Rethinking Lyell's Principles of Geology

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 picture of the earth based upon a priori methodological assumptions, as is usually 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 intensities, 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 directly observable (or at least this is the view most often attributed to Lyell). This interpretation of Lyell begins with Herschel (1830).

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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 geologist 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 methodology in geology, based upon empirical conclusions about the earth and its past. That is, on the interpretation to be defended here, Lyell’s core argument is that geological explanation should henceforth appeal only to presently acting 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 rethinking 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

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the Principles betrays a misunderstanding of the very nature of an empirical 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 sometimes 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 evidence. 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 nature 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 phenomena 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

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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 intensity 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 methodological 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 explanations by way of some antecedently known cause can always be made to explain the facts; and, on the other, that such explanations are always to be preferred to because of their greater “a priori probability.”3 Accordingly, Whewell’s assessment is that there really are no positive arguments in favor of uniformitarianism, nor need there be given such a strategy; all the uniformitarian need do, in light of such assumptions, is explain away the manifest evidence of catastrophe: The Catastrophist is afªrmative, the Uniformitarian is negative in his assertions....Theoneisconstantly bringing fresh evidence of some great past event, or series of events, of a striking and deªnite kind; his antagonist is at every step explaining away the evidence, and showing that it proves nothing (Whewell [1840] 1996, p. 129) 3. Whewell writes, “The doctrine of Uniformity in the course of nature has sometimes been represented by its adherents as possessing a great degree of a priori probability. It is highly unphilosophical, it has been urged, to assume that the causes of the geological events of former times were of a different kind from causes now in action, if causes of the latter kind can in any way be made to explain the facts” (Whewell [1840] 1996, pp. 125– 126).

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This basic characterization that we ªnd in Sedgwick and Whewell has, I submit, been carried over almost unchanged to the present day. What has been added, however, is a peculiar diagnosis of Lyell’s error—namely, that he is conºating a methodological principle governing scientiªc explanation with a substantive empirical claim about the world. Here, for instance, is the 20th Century historian Reijer Hooykaas making the accusation: Lyell conceived uniformity as a methodological principle, but showed a tendency to take it as a law of nature (Hooykaas 1963, p. 47). This is held to be true a priori. . . . However...theprinci- ple of uniformity is not a law, not a rule established after comparison of facts, but a methodological principle, preceding the observation of facts. (Hooykaas 1963, pp. 38–39) On Hooykaas’ reading, then, Lyell begins with a methodological principle of uniformity “preceding the observation of facts,” but somewhere along the way begins to take the uniformity of nature as a matter of fact. What- ever might have brought about this conºation, it cannot have been a consideration of the evidence, for this fact “is held to be true a priori.”Myim- pression is that Hooykaas is tacitly adopting Whewell’s line: since Lyell thinks that known causes at present magnitudes can always “be made to explain the facts,” he infers that only such causes have ever acted, on the grounds that such explanations are a priori more probable. Similar charges are made by modern earth scientists. According to Ste- phen J. Gould, “Lyell conºated [a] testable and controversial theory about the nature of things with methodological canons that all scientists accept, thereby attempting to secure an a priori status for time’s cycle”—“time’s cycle” being Gould’s metaphor for Lyell’s picture of earth’s history (Gould 1987, p. 124). Lyell’s world-picture is said to have an a priori status because, on the one hand, he argues against competing theories “not by cit- ing contrary evidence, but by holding that their claims for a different earth in the past could not be rendered accessible to inquiry” (Gould 1987, p. 124); and on the other, because he “believed that modern causes would sufªce to explain everything about the past” (Gould 1987, p. 128) and that we are to preserve “uniformity ...intheface of almost any con- ceivable evidence [to the contrary]” (Gould 1987, p. 125). Thus, just as Whewell charged over a century earlier, it is not any positive evidence that conªrms Lyell’s view of the world; all competitors are eliminated on methodological grounds, and any threatening evidence is “explained away” because present causes can always “be made to explain the facts.” Taking these claims even further, the geomorphologist Victor R. Baker goes so far as to insist that, on Lyell’s conception of methodology, the “veriªcation of theories against nature is logically precluded” (Baker 1998, p. 177, empha-

