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Preface vii
Acknowledgments xiii
About the Contributors xv
Part I | General Philosophy of Science
1. Scientific Explanation—Denis Bonnay 3
2. Confirmation and Induction—Mikaël Cozic 53
3. Causality—Max Kistler 95
4. Metaphysics of Science as Naturalized Metaphysics—Michael Esfeld 142
5. Theories and Models—Marion Vorms 171
6. Scientific Change— Anouk Barberousse and Marion Vorms 225
7. Philosophy of Science and Science Studies— Anouk Barberousse 259
8. Reduction and Emergence—Pascal Ludwig 285
Part II | Philosophy of the Special Sciences
9. Philosophy of Logic—Philippe de Rouilhan 319
10. Philosophy of Mathematics—Denis Bonnay and Jacques Dubucs 349
11. Philosophy of Physics— Anouk Barberousse 405
12. Philosophy of Biology—Thomas Pradeu 430
13. Philosophy of Medicine—Élodie Giroux and Maël Lemoine 464
14. Philosophy of Social Sciences— Jon Elster and Hélène Landemore 510
15. Philosophy of Economics—Mikaël Cozic 542
16. Philosophy of Cognitive Science—Daniel Andler 595
17. Philosophy of Linguistics—Paul Égré 654
Index 727
Preface
General Introduction
Philosophy of science has the aim of answering those questions raised by scientific activity that are not directly addressed by science itself. Among such questions, we can mention: What are the overall goals of science, as well as the specific goals of its various branches? By what means are these goals pursued? What basic principles does it put into practice? Philosophy of science also tries to understand the relationships that exist between the scientific disciplines. To what extent, and in what sense, are they, and should they be, unified? Also belonging to its domain is the relationship between science and reality. What, if anything, does science tell us about reality? And to what extent is it justified in making the claims it does?
Just like the sciences themselves, current philosophy of science is multifaceted and specialized. A philosopher of science may embark on projects as diverse as the development of a formal analysis of the concept of confirmation using probability theory and the study of the potential contribution neuroscience may bring to our understanding of consciousness. Thus, it becomes difficult for both students and researchers within a given domain to be aware of the advances and challenges arising in any specific area in philosophy of science.
The aim of the present book is to expose the main questions, as well as some of the answers, being discussed in today’s philosophy of science. We view it as the “missing link” between introductions and research, and our own goals will have been met if this book successfully bridges the gap between introductions to the philosophy of science meant for a general audience on the one hand, and research articles and monographs
on the other. It is therefore primarily intended for the use of advanced undergraduate or graduate students who, after a first introduction to the area, may now wish to deepen their knowledge. We also hope that The Philosophy of Science: A Companion will be useful to both junior and senior researchers in philosophy of science wishing to familiarize themselves with areas outside of their own.
Philosophy of science has become too specialized for this goal to be achieved by any one person. Thus, our book is a collective effort. We have nevertheless endeavored to present the basic problems that shape contemporary philosophy of science in a coherent way. In contrast with encyclopedias, where contributions tend to simply coexist and thus lack organic unity, we have tried to maximize complementarity and crossreferencing between the chapters. Our hope is that this has favored a strong sense of unity, something that is always hard to attain in such collective undertakings.
Part I: General Philosophy of Science
The two parts of The Philosophy of Science mirror the traditional distinction between general philosophy of science and philosophy of the special sciences. General Philosophy of Science (Part I) deals with generic issues raised by scientific activity, independent of specific disciplines. General philosophy of science was the very core of philosophy of science up to the middle of the twentieth century. Philosophy of science itself has dramatically evolved over the last several decades, becoming increasingly devoted to issues raised by specific scientific disciplines. The study of general problems nevertheless remains a highly active element of philosophy of science. Moreover, it is our opinion that the study of these general problems is indispensable to those who focus on the philosophy of some particular scientific discipline or area, since they represent a set of tools invaluable to understanding their own, specific objects of study.
