This is usually taken to mean that causal agents need to be di- rectly observable (or at least this is the view most often attributed to Lyell). This interpreta- tion of Lyell begins with Herschel (1830). Perspectives on Science 2011, vol. 19, no. 2 ©2011 by The Massachusetts Institute of Technology 136 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/POSC_a_00034 by guest on 30 September 2021 Perspectives on Science 137 are always more probable than explanations which do not (Sedgwick 1831, Whewell [1840] 1996); that he likewise assumes a priori that these causes will always be sufªcient to explain phenomena, so long as the geol- ogist can help himself to an indeªnite stretch of time (Rudwick 1971, p. 221); that he therefore does not base his conclusions on the evidence, but was a covert a priorist (Sedgwick 1831; Whewell [1840] 1996; Gould 1987); that his methodology therefore does not require, and that it even precludes, empirical veriªcation for his hypotheses (Baker 1998); and that underlying all of these startling positions is a simple-minded confusion between the uncontroversial methodological injunction that one should appeal to known before unknown causes, and the substantive empirical claim that only known causes have ever acted upon the earth (Hooykaas 1963; Gould 1987; Baker 1998). But Lyell would have disputed all of these charges, and would have been right to do so. They show, in fact, that the argument of the Principles was and continues to be fundamentally misunderstood; indeed, it has been turned upside down. The Principles is not an argument for a picture of the earth founded upon contentious a priori methodological assumptions, as these claims suggest. Exactly the contrary: it is an argument for a method- ology in geology, based upon empirical conclusions about the earth and its past. That is, on the interpretation to be defended here, Lyell’s core argu- ment is that geological explanation should henceforth appeal only to presently act- ing causes at current intensities, because the weight of the evidence he presents shows that such causes are the only ones that have been active on the earth. Initial resistance towards this interpretation may have a philosophical as well as an exegetical basis. After all, how can a methodology be grounded in an empirical conclusion about the earth, when we normally think that it is only in light of a methodology that empirical conclusions have any credibility in the ªrst place? But one of the rewards in rethink- ing Lyell’s arguments will be to bring such assumptions about method to light, exposing them for what they are: mere presuppositions that are by no means inevitable. 2. Lyell and His Critics Deep into his 1831 presidential address to the London Geological Society, Adam Sedgwick commences a discussion of the Principles, only the ªrst volume of which had appeared by that time. Nevertheless, we ªnd there the various lines of criticism that William Whewell would later reªne and expand upon, and that would be uncritically repeated by commentators much nearer our own day. Sedgwick’s central accusation is nothing less than that the geology of Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/POSC_a_00034 by guest on 30 September 2021 138 Rethinking Lyell’s Principles of Geology the Principles betrays a misunderstanding of the very nature of an empiri- cal investigation of the earth: The study of the great physical mutations on the surface of the earth is the business of geology. But who can deªne the limits of these mutations? They have been drawn by the hand of Nature, and may be studied in the record of her works—but they never have been, and never will be ªxed...byanytrains of a priori reasoning, based on hypothetical analogies [with the present]. We must banish all a priori reasoning from the threshold of our argument; and the language of theory can never fall from our lips with any grace or ªtness, unless it appears as the simple enunciation of those general facts, with which, by observation alone, we have at length become acquainted. I should not have detained you one moment in enunci- ating propositions such as these, had I not believed that their true import had been partially misunderstood, and their spirit some- times violated in a recent work on the “Principles of Geology.” (Sedgwick 1831, p. 302) Compressed in this short statement we ªnd some of the most inºuential and oft-repeated criticisms that will be leveled at Lyell. The author of the Principles, it is suggested, seeks to purge geological explanation of certain causes antecedent to any empirical investigation at all. Yet the sorts of changes that have taken place on earth are not to be determined a priori by us, but by nature itself, and thus can only be gleaned by a study of the evi- dence. Hence to claim that geological causes are the same in kind and intensity as those at all anterior periods is, in Sedgwick’s analysis, “an un- warrantable hypothesis with no a priori probability, and only to be main- tained by an appeal to geological phenomena.” For “we know nothing of . causes, but by the effects they have produced” (Sedgwick 1831, p. 304). Accordingly, Lyell must be assuming that we can know about na- ture through some other means than considering the evidence at hand, thereby “violating” the very “spirit” of empirical, natural philosophy. The subsequent appearance of the remaining two volumes of the Princi- ples did nothing to deºect this line of criticism. Indeed, by the time Whewell published his History and Philosophy of the Inductive Sciences—in 1837 and 1840, respectively—all three volumes of the Principles had long been in print; and in those two works we ªnd Whewell only sharpening the sorts of criticisms given previously by Sedgwick. What had changed by this time, however, was the vocabulary of the debate, thanks to Whewell himself: those geologists who hold that the geological phenom- ena have been of the same kind and intensity as those at present are called “uniformitarians,” while those who think that earth’s history has been Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/POSC_a_00034 by guest on 30 September 2021 Perspectives on Science 139 punctuated “by epochs of paroxysmal and catastrophic action” are called “catastrophists” (Whewell 1832). Whewell agrees with the uniformitarian that we should “avoid all bias in favour of powers deviating in kind and degree from those that act at present” (Whewell [1857] 1967, 3:513). However, when My. Lyell goes further, and considers it a merit in a course of geological speculation that it rejects any difference between the in- tensity of existing and of past causes, we conceive that he errs no less than those whom he censures. [Quoting Lyell] ‘An earnest and patient endeavor to reconcile the former indications of change’ with any restricted class of causes,—a habit which he enjoins—, is not, we may suggest, the temper in which science ought to be pursued. The effects must themselves teach us the nature and intensity of the causes which have operated. (Whewell [1857] 1967, 3: 513) On Whewell’s view, then, it would be unscientiªc to place any method- ological restriction on causes, precisely because he claims, with Sedgwick, that the evidence alone is supposed to indicate what sorts of causes there have been; and if that evidence indicates that there has been a catastrophe, then we ought to infer a catastrophe. What then motivates the uniformitarian restriction on causes, if not the evidence? On Whewell’s reconstruction, it appears to fall out from the combination of two contentious assumptions: on the one hand, that expla- nations by way of some