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sis added). On Baker’s view, Lyell is not interested in understanding nature, for that “has to do with what nature says to us” (Baker 1998, p. 178). In- stead, uniformitarians want to “overstand” nature, taking their a priori methodological restrictions to the evidence, and contorting the evidence to ªt them. Accordingly, Baker (with admirable charity) actually deªnes uniformitarianism as a conºation of the methodological with the factual: Nineteenth century advocates of a ‘more scientiªc’ geology con- fused simplicity of logical expression with intrinsic qualities of nature. The resulting doctrine, named ‘uniformitarianism’, asserted that the relatively low-intensity, frequently occurring processes in evidence today must be the class of processes generally operating in the past. (Baker 1998, p. 173)

3. The “Standard View” and Lyell’s Response In virtue of its ubiquity, I will refer to the interpretation of §2 as the “standard view” of Lyell’s argument in the Principles. Although I shall argue that the interpretation is fundamentally wrong, it is easy to see how, once arranged in the proper order, its claims can present a tidy little picture of Lyell’s argument strategy. In the ªrst place, commentators are all agreed that Lyell begins his argument with a methodological claim “preceding the observation of facts” (as Hooykas puts it). In particular, Lyell begins by assuming that 1. We should only explain geological vestiges of the past by reference to causes of the same kind and intensity as those in evidence today. Secondly, the geological reconstructions of the Principles are supposed to demonstrate that 2. Known causes at present intensities can explain any geological vestige of the past, so long as we grant enough time for those causes to operate. Of course, Lyell does not merely want to argue that these causes can explain such phenomena, but that they do; yet all are agreed that Lyell either does or must make this additional step independently of any empirical considerations at all. Accordingly, he has to at least covertly assume: 3. Explanations appealing to known causes at present intensities are inherently (a priori) more probable than those that do not. Granted this, as well as the assumption that causes are either known or unknown, Lyell can now go on to conclude:

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4. Probably, only known causes at present intensities have ever acted on the face of the earth. Thus Lyell slides from the methodological claim (1) to the substantive empirical claim (4); but crucially he does so without ever seeking empirical veriªcation or support for his speciªc explanations. Indeed, it is the claim to greater “a priori probability” that greases the rails, here. For our modern commentators this slide represents Lyell’s “conºation” of methodological and empirical claims; for Sedgwick and Whewell, it represents Lyell’s re- fusal to let the evidence reveal what sorts of causes are acting. It looks like Sedgwick was right all along then: Lyell thinks he can glean information about nature independently of any evidence at all, violating the very spirit of empirical science. My position, again, is that this interpretation is dead wrong. That it is dead wrong is suggested by something it should be difªcult to ignore: Lyell claims that it is dead wrong. In a letter to Whewell, in which Lyell complains about Sedgwick’s address to the Geological Society, he responds directly to the charge of a priorism: Sedgwick was wrong if he imagined that I began with a priori reasoning on any assumed uniformity of physical events....Ididnot lay it down as an axiom that there cannot have been a succession of paraoxysms and crises, on which ‘a priori reasoning’ I was accused of proceeding, but I argued that other geologists have usually pro- ceeded on an arbitrary hypothesis of paroxysms and the intensity of geological forces, without feeling that by this assumption they pledged themselves to the opinion that ordinary forces and time could never explain geological phenomena. (Lyell 1881, 2:3) Lyell thus contends that no a priori assumptions about nature or method are assumed from the start—only that we should not settle on extraordinary causes without ªrst trying our best to see if “ordinary forces and time” could account for the phenomena as well. We make this effort, then, not because these uniformitarian explanations are inherently more probable, for that would just reintroduce the sort of bias that obviously bothers Lyell, but presumably because we should not settle on any hypothesis before ruling out other, potentially plausible alternatives. Granted this, Lyell concedes to Whewell that “the former intensity of...terrestrial forces may be true; I have never denied its possibility; but it is conjectural” (Lyell 1881, 2:3). And indeed it is, at least until plausible alternatives are eliminated. To be sure, Lyell also wants to convince Whewell that explanations em- ploying ordinary causes are generally more plausible, but the grounds he