The objective of the first part of the book is twofold. We intend to both take stock of the traditional questions which have shaped analytic philosophy of science and to introduce certain problems that have been raised more recently. Thus the first two chapters, bearing upon explanation and confirmation, respectively, tackle issues that were the subject of intense debate in the middle of the twentieth century—notably among philosophers of science influenced by logical empiricism—and which, as we shall see, are still much studied today. With causality, chapter 3 also focuses on a traditional concept, though one to which logical empiricism has been rather hostile. Causality is now at the epicenter of a very vibrant area, straddling the borders of philosophy of science and metaphysics. Metaphysics is also at the heart of chapter 4, which deals with scientific realism (an issue that underwent a thorough overhaul during the 1980s) and the metaphysics of science, constituting a topic that is much discussed today. Chapter 5 addresses the issue of knowing how best to analyze some of science’s primary products, namely theories and models. Starting from the “received view” of scientific theories, inherited from logical empiricism, it discusses the objections that have been raised against this view while also looking at alternative conceptions. Lastly, chapter 8 deals with issues surrounding the reduction and emergence of properties
and/or theories coming from distinct scientific disciplines. Logical empiricism also contributed greatly to this research area. We shall see that current reflection on the matter is closely related to metaphysics, philosophy of knowledge, and sometimes also to the philosophy of the special sciences (particularly the philosophy of mind).
In our view, these six topics—explanation, confirmation, causality, scientific realism, the nature of theories and models, and reduction— constitute the core of general philosophy of science, even if they do not exhaust it. This latter consideration in mind, two further issues are also touched on in Part I. Chapter 6 studies the diachronic dimensions of scientific activity, a topic made famous by Kuhn’s much celebrated book (The Structure of Scientific Revolutions, 1962/1970). Chapter 7 is more meta-philosophical in character: it reviews the relations between philosophy of science and other approaches (notably historical and sociological) which share in the aim of analyzing scientific activity and which are currently referred to as sciences studies Although comprehensive, this does not cover all topics having a justifiable claim to the label of general philosophy of science. For instance, the growing literature on statistics and statistical reasoning is not represented. But it is our contention that Part I of The Philosophy of Science will provide the reader with a satisfyingly complete survey of contemporary general philosophy of science.
Part II: Philosophy of the Special Sciences
For several decades, philosophers of science have increasingly directed their attention toward the finer details of scientific activity, in particular to issues exclusive to specific disciplines. These issues are the object of the philosophy of the special sciences, to which the second part of The Philosophy of Science is devoted.
Compared with general philosophy of science, philosophy of the special sciences appears two- sided. Certain problems are essentially instances or applications of issues belonging to general philosophy of science. In this case, more often than not, the targeted area of knowledge requires some reconsideration of the issue on the part of the philosopher. For instance, the issue of justification or confirmation of theories raises specific problems when one studies, let’s say, economic or mathematical theories, as opposed to theories from physics, which often serve to illustrate confirmation theories. By contrast, certain other issues in the philosophy of the special sciences are entirely generated by the specific concepts and methods of a given field. The discussions on the concept of function (in biology) or on the nature of linguistic universals (in linguistics) are two cases in point. The main objective of the second part of this volume is to introduce the reader to a representative sample of the issues that currently structure the philosophy of the special sciences. We have done our best to respect this two- sided character, i.e., to show how some of the issues are very closely linked to the “big” issues in general philosophy of science while others are specific to certain specialized domains of science.
The first two chapters of Part II are devoted to the philosophy of the formal sciences. More precisely, chapter 9 is concerned with logic and chapter 10 with mathematics. The
philosophy of the formal sciences has often been left out of handbooks or textbooks on the philosophy of science. One of the reasons that implicitly underpins this state of affairs is that the issues raised by these formal sciences can seem remote from those raised by bona fide empirical sciences. But there are other reasons that speak in favor of integrating philosophical discussion on these disciplines. First, there is some interesting convergence between certain issues in the philosophy of the formal sciences and other issues in general philosophy of science, for example, those related to the nature of explanation. Second, there are certain other issues which call for a unified and coordinated answer from both the philosophy of the formal sciences and other branches within philosophy of science. For example, understanding why mathematics fits into the physical world so well—an issue that lies at the border between the philosophy of mathematics and the philosophy of physics. Or the problem of understanding mathematical cognition, which is of interest to both philosophy of mathematics and cognitive science.
