Regressus and Empiricism in the Controversy about Galileo’s Lunar Observations David Marshall Miller Iowa State University This paper defends a version of J. H. Randall’s thesis that modern empiricism is rooted in the Scholastic regressus method epitomized by Jacopo Zabarella in De Regressu (1578). Randall’s critics note that the empirical practice of Galileo and his contemporaries does not follow Zabarella. However, Zabarella’s account of the regressus is imprecise, which permitted an interpretation intro- ducing empirical hypothesis testing into the framework. The discourse surround- ing Galileo’s lunar observations in Sidereus Nuncius (1610) suggests that both Galileo and his interlocutors amended the regressus method in this way, such that a developmental narrative links Scholastic logic to Galilean science. 1. Introduction One of the distinctive features of modern science is a commitment to empiricism—a fundamental expectation that theoretical hypotheses will survive encounters with observations. Those that comport with the theory’s explanations and predictions confirm the theory. Anomalous observations that do not fit theoretical expectations disconfirm it. Moreover, experiments can be contrived to generate observations that might serve to confirm or disconfirm a theory. Philosophers of science may disagree as to how exactly all of this is supposed to work, but the basic empiricist expectation almost goes without saying. To deny it is to rule one’s self out of the bounds of the scientificenterprise. Strange as it might seem to modern readers, empiricism in this guise was not always a feature of attempts to understand the natural world. It This paper is partly the result of work supported by the NEH Summer Institute “Between Medieval and Early Modern, Philosophy from 1300–1700.” Anonymous referees made cru- cial contributions, for which I am very grateful. I am also thankful for the feedback from audiences in Hanover (NH), Alba Iulia, Minneapolis, and Padua. Perspectives on Science 2018, vol. 26, no. 3 © 2018 by The Massachusetts Institute of Technology doi:10.1162/posc_a_00276 293 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/posc_a_00276 by guest on 02 October 2021 294 Regressus and Empiricism was not a prominent feature of Scholastic Aristotelianism, the preeminent philosophical tradition of the pre-modern era. The Scholastics held that it was possible to attain certain knowledge—scientia—about the natural world without a commitment to empirical adequacy in the face of specific observations. This is not to say that Scholastic natural philosophy was not at all empirical, as we shall see. Yet, in the view of Scholastic philosophers, individual experiences typically could not confirm or disconfirm a theory. Consequently, there was little point in appealing to discrete observations or devising experimental tests to establish grounds for accepting or reject- ing theoretical hypotheses (Schmitt 1969; McMullin 1978; Dear 1987; Dear 1995). This raises a historical question: how did natural philosophers come to value empiricism in the modern sense? When did observations become probative? Naturally, this is a complicated question that has long exercised scholars. Much of the relevant literature, however, focuses on northern Europe in the middle-to-late seventeenth and eighteenth centuries, and espe- cially on the influence of Francis Bacon and Baconian thought. Certainly, Baconianism is an essential feature of the broader story, especially regarding inductive confirmation of theories, but empiricism of a sort also emerged outside and prior to the Baconian sphere of influence. One finds, in par- ticular, that Galileo and his interlocutors considered specific observations as reasons to reject theoretical hypotheses at the beginning of the seven- teenth century. That is, they took incompatible observations as disconfirma- tions of a theory. This essay attempts to illuminate this cisalpine Baconianism avant la lettre, employing the debates surrounding Galileo’s lunar observations as a paradigmatic example. In that debate, both Galileo and his opponents used particular observations—as opposed to general experience—to dis- criminate between competing theoretical hypotheses. Anomalous obser- vations discounted possible explanations in favor of an alternative. This shows that eliminative empiricism (for lack of a better term)1 was an agreed- upon method by the time Galileo and his opponents entered the lists. This narrows the framing question to the use of observations as discon- firmations in Galileo’s milieu. That is, how did Galileo and his contem- poraries make observations probative—at least negatively—in the pursuit of natural science? One thread of an answer links empirical hypothesis testing to Aristotelian Scholastic logic, which was already undergoing significant reconsideration during the sixteenth century. In particular, the “demonstrative regress” or 1. The term is chosen to echo “eliminative induction,” used to characterize aspects of Bacon’s method (Schwartz 2017). Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/posc_a_00276 by guest on 02 October 2021 Perspectives on Science 295 regressus had been incorporated into the logic curriculum by which Galileo and his contemporaries were schooled. The regressus method reconfigured the relationship between theoretical explanations and observed effects. Yet regressus also inserted an ambiguity into the understanding of this relation- ship, in the form of the negotiatio intellectus. Though an essential step in regressus, there was no consensus as to how the negotiatio was supposed to proceed. The discriminatory use of observations could be seen as a natural way to resolve this ambiguity. Thus, the regressus method suggested a novel method- ology in natural science that admitted observations as epistemic grounds for accepting and rejecting theories. Hence, Scholastic Aristotelian resistance to the probity of individual experiences was partly overcome by developments within the tradition of Scholastic Aristotelianism itself. The regressus method has been an object of scholarly attention for a long time. In 1940, John Herman Randall, following Ernst Cassirer’s earlier suggestion, famously asserted that, with the addition of mathematics, it could be counted as the methodology of modern science (Cassirer 1906, pp. 134–41; Randall 1940, esp. p. 205; Randall 1961).2 Subsequent authors harshly criticized Randall’s view, showing that his radical claim had to be tempered, at least, if any connection between regressus and modern science was to be retained. In particular, Randall’s critics emphasized dis- similarities between Galileo’s method and Zabarella’s, concluding that the former was not the latter (Gilbert 1963; Schmitt 1969; Jardine 1976; Wisan 1978).3 This can be granted, but it does not preclude the possibility that Galileo’s method was a development of Zabarella’s. As discussed below, William Wallace and others have documented that Galileo and his contem- poraries were at least well-acquainted with regressus (Wallace 1984, 1988, 1992a, 1992b).4 Thus, even if Randall’s original claim overreached, it might nevertheless be possible to show that regressus contributed to modern, Galilean science.5 2. See also Edwards 1960, p. 283. 3. Schmitt (1969) and Jardine (1976), in particular, contrast early modern experimen- talism with the regressus. Their argument is that Galileo used particular observations to test hypotheses, while Zabarella only appealed to experience in general, so the latter could not have influenced the former. What follows is an attempt to weaken this contrast by suggest- ing a path from regressus to an experimental method. 4. The discussion that follows is indebted especially to Wallace’s work. See also Crombie 1996; Crombie and Carugo 1996. 5. Wallace’s close reading of Galileo’s early logical manuscripts is not matched in his at- tempts to locate Galileo’suseofregressus in his later scientific work (Wallace 1992a, chs. 5–6; Laird 1997, p. 258). That is, in part, what I intend to do here. Others have also connected the regressus to non-Galilean contexts—including Italian vernacular logics, Harvey, Hobbes, and British empiricism (Edwards 1983; Schmitt 1983; Sgarbi 2013; Hattab 2014; Sgarbi 2014). Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/posc_a_00276 by guest on 02 October 2021 296 Regressus and Empiricism The present essay is an attempt to modulate Randall’s claim along these lines. I aim to establish a more limited, yet more precise connection be- tween Scholastic logic and Galilean-style empiricism by closely examining uses of the regressus by those in Galileo’s intellectual orbit, including—but not limited to—Galileo himself. Galileo’s deployment of a regressus-like argument, and his interlocutors’ responses in kind, shows that the method, and the eliminative empiricism associated with it, had entered the natural scientific discourse of Galileo and his contemporaries. In this regard, Galileo’s work represents the development of natural scientific discourse more generally.6 The discussion begins by briefly stating the problem—the Aristotelian emphasis on certainty and what is generally known to be the case stands opposed to argumentative appeals to specific observations. The regressus theory is then explained, using the authoritative De Regressu (1578) by the Paduan Scholastic Jacopo Zabarella. This discussion highlights the ambigu- ity surrounding one stage of the regressus method—the so-called negotiatio intellectus. Two successive