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gives are certainly not a priori. Lyell cleverly offers Whewell an analogy within the latter’s principle domain of research. He asks Whewell to sup- pose that “at a certain period of the past...anunusual rise or fall of the tides took place,” and that an explanation featuring an extraordinary cause is immediately proposed: a “periodical increase or diminution in the quan- tity of matter contained in the sun or moon, or in both heavenly bodies.” Surely, Lyell insists, even you, Whewell, would consider it “unphilosophical” to resort to any such guesses, especially until we have exhaustively considered what combination of astronomical and terrestrial forces ordi- narily affects the tides. (This is just the point he made earlier). Yet Lyell now goes further: he supposes that Whewell would “feel much more conªdence in the probable uniformity in the condition of the heavenly bodies for an indeªnite period of time, past or present . . . especially when [he] recollect[s] that several problems in the former state of the tides...can now be explained without resorting to such expedients, and are in fact found to be the consequences of known and regular causes” (Lyell 1881 2:6, emphasis added). In other words, Lyell’s bet is that Whewell would be antecedently more conªdent that ordinary causes are the correct explanation, not out of any a priori bias towards such causes, but because anomalous phenomena once thought to be due to extraordinary agents have in fact been found to be due to the un-extraordinary operations of nature. This, I submit, neatly encapsulates Lyell’s strategy in the Principles— indeed, precisely as it was intended to. Lyell will argue, on a case-by-case basis, that many geological vestiges of the past are in fact the consequences of presently acting causes, and therefore that we ought to “feel much more conªdence in the probable uniformity” between geological causes in the past and in the present. It is on these grounds that he will derive the methodological principle that geologists should henceforth explain such phenomena only by reference to these causes. This is a far cry from reading-off substantive conclusions from a priori methodological assumptions. On the contrary, as he puts it in a letter to Roderick Murchi- son, the aim of the Principles is “to establish the principle of reasoning in the science”; that is, the Principles is an argument for a methodology—and an empirical one (Lyell 1881, 1:234, emphasis added). Over the next two sections, we shall consider portions of Lyell’s empirical argument for his methodological principle.

4. The History of Geology Lyell begins his argument, not with any geological reconstructions of the past, but with a sweeping history of theories of the earth from the ancient Egyptians to the late eighteenth century. From this history, he tries to iso- late those factors that (he believes) have retarded progress in geological

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science and those that have facilitated it. It turns out that trying to reconcile geological evidence with Scripture and speculating on the origin of the earth have been spectacularly unproductive, while conclusions on which there has been lasting consensus have been obtained (of course) by explaining phenomena by reference to present causes. Accordingly, on Lyell’s reading of history, the tendency has gradually been away from cos- mogony and the postulation of catastrophes and towards the uniformitarian approach. Moreover, he notes that some of the traditional objections to pursuing this as a general strategy in geology are beginning to give way— for example, the notion that time is insufªciently deep. What does the standard view make of the history? Since he is supposed to be assuming an a priori principle of explanation from the very start of his argument, Lyell’s description of the historical rise of uniformitarian methodology would have to be a description of the dawning recognition by nascent geologists of where the bounds of admissible explanation are in their science, and the gradual crumbling-away of the biases that have ob- scured that demarcation from view. As Martin Rudwick puts it, the aim of the history is to show “that the slowly increasing reliance on [ordinary] causes [in geology] has been similar to the growth of the scientiªc attitude as a whole; [it is meant to show that] to be Lyellian in geology is, in effect, to be scientiªc” (Rudwick 1990, p. xvii).4 The idea, then, is that very general conceptual factors determine what the nature of scientiªc explanation is, and Lyell’s history describes the arduous process of discovering what such explanations should look like in geology. If so, exactly what the history couldn’t be is the beginning of one long empirical argument for a geological method that isn’t settled by purely conceptual factors in ad- vance, as I want to insist. The problem for the standard view, however, is that Lyell is quite ex- plicit that the latter is what he intends the history to be. That this is his intention is clariªed in the introductory chapter to volume 3, which he wrote in response to objections and misunderstandings after the appearance of the ªrst two volumes (Rudwick 1990, p. xxxvii). Here he reºects on the two “methods of theorizing in geology” in light of three historical cases he had given in volume one. Such historical cases are valuable, he says, because they enable us “to judge by experience” the “respective mer- its” of the two methodologies and “the relative value of the fruits which

4. Indeed, it is in the history that Lyell ªrst compares the “uniformitarian” method with Isaac Newton’s, relating the former’s attempt to understand the past by studying present causes to the latter’s attempt to understand the celestial by studying terrestrial causes and regularities. According to the standard view, with this comparison Lyell would be trying to show that, inasmuch as geological method has approximated Newton’s method, it has approximated the scientiªc method.