Chapters 11 and 12 are devoted to the philosophy of physics and the philosophy of biology, respectively. These two areas have a special status in philosophy of science. Philosophy of physics is considered basic because physics is viewed as the fundamental scientific discipline. This means at least two things. First, that physics is an area where scientific reasoning is supposed to reach its zenith, and thus, in particular, that it is indispensable to be at least minimally familiar with it if one wishes to gain an understanding of scientific reasoning in general. And, second, that it is crucial to clarify the picture of the world as it is depicted by the physical sciences. Philosophy of biology has become an extremely active field, such that there is probably no other area in the philosophy of the special sciences whose importance has grown more over the last two decades.
An entire chapter is devoted to the philosophy of medicine. Our main reason for this is that philosophy of medicine is an area where philosophy of science overlaps with normative and practical philosophy. This reveals itself with respect to the question of whether the concepts of health and illness have an essential normative dimension, and also as regards the study of clinical reasoning. In both cases, the discussion goes beyond the purely epistemic point of view dominant in the philosophy of the natural sciences.
Another particular feature of Part II is the space we have devoted to philosophy of the human and social sciences (chapters 14 to 17). Interestingly, in these areas the philosopher’s stance and corresponding expectations may differ from those that are generally endorsed in the philosophy of the natural sciences. In the former area, philosophers often assume that there is nothing wrong with the way science is done and thus refrain from making recommendations to scientists or from criticizing their methods. Not so in the latter case, and this is to be expected, since there are far more methodological uncertainties, debates, and disagreements involved in the human and social sciences.
Chapters 14 and 15 broach the social sciences. Chapter 14 deals with general issues in the philosophy of the social sciences, for example, methodological individualism
and the relations between social sciences and cognitive sciences. Chapter 15 focuses on one specific social science, economics. This emphasis is to be welcomed, in light of the scientific and social impact of economics, and all the more so since it currently constitutes a particularly active field of study for philosophers.
The last two chapters are organized in a similar way. Both are devoted to disciplines that study human cognition. Chapter 16 is a general presentation of the issues raised by cognitive science from the point of view of philosophy of science. Chapter 17, on the other hand, bears on one specific discipline—linguistics. While philosophy of language is a well- structured and well-known area in philosophy, there are relatively few philosophical discussions on linguistics as a science. Both for this reason and for the fact that the philosophy of cognitive science focuses more on disciplines like psychology and neuroscience, we deemed it fitting to devote a whole chapter to linguistics.
Acknowledgments
We are grateful to the contributors to this volume, to Thierry Martin (the editor of the series in which an earlier version of The Philosophy of Science was published, under the title “Précis de Philosophie des Sciences” in 2011), to Daniel Andler who brought financial support to this initial version through his Senior Fellowship of the Institut Universitaire de France and to Christopher Robertson, who translated many of the chapters’ earlier versions. The current version has benefited from the comments of two anonymous referees. It was notably supported by the Institute of Cognitive Studies at Ecole Normale Supérieure (Paris) under grant ANR-10-LABEX-0087 IEC and by Mikaël Cozic’s Junior Fellowship of the Institut Universitaire de France. Lastly, we wish to thank the Institut d’histoire et de philosophie des sciences et des techniques (UMR 8590, Paris I—ENS Ulm— CNRS), which has provided us with a highly stimulating scientific environment for 15 years.
Anouk Barberousse, Denis Bonnay, and Mikaël Cozic, Paris, January 2018.
About the Contributors
Daniel Andler is a philosopher of science focusing on cognitive science. His interests include the philosophical issue of naturalism, the impact of cognitive science on the social sciences and their relevance for societal issues like education and public policy, and artificial intelligence. He is professor emeritus at the Sorbonne and a member of the Académie des Sciences Morales et Politiques.
Anouk Barberousse is a professor of philosophy of science at Sorbonne Université, Paris, where she teaches general philosophy of science, philosophy of physics and philosophy of scientific expertise. She has recently written on the epistemology of computer simulation, the philosophy of probability, and the role of databanks in our knowledge of biodiversity.
Denis Bonnay is an assistant professor in philosophy at Université Paris Nanterre, working in logic, philosophy of science and philosophy of cognitive science. His research ranges from works on the nature of logic and the boundaries between logic and mathematics to studies on judgment aggregation and group beliefs.
Mikaël Cozic (Paris- Sorbonne University, PhD, 2005) is an assistant professor at the Paris-Est University, a researcher and head of the group “Decision, rationality and interaction” at the Institut d’Histoire et de Philosophie des Sciences et des Techniques and a member of the Institut d’Universitaire de France. He studied philosophy (Ecole Normale Supérieure de Paris, 1997–2002), logic (University Paris-Diderot, MSc, 2002), and cognitive science (Ecole des Hautes Etudes en Sciences Sociales, MA, 2001) in Paris. Professor Cozic’s research focuses primarily on philosophy of economics and
formal theories of rationality. His current research concerns the relationship between cognitive science and positive and normative economics, as well as several issues in Bayesian epistemology, including the revision of one’s beliefs upon learning the opinion of others.
Jacques Dubucs is a senior scientist at the Centre National de la Recherche Scientifique and the head of the Social Sciences and Humanities Department at the French Ministry of Higher Education, Research, and Innovation. His scientific work deals with logic and philosophy of science.
Paul Égré (born 1975; PhD, 2004) is directeur de recherche at Institut Jean-Nicod (CNRS) and an associate professor in the Philosophy Department of Ecole Normale Supérieure in Paris. Besides work in formal semantics and on the epistemology of linguistic theory, a large part of Paul Egré’s work over the last decade has been on the topic of vagueness in language and in perception, dealing with semantic, logical, and psychological aspects of the phenomenon. Since 2012, Egré is also the editor-in- chief of the Review of Philosophy and Psychology.
Jon Elster is the Robert K. Merton Professor of Social Science at Columbia University. He is the author or editor of more than thirty- five books translated into more than seventeen languages on the philosophy of social sciences, the theory of rational choice, political psychology, deliberative democracy, and the history of political thought (Marx and Tocqueville), to name a few of their subjects. He is currently working on a comparative study of the Federal Convention (1787) and the first French constituent assembly (1789–1791).
Michael Esfeld is full professor of science at the University of Lausanne. His research is in the metaphysics of science, the philosophy of physics, and the philosophy of mind. His latest book publication is A Minimalist Ontology of the Natural World, with Dirk- André Deckert (New York: Routledge, 2017).
Élodie Giroux is an assistant professor at Jean Moulin Lyon 3 University, where she teaches philosophy of science and philosophy of medicine. She is director of the master’s in “Culture and Health.” Her main research interests are the history and epistemology of “risk factor epidemiology”; causation in medicine and public health; and risk, health, and disease concepts. She is currently working on precision medicine. Besides several papers on modern epidemiology, she published Après Canguilhem, définir la santé et la maladie (Paris: PUF, 2010) and Naturalism in the Philosophy of Health (Cham: Springer, 2016), and she edited a special issue on the history of risk factor epidemiology in Revue d’Histoire des Sciences (2011) and on precision medicine in Lato Sensu (2018).
Max Kistler is professor at the Department of Philosophy at University Paris 1 Panthéon–Sorbonne and head of IHPST (Institut d’Histoire et de Philosophie des Sciences et des Techniques). His research topics include causation, powers and dispositions, laws of nature, natural kinds, and reduction. He is the author of Causation
and Laws of Nature (Routledge, 2006), L’esprit matériel. Réduction et émergence (Ithaque, 2016), and coeditor (with B. Gnassounou) of Dispositions and Causal Powers (Ashgate, 2007).
Hélène Landemore is an associate professor of political science at Yale University. She is a political theorist interested in democratic theory, theories of justice, Enlightenment thinkers, and the philosophy of social sciences. Her book Democratic Reason (Princteon, NJ: Princeton University Press, 2013) was awarded the 2015 David and Elaine Spitz Prize for best book in liberal and/or democratic theory published two years earlier. She is currently writing a new book on postrepresentative or “open” democracy.
Maël Lemoine is a professor at the University of Bordeaux, France, where he teaches philosophy of medical science. He published an introductory essay in the philosophy of medical science in 2017 and has recently published various articles on biological research in psychiatry, animal models, and precision medicine.
Pascal Ludwig is an associate professor in the Department of Philosophy, Sorbonne Université, Paris. He has coauthered several books on the philosophy of science and the philosophy of the mind.
Thomas Pradeu is a CNRS senior investigator in philosophy of science (permanent position) at ImmunoConcept (CNRS and University of Bordeaux), and associated member at the Institut d’Histoire et des Philosophie des Sciences et des Techniques (CNRS and University Pantheon– Sorbonne). His research focuses on biological individuality, immunology, the microbiota, and the interactions between philosophy and science.