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they have produced” (Lyell [1833] 1991, 3:3). And after recounting the cases he concludes that We shall adhere to this plan [of explaining the past in terms of the present]...because history informs us that this method has always put geologists on the road that leads to truth....Ontheother hand, the opposite method, that of speculating on a former distinct state of things, has led invariably to a multitude of contradictory systems, which have been overthrown one after the other. (Lyell [1833] 1991, 3:6, emphasis added) To be sure, many historians of geology have disputed Lyell’s history, and I do not question their conclusions. But the point is that, in disputing Lyell’s history, they are disputing his evidence—evidence that one method in particular has so far been truth-tropic, while the other not. The history, then, is not a description of the gradual awakening by proto-geologists to where the a priori bounds of scientiªc explanation are in their science; rather, it stands at the very beginning of one long empirical argument for where those bounds should be. It is meant to “soften the reader up”—to suggest that this may well be a plausible approach to geology, since it has yielded considerable fruit up to this point; but any stron- ger conclusion will have to await successful geological reconstructions of the past.5 It is to these we now turn.

5. Lyell’s Geological Reconstructions Lyell presents dozens of geological reconstructions over the three volumes of the Principles and it would be both impossible and tedious to discuss them all. Instead, I will present two such reconstructions, which will suf- ªce to show that the standard view radically distorts Lyell’s argument strategy, and that his actual strategy resembles that indicated in his letter to Whewell.

5.1. Lyell on Climate Change Before Lyell weighed in on the subject, the evidence that there had been a gradual diminution in mean global temperature from deep into the past to the present primarily consisted of fossil remains found in the Secondary 5. A similar story can be told, I think, for Lyell’s comparisons between the methodology he recommends and Newton’s (see fn. 4). More than anything, Newton’s methodology was successful: with it, he hit upon the uncontestable truth, or so it was believed. Accord- ingly, if using the present to illuminate the past is importantly analogous to Newton’s method, then perhaps such a method stands a chance of delivering the truth as well. But this is something that Lyell recognizes he will have to show, not assume a priori.

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Strata of Europe. These remains bore a generic resemblance to ºora and fauna now inhabiting tropical regions, strongly suggesting that Europe’s climate was once warmer.6 This would already pose a challenge to Lyell, for he must ªnd some explanation by way of “ordinary causes” to account for this dramatic change. But the problem is much deeper, for the northern hemisphere’s decline in temperature was taken as grounds for adopting a particular picture of the earth and its original state. As Lyell puts it, the widely accepted explanation for the temperature decline was that “the earth at its creation was in a state of ºuidity, and red hot, and that ever since that era it had been cooling down, contracting its dimensions, and acquiring a solid crust” (Lyell [1830] 1990, 1:104). If this explanation is correct, the decline in temperature recorded in Europe’s Secondary Strata would be a striking snapshot of this “gradual refrigeration of the globe from a molten state”; and if this is what the evidence indicates, then explaining past phenomena by reference to causes operating at present is simply a non-starter. As Sir Humphrey Davy remarked, as the earth gradually cooled “there was no order of events similar to the present” (Davy 1830, p. 321). How does Lyell handle the problem of climate change? If the standard view is correct, we should ªnd him blocking the catastrophist explanation by showing that an “ordinary cause” of climatic variation could account for the phenomena just as well. And this he certainly does. But we should also ªnd him inferring to his own explanation on these grounds alone, since he allegedly assumes that explanations featuring ordinary causes are inherently more probable; again, no “positive evidence” for the correctness of his explanation is or even can be given. But is this how the argument really goes? Naturally, Lyell’s strategy is to begin by considering those factors that contribute to the variations in climate on the earth at present. He ªrst notes that latitude alone does not sufªce, since “zones of equal warmth, both in the atmosphere and in the waters of the ocean, are neither parallel to the equator nor to each other” (Lyell [1830] 1990, 1:106). In place of this, Lyell spends a considerable time arguing that variations in climate in the present state of the earth are primarily accounted for by differences in the distribution of land and sea. And this, of course, yields his famous pro- posal for explaining climate change: the gradual decline in the northern hemisphere’s mean annual temperature was simply due to a change in the distribution of land and sea occurring from the deposition of the Second- ary Strata into the Tertiary. Lyell’s next step, then, is to consider the

6. Lyell actually supports the case for climate change by adding his own evidence in vol. 1.

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changes in climate that would be brought about by different hypothetical distributions. As he put it to a friend, here he is demonstrating that he now has a “receipt for growing tree ferns at the pole, or if it suits me, pines at the equator; walruses under the line, and crocodiles in the Arctic Cir- cle” (Lyell 1881, pp. 261–262). In the ªrst scenario, he considers the arrangement of land and sea that would cool the entire globe, while in the second he considers the arrangement that would warm it. Now note that it is precisely here that we should expect Lyell’s argument to terminate if the standard view is correct. For if Lyell believes he has “receipt for growing ferns at the pole,” he believes he is in a position to “explain away” the evidence for climate change by constructing a speciªc uniformitarian hypothesis detailing the changes in the distribution of land and sea that, if they obtained, would account the decline in temperature reºected in the European strata. And since he is supposed to be assuming that his own hypothesis is a priori more probable, he can go on to infer that it is probably true, so long as he is assuming, as he appears to, that one or the other explanation is likely correct. If so, then he can draw a substantive conclusion about the earth’s past, bypassing any positive evidence that his proposed explanation is actually the case—which is precisely how his arguments are supposed to go on the standard view. Unfortunately for the standard view, however, this is not at all how the argument goes. Lyell in fact devotes an entire chapter to showing that the causal conditions described by such a uniformitarian hypothesis actually obtain. As he puts it there: It next remains for us to inquire whether the alterations, which the geologist can prove to have actually taken place at former periods, in the geographical features of the northern hemisphere, coincide in their nature, and in the time of their occurrence, with the revolu- tions in climate as would naturally have followed, according to the meteorological principles already explained [in the previous chapter]. (Lyell [1830] 1990, 1:125) In the ªrst place, he presents a variety of evidence demonstrating that the northern hemisphere was once a large ocean, interspersed with numerous islands. According to his theory of climate, the temperature of the northern hemisphere would have been considerably warmer than it is now, analogous to the current climate of the archipelagos of the South Paciªc. Moreover, evidence from organic and inorganic remains shows that this subaqueous land was elevated as the tertiary strata were laid down; and with this elevation would come a gradual decline in mean temperature. Once this crucial additional step is taken into account, it should be clear that no assumption of greater a priori probability is required or in-

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volved. Lyell is simply arguing, ªrst, that there is a uniformitarian hypothesis which could, if true, account for the evidence of climate change; and then he simply presents evidence that the causal conditions described by the hypothesis actually obtain.7 This seems to me to be a pretty unex- ceptionable empirical strategy. Lyell is therefore not merely trying to explain away putative evidence against uniformitarianism. He is doing something much bolder. He is arguing that what was once considered evidence against a uniformitarian picture of the earth turns out to be a conªrming instance of it: the gradual diminution in temperature is in fact the result of causes of a kind and magnitude now in operation—in this case, speciªc gradual changes in the distribution of land and sea.

5.2. The Paris Basin There is space enough to brieºy consider one more geological reconstruction that is particularly telling. According to the French geologists Geor- ges Cuvier and Alexandre Brongniart, the Paris basin is a depression in the chalk that has been ªlled up in succession by alternating groups of marine and freshwater strata. The junctions between the strata are abrupt however, and had therefore been taken as evidence of sudden changes in physical geography at earlier epochs, with catastrophic marine incursions wip- ing out any terrestrial fauna. In this case, Lyell combats the catastrophist’s explanation simply by disputing the evidence: the marine and freshwater formations that Cuvier and Brongniart had taken to be successive actually turn out, upon more extensive consideration, to be contemporaneous. In particular, a closer investigation of the region conducted by Constant Prevost had shown that freshwater formation is actually intercalated with the marine formation, thus rendering any hypothesis of a catastrophic marine incursion wholly inappropriate. In place of this, Lyell conjectures that there was once a river charged with clay particles that entered a bay of the sea, “drifting down, from time to time, fresh-water shells and wood.” And when we actually look at the testacea found in the marine formation, they belong to a genus inhabiting the brackish waters of river-mouths that drain into the sea, “in perfect harmony with the hypothesis before advanced, that a river ºowed into a gulf” (Lyell [1833] 1991, 3:245). Moreover, when we then examine 7. Proponents of the standard view sometimes acknowledge this additional step, but only in passing. In extended treatments of Lyell’s theory of climate change, Gould (1987, p. 145) devotes a single sentence to it, while Laudan (1987, p. 214) a short paragraph. This neglect is understandable, for on their views the additional step of showing empirically that the uniformitarian hypothesis is actually true would either be unnecessary or impossible. It seems to me neither, which is why Lyell devotes a chapter to it.