Philippe de Rouilhan is a senior researcher emeritus at the CNRS and a member of the Institut d’Histoire et de Philosophie des Sciences et des Techniques (CNRS and Université Panthéon– Sorbonne), of which he was the director for a long time. His work pertains to logic lato sensu or, more specifically, to formal ontology, formal semantics, philosophy of logic, philosophy of mathematics, and philosophy of language. He is currently preparing a book on truth, logical consequence, and logical universalism.
Marion Vorms is a lecturer (maître de conférences) in philosophy at University Paris 1 Panthéon– Sorbonne and a Marie Curie fellow at Birkbeck College, London, psychology department. Her past work in philosophy of science concerns the nature of scientific theories and representations. Her new project, which is at the crossroads of epistemology and the psychology of reasoning, bears on the notion of reasonable doubt; she is particularly interested in judicial reasoning and decision-making.
General Philosophy of Science
SCIENTIFIC EXPLANATION
Denis Bonnay (Université Paris Nanterre, IRePh & IHPST)
Why is Nicolas angry? Because he thinks Dominique wanted to play a nasty trick on him. Why was Gomorrah destroyed? Because God wanted to punish its inhabitants. Why did the dinosaurs disappear? Because a giant asteroid crashed into the earth. In asking the question “why?” we bring a real or reputed fact—Nicolas’s anger, the destruction of Gomorrah, dinosaur extinction— to the attention of our interlocutor, and we ask for an explanation of that fact. These explanations may rely on simple everyday knowledge: it is well known that people do not like having nasty tricks played on them. Explanations can be of the religious sort: the biblical account tells not only of Gomorrah’s existence but also of the sins of its people, going on to explain the destruction of the city by an act of divine retribution. And then there are the explanations offered to us by science: thus, the extinction of the dinosaurs being one of the enigmas that paleontology faces, an asteroid strike is one of the explanations put forward.1
More than just a simple side issue of scientific activity, explanation takes its place as one of the specific goals of science. Of course, as we have just seen, it is not just science that claims to offer explanations. And, conversely, science certainly has goals other than explanation too. Science enables us to describe and classify phenomena, as well as
1 I thank Anouk Barberousse, Mikaël Cozic, Henri Galinon, Marion Vorms, and Kenneth Waters for various discussions, comments, and re-readings, which were of help to me. I also wish to thank Christopher Robertson, who translated the French version. This work received funding from the ANR (The IHPST’s Logiscience program) and from the Institut de Recherches Philosophiques (Université Paris Nanterre). The survey on theories of explanation is also obviously indebted to some other, similar enterprises, in particular the surveys by Salmon (1989) and Woodward (2009).
enabling us to predict and control them. Nevertheless, one of the motivations, be they individual or collective, to “do science” in the first place seems to be to find explanations that cannot be found elsewhere—for example, research on electricity and magnetism, and also work on the electromagnetic theory, that is developed to explain a group of mysterious phenomena such as static electricity, the properties of Magnesia stones, or lightning and its effects. In contrast, it is not easy to imagine what sort of thing a scientific theory that explained nothing would be. A strict typology, say a botanical classification of different plant species according to their phenotype for example, doesn’t strike us as being a bona fide scientific theory, insofar as it lacks any explanatory power. Not lacking, however, are opponents to the idea that the aim of science is to provide explanations. Pierre Duhem, in The Aim and Structure of Physical Theory, opposes the idea that the object of a scientific theory is to explain a set of observable regularities, an opinion shared by other physicists of his time such as Ernst Mach. But this refusal is primarily grounded in Duhem’s own concept of explanation. To explain would be “to strip reality of the appearances that envelop it like a veil, in order to see the bare reality itself” (Duhem, 1908); Duhem considers that attaching an explanatory ambition to science makes it subservient to metaphysics, the only domain to claim possession of the keys to the ultimate essence of things.2 The approach that we will follow here is not quite the same. In determining whether science provides explanations or not, we will not start out with some overly demanding concept of explanation. We will set out from the intuition that science provides explanations, and we will try to identify a concept of explanation such that this concept would enable us to account for the explanatory power of science. What can be expected from this line of enquiry? What goals are we pursuing? In a good concept of explanation, we expect first of all that it be adequate; that is, that it will allow us to understand which elements provided by science constitute explanations and by what virtue they come to possess their explanatory power. For example, if an explanation has some epistemological virtue, in that it allows us to “understand what is happening,” then a good concept of explanation must tell us how scientific explanations allow us to “understand what is happening.” We would hope then, off the back of this, to be in a position to evaluate explanations, that is to say, to have the capacity to distinguish between good and bad explanations. An analysis of the concept of explanation will obviously not tell us if the explanation is right, in the sense of its expressing truth, but it should be able to tell, or at least indicate to us, whether some explanation would be a good explanation, presuming that it does express the truth. And lastly, we would like some insight regarding the relationship between the explanatory aim of science and its other aims—prediction, control, and so on.