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what Cuvier had claimed to be the next freshwater formation, we ªnd that it too is intercalated with the marine formation, further supporting Lyell’s hypothesis that the region consisted of a gulf or estuary into which streams and rivers had ºowed. It is important to point out that Lyell never doubts that Cuvier’s own explanation of the Basin is well-supported by the original evidence, and thus that any uniformitarian account would be far less probable. Crucially, this shows that there is no hint of an a priori bias for a uniformitarian explanation of that evidence—no assumption that a uniformitarian account could “explain the evidence away” if granted enough time; it cannot. In- stead, Lyell argues that the evidence itself should be supplanted by more accurate evidence that renders any catastrophist explanation entirely irrel- evant: there simply aren’t successive changes that are in need of an explanation, much less a catastrophic one. His own explanation for the region is then originally ºoated as a hypothesis, but one that he thinks is subse- quently rendered probable by the brackish testacea found in the marine beds, as well as the additional freshwater formations he considers. For Lyell, then, the Paris Basin is another apparent catastrophe that turns out in fact to be the result of ordinary causes; it is another conªrming instance of the uniformitarian picture of the earth.8

6. Scientiªc Method and Its Empirical Grounds On the view for which I am arguing, it is from such positive instances that Lyell generalizes to the conclusion that, probably, only present causes have ever acted on the earth; and it is from this substantive empirical claim that Lyell derives his methodological principle—that geologists reconstructing the past should only appeal to such causes. This, incidentally, explains those otherwise puzzling passages in which Lyell makes clear that his method permits an appeal not just to known or observed causes, as on the standard view, but to present causes more generally, whether known or unknown. For instance, he says that his method “seeks an interpretation of geological monuments by comparing the changes of which they give evi- 8. To be clear, I am not claiming that all of Lyell’s conclusions are as strong as the ones he draws with climate change or the Paris Basin, where he appears conªdent that his own explanation is correct. Sometimes he is merely out to show that his own explanation is only more probable than the leading catastrophist explanation, while at other times he only argues that ordinary causes could explain a phenomenon, without explicitly ruling out alternative catastrophist or uniformitarian explanations. But there is nothing even remotely perni- cious about these weaker sorts of argument. Sometimes, when more decisive evidence is unavailable, it is necessary to show that one’s own explanation is at least as likely as its competitors, if not more so. While such arguments might not count in favor of one’s over- all case, they certainly do not count against it. And as I have been trying to show, these weaker arguments are by no means Lyell’s main strategy.

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dence with the vicissitudes now in progress, or which may be in progress” (Lyell [1833] 1991 3:3, emphasis Lyell’s). Now why would Lyell favor present causes even if they are unknown? I think we already have the an- swer: since he is deriving his methodological constraint from the conclusion that only present causes have ever acted, geological explanation will have to feature presently acting causes, even if they need to be hypothetical. On the standard view, this stipulation would be a complete mystery. But if the standard view can be so forcefully dispatched, why has it per- sisted for centuries? The reason, I would now like to suggest, is that Lyell is contravening long-held assumptions about scientiªc method that had become ubiquitous by the mid twentieth-century and that remain domi- nant today; and instead of recognizing that these assumptions are being challenged, critics have read those assumptions onto the Principles, resulting in a reconstruction of an argument that no intelligent person would give. I want to conclude by brieºy considering Lyell’s challenge to what are now near universal views on scientiªc method.9 It is widely believed that Lyell’s principle of uniformity is nothing more than a principle of causal parsimony, and thus at bottom an appeal to simplicity: all else being equal, it is simpler or more parsimonious to pos- tulate that past geological causes are no different in kind or magnitude from those in evidence today.10 But simplicity or causal parsimony, the usual view goes, function only as non-empirical virtues that explanations possess. In particular, they allow us to select one explanation over a less simple or parsimonious one when the available evidence is insufªcient for deciding between them. So understood, though, nothing at all would follow about nature when we select an explanation on these grounds; to think otherwise is to attempt to read-off something about the world from a mere prescription relating to scientiªc procedure. Eliot Sober has summed up this view quite nicely. Simplicity—a notion which for him includes causal parsimony and uniformity—is nowadays taken to be