We will begin by looking in detail, during the first section, at the theory of scientific explanation proposed by Hempel and Oppenheim known as the deductive-nomological model (DN). The importance of place we give it here is justified conceptually by the rigor of the analysis it proposes and historically by the role of cardinal reference it
2 On the question of realism—does science give us access to the very nature of things or not?—and on the metaphysical scope of science, see chapter 4 of the present volume.
Explanation 5
continues to play in contemporary debates on explanation, despite its no longer being the dominant model. In the second section, and in light of the DN model, we will revisit the general properties of explanation, discussing the link between explanation and prediction, the temporal conditions that weigh, or do not weigh, on explanation, as well as the characterization of the laws of nature. The third section is devoted to an examination of the classic objections brought against the DN model, these taking the form of a list of counter-examples. The main rival theories that have emerged to resolve these problematic examples in the DN model’s stead— causal theory and unificationist theory—are presented and discussed in the fourth section. In the closing section, we will sketch out some other approaches toward contemporary reflection on explanation.
1. The Deductive-Nomological Model
1.1 To Explain i s T o D ED uc E from a l aw
Let us begin then by looking at the inaugural example given by Hempel and Oppenheim (1948). A mercury thermometer is rapidly immersed in a basin of hot water. The level of the mercury column falls slightly at first before rising swiftly. Why? Here we have a little puzzle to solve— we were expecting that the level of the mercury would simply rise, though this is not exactly what has happened. In fact the explanation is quite simple. The rise in temperature, at first, affects only the standard quality glass tube which contains the mercury. Expanding, the tube leaves more room for the mercury, whose level promptly drops. Then, rapidly, the heat spreads out and the mercury expands in turn. As its coefficient of expansion is much higher than that of glass, the mercury level rises and exceeds its own initial level.
Analyzing this example makes the distinction between the explanandum, what is to be explained, namely the slight decrease followed by rapid rise in the level of the mercury, and the explanans, which does the explaining, immediately clear. Under explanans we see, first, the initial conditions, the particular facts reported in the explanation, such as the set-up involved— the glass tube, the mercury column, the bowl of hot water—and the act of immersing the tube in hot water itself. Then too, the general laws come into effect, such as the laws governing the thermal expansion of glass and mercury, and a statement regarding the relatively low thermal conductivity of glass. The explanandum is subsumed under the general laws, in the sense that it can be deduced from these laws and the initial conditions.
Hempel and Oppenheim’s theory is that the full generality of scientific explanation can be read in this particular case. To explain, one need not do anything other than deduce the phenomenon to be explained by using general laws and the initial conditions, which justifies the labeling of their model as the deductive-nomological (DN) model of explanation. Thus, the general form for scientific explanation that we draw from Hempel and Oppenheim is as follows:3
3 The double-lined bar ==== indicates that the statement below follows on logically from those statements above it.
Initial conditions Explanans L1, . . . , Ll
General laws
E Empirical phenomenon to be explained Explanandum
For there to be explanation, certain conditions must be met by the explanans and by the explanandum (the explanandum is a statement describing the phenomenon to be explained, the explanans is a set of statements describing the initial conditions and the laws involved):
Logical Conditions of Adequacy
(R1) The explanandum must be a logical consequence of the explanans (R2) The explanans must contain general laws whose presence is necessary for the explanandum to be a logical consequence of the explanans (R3) The explanans must have empirical content.
Condition of Empirical Adequacy
(R4) The statements making up the explanans are true.