“purely methodological”; it guides the way we allow observations to shape our judgments about the plausibility of hypotheses, but 9. Below I concentrate on the notion of simplicity rather than some other concept in philosophy of science mainly because, as I note in a moment, many commentators take Lyell’s methodological principle of uniformity to be a principle of simplicity or causal parsimony, making simplicity the most direct link between Lyell and modern philosophy of science. Moreover, I take Lyell’s treatment of simplicity to provide historical grounding for Eliot Sober’s interesting views on simplicity, as well as John Norton’s more general views on the “locality” of inductive principles; see fn. 11 and fn. 13. 10. For instance, Hooykaas (1963, pp. 38–39) and Goodman (1967) both claim that the geological principle of uniformity is just an instance of a principle of simplicity or causal parsimony; see also the quote from Baker at the end of §2.

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make no substantive assumptions about the world those hypotheses purport to describe....[S]implicity, whatever its detailed charac- ter turns out to be, is an a priori constraint on rational investigation....This, I believe, is the status that many philosophers now assign the simplicity concept. (Sober 1988, p. 58)11 It is easy to see that the received view of Lyell would fall directly out of these assumptions. In the ªrst place, the principle of uniformity is only a special case of an appeal to simplicity; but simplicity is “an a priori constraint on rational investigation.” Accordingly, Lyell must be working from a general a priori notion of what it is for an investigation to be rational—and thus scientiªc—and simply extends that general view to the particular case of geology by setting out his more speciªc uniformity principle. His great confusion, then, lies in thinking that this methodological prescription—which is merely constitutive of scientiªc procedure—im- plies something about the world. In doing so, he accords a higher probability to any hypothesis featuring uniformitarian causes; and once he shows in the Principles that these causes can account for any geological vestige, the principle of uniformity is activated—as any appeal to simplicity is— to choose between the competing theories equally consistent with the evidence. This allows to him draw speciªc substantive conclusions about the earth while bypassing any positive empirical considerations. But if my interpretation of Lyell is correct, we are left with a very different picture of methodology and its relation to empirical science. Lyell would clearly not be thinking of the postulation of a uniformity—or of an explanation’s simplicity or causal parsimony, if you like—as a non-empirical virtue that geological explanations would possess. Quite the contrary: it is only in light of a prior empirical investigation of nature that we are autho- rized to favor the “simpler, i.e., uniformitarian” theories. Accordingly, if uniformitarian geological theories now have a greater prior probability than those postulating catastrophes, then that is only because of the back- ground empirical information that we are bringing to bear.12 To put the point as starkly as possible: if the principle of uniformity is nothing more an appeal to simplicity or causal parsimony, then Lyell’s striking view is that these are to be included among the empirical virtues of a geological theory. Moreover, it is crucial to emphasize that this would apply only to geological theories. That is to say, no completely general notion of what it is for an explanation to be scientiªc could possibly sire Lyell’s methodological principle, so long as we are thinking of that principle as underwritten by speciªc empirical in- 11. Naturally, Sober names Lyell as someone who takes simplicity to be an “a priori constraint on rational investigation” (Sober 1988, p. 69, fn. 20). 12. Ironically, this is the very view about simplicity that Sober (1988) is arguing for.

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formation about geological causes.13 On this view, then, methodological restrictions on causes will have to be established piecemeal, based upon the speciªc empirical information brought in to support those restrictions.14 There is therefore no general methodological restriction that is to be instantiated in particular scientiªc domains, independently of any consideration of how the tract of nature covered by that domain behaves. There is thus no a priori essence to what legitimacy in scientiªc explanation would be; if the world had been otherwise, Lyell would think that adverting to catastrophes may well have been the “philosophical” thing for the geologist to do. This means, then, that the boundaries between science and non-science on Lyell’s view are contingent and empirical. And this means, of course, that those boundaries may have to be revised in light of compel- ling future experience. Methodological principles can certainly change over time, then; but these shifts track changes in how we take certain portions of nature to be. Of course, we are only drawing out Lyell’s views here—they may or may not be correct. It may be that simplicity or causal parsimony are indeed non-empirical virtues; and it may be that speciªc methodological constraints are merely broken-off from more general constraints on all scientiªc explanation-giving. The point has merely been that an unques- tioned allegiance to these latter views by historians and philosophers of science has been responsible for a distorted interpretation of Lyell. But we now know at least what the offending views are, and that one deservedly inºuential scientist does not share them.

References Baker, Victor R. 1996. “Catastrophism and Uniformitarianism: Logical Roots and Current Relevance in Geology.” Pp. 171–182 in Lyell: the Past is the Key to the Present. Edited by D. J. Blundell and A. C. Scott. Bath, UK: The Geological Society Publishing House. Davy, Humphrey. 1830. “On the formation of the earth.” Edinburgh New Philosophical Journal 8: 321–325. Goodman, Nelson. 1967. “Uniformity and Simplicity.” Geological Society of America, Special Papers 89: 93–99. Gould, Steve J. 1987. Time’s Arrow, Time’s Cycle: Myth and Metaphor in the Discovery of Geological Time. Cambridge, Mass: Harvard University Press. 13. This point is especially aimed at those who claim that Lyell adheres to the so-called vera causa principle—a very general principle governing scientiªc explanation—which he simply extends to the speciªc case of geology. See fn 2. 14. The idea that methodological rules are valid only “locally,” in light of empirical information speciªc to a particular domain, is defended in Norton (2003).

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Herschel, John F. W. [1830] 1996. Preliminary Discourse on the Study of Natural Philosophy. London: Routledge/Thoemmes Press. Hooykaas, Reijer. 1963. The Principle of Uniformity in Geology, Biology and Theology. Leiden: E.J. Brill. Laudan, Larry. 1981. Science and Hypothesis. Dordrecht, Holland: D. Reidel Publishing Company. Laudan, Rachel. 1972. “The Role of Methodology in Lyell’s Science. Studies in the History and Philosophy of Science 13: 215–249. ———. 1987. From Mineralogy to Geology: the Foundations of a Science, 1650–1830. Chicago: University of Chicago Press. Lyell, Charles 1881. The Life, Letters and Journals of Sir Charles Lyell, Bart., 2 vols. Edited by Katherine Lyell. London: John Murray. ———. [1830–1833] 1990–1991. The Principles of Geology: an Attempt to Explain the Former Changes of the Earth’s Surface by Reference to Causes Now in Operation, 3 vols. Chicago: University of Chicago Press. Norton, John. 2003. “A Material Theory of Induction.” Philosophy of Sci- ence 70: 647–670. Rudwick, Martin J. 1971. “Uniformity and Progression”. Pp. 209–227 in Perspectives in the History of Science and Technology. Edited by Duane Roller. Norman, Oklahoma: University of Oklahoma Press. ———. 1990. “Introduction.” Pp. vii–lviii in Principles of Geology: an At- tempt to Explain the Former Changes of the Earth’s Surface by Reference to Causes Now in Operation, 3 vols. By Charles Lyell. Chicago: University of Chicago Press. Ruse, Michael 1976. “Charles Lyell and the Philosophers of Science.” Brit- ish Journal for the History of Science 9:120–131 Sober, Eliot. 1988. Reconstructing the Past: Parsimony, Evolution, and Infer- ence. Cambridge, Mass: MIT Press. Whewell, William. 1832. “[Review of] Principles of Geology, vol. II, by Charles Lyell.” Quarterly Review 47: 103–32. ———. [1857] 1967. History of the Inductive Sciences, 3rd Edition, 3 vols. London: Frank Cass and Company Limited. ———. [1840] 1996. The Philosophy of the Inductive Sciences, Founded Upon Their History, 2 vols. London: Routledge/Thoemmes Press.

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