The logical conditions of adequacy are purely formal. They specify the properties of the explanans and of the explanandum, which do not depend on the actual state of the world. This is not the case with the condition of empirical adequacy, which states that a supposed explanation is not truly an explanation unless one additional condition is satisfied: the statements contained in the explanans must be true. (R1) and (R4) together imply that the statement, which is the explanandum, is also true.
Condition (R1) carries the full weight of the analysis. When we are given the explanation of a phenomenon, we understand why this phenomenon occurred, in the sense that we have an argument that shows that it was to be expected that the phenomenon would occur (see Hempel, 1965b, p. 337). Salmon (1989) summarizes this point by saying that the essence of scientific explanation, according to Hempel, lies in nomic expectability.4 The initial conditions being in place, the phenomenon could only but occur, since it follows on logically from the initial conditions using general laws.
Note that Hempel’s model does not leave room for the common idea that to explain is to explain surprising or unfamiliar phenomena by reducing them to facts and principles with which we are already familiar (Hempel, 1966). To explain is to bring everything back to laws. If these laws are familiar, then the explanation will equal reduction to the familiar, but this is not necessarily the case. An example of the first sort of explanation would be the kinetic theory of gases: the behavior of the molecules of a gas, with which we are not familiar, is explained by subsumption under laws that also apply to the movements of things with which we are familiar, such as billiard balls. But science is overflowing with examples of the second sort. Very often, familiar
4 In this context, nomological simply means “relative to the laws of nature.”
Explanation 7 phenomena are explained by less familiar things, such as when we explain the range of colors of the rainbow, with which we are very familiar, using the laws of reflection and refraction of light, with which we are certainly less familiar. That the proposed model of what a scientific explanation is does not imply that these explanations work by reduction to the familiar is a good thing if it is simply not true that all scientific explanations work by reduction to the familiar.
Condition (R2) enables the distinction of scientific explanations from pseudoexplanations. Carnap (1966) explores the example of the vitalist theories of German biologist and philosopher Hans Driesch. Driesch proposed explaining the various phenomena of life by means of the notion of entelechy. The entelechy is “some specific force that makes living beings behave in the way they behave.” The various levels of complexity in organisms correspond to various types of entelechies. What we call the spirit of a human being is nothing other than a part of its entelechy. It is this same entelechy, the vital force, that explains, for example, that skin heals over after an injury. To those who criticize the mysterious nature of the concept of entelechy, Driesch replies that it is no more mysterious than the concept of force used in physical theory. Entelechies are not visible to the naked eye, but electromagnetic force is no more observable—in both cases, we see only the effects. But, as Carnap highlights, there is a crucial difference between Driesch’s entelechies and the forces of physics. The concept of force used by physical theories is called on from within a set of laws, whether this be the general laws of motion and the law of gravitation in regards to gravitational force, or Coulomb’s law when regarding electrical force. If the concept of force has explanatory virtue, in the sense that it can be included in scientific explanations, such as the explanation of an eclipse based on the antecedent position of celestial bodies, the laws of motion, and the law of gravitation, then it is precisely because it plays a crucial part in the formulation of these general laws. No such thing occurs in the case of the entelechy: there are no laws of the entelechy. Driesch offers many zoological laws that are indeed bona fide laws, but the concept of the entelechy is nowhere to be seen, it appears at the end as something of a deus ex machina expected to explain away the mystery of life. For Carnap this firmly establishes that entelechy explanations are mere pseudo-explanations, so that a virtue of Hempel’s analysis of scientific explanation is precisely that it allows us to establish this.
Condition (R3) means that the statements in the explanans can be tested, at least in principle. It is redundant if the explanandum is indeed an empirical fact, since in that case the very fact that the explanandum is a consequence of the explanans enables it to be tested. Its inclusion alongside (R1) and (R2) is no doubt a sign of Hempel and Oppenheim’s resolutely empiricist mindset.
Condition (R4) makes the concept of explanation an objective one. Without (R4), the concept of explanation is relative to a theoretical framework. The flaming of a match can be deduced from the presence of phlogiston5 and the law dictating that phlogiston
5 In the chemical theory preceding Lavoisier’s modern theory, phlogiston was a hypothetical substance supposedly found in all flammable materials and would dissipate into the air during combustion, thus explaining the decrease in mass observed subsequent to combustion.
