CHEMISTRY, PHILOSOPHY OF

Ideas about the diversity of matter in terms of elements and compound substances and their transformations have been pivotal to any scientific or prescientific approach to nature. From ancient natural philosophy and alchemy to modern nineteenth-century chemistry, these ideas were made the basis of philosophical systems and became the target of critical reflection. After a temporary interruption when modern philosophy of science focused on mathematical physics, philosophy of chemistry emerged anew in the 1980s and has become a flourishing field in which philosophers, chemists, and historians of chemistry are engaged. While many of the old philosophical issues have been rediscovered and discussed, new issues have also appeared as a result of shifts of general philosophical focus, alliances with historians and sociologists of science, the development of chemistry, and changes in its role in society.

Ontological Issues

The objects of chemistry are subject to many ontological debates beyond simple issues of definition, and these debates also have an impact on epistemological and methodological issues. Following the example of microphysics, many philosophers and chemists take atoms and molecules as the basic objects of chemistry. Yet despite the numerous techniques available to visualize molecules, the notion of a molecule is a theoretical concept with many model assumptions that do not apply to nonmolecular substances, such as water, metals, and salts. It is not so much the lack of optional microstructural descriptions for these substances, but the variety of models, which are continuously refined and adapted to certain contexts and problems, that makes such models a weak basis for defining the basic objects of chemistry. Another option is to take material substances, either elementary or compound, as the basic objects. Yet, far from being phenomenologically given entities, pure substances are the final results of infinite purification operations; that is, they are ideal laboratory artifacts. This fact has in turn inspired operational definitions. Whether one takes microstructures or pure substances as basic is not an arbitrary decision, but rather has direct impact on chemical classification and all derived concepts, because there is no simple one-to-one relationship between the two kinds of entities. There are microstructures without corresponding pure substances, and there are substances with many different microstructures.

A second but related ontological issue is about natural kinds in chemistry. Microstructuralists, following Hilary Putnam, have claimed that water is a natural kind because it is determined by a microstructural essence. This claim faces the problems mentioned above. Yet the substance-based approach to natural kinds is confronted not only with a potentially infinite number of possibly essential properties (see below) but also with the artificiality of pure substances. Even if pure substances were stable kinds independent of our conceptualization, they are not independent of laboratory purification. Nonetheless, the experimental reproducibility of sufficiently pure substances provides, within limits, a successful operation to ensure relatively stable kinds.

A third ontological issue is about whether substances (or microstructures) or transformations are the basic objects of chemistry. This issue refers to the general debate between substance and process philosophy. If not closed in bottles, substances continuously undergo chemical reactions and are only intermediate states in an ongoing process. Quantum chemistry describes even these states as processes. Furthermore, traditional chemical characterization of substances goes by chemical properties, that is, by all the dispositions of substances to transform into other substances under certain conditions, including the presence of still other substances as reactants. Substance philosophers define a chemical reaction as the change of certain substances, whereas process philosophers define a substance by its characteristic chemical reactions. A third option, proposed by Joachim Schummer, combines substances and processes in a network of dynamic relations, as the proper object of chemical research. On this view, substances and reactivities mutually define each other. Answering the ontological question has direct consequences on whether chemists can best organize their knowledge in the form of substance databases, reaction databases, or combined substance-reaction databases.

Although all substances and transformations are usually considered objects of chemistry, the metaphysical distinction between natural and synthetic pervades both commonsense and chemical reasoning. Yet the notion of natural substances—substances that can be isolated from natural resources by purification—is questionable. Not only is purification a technical operation; also, most elements would have to count as synthetic when natural resources are lacking. On the other side of the ledger, all substances that can be isolated from natural resources can also be synthesized in the early twenty-first century, which undermines the distinction. Furthermore, we have little evidence to claim that a synthetic substance will never be isolable from natural resources in the entire universe.

Epistemological and Methodological Issues

A central epistemological issue is whether chemical knowledge can be complete or not. Microstructural essentialists claim that a perfect microstructural description of any substance yields complete chemical knowledge. However, chemical properties are not manifest properties but dispositional relations (that is, relations of the form "A under certain conditions is disposed to react with B to form C and D "). This means that the structure of experimental chemical knowledge is relational, dispositional, and open-ended. Because new properties are defined by new conditions and new potential reactants (currently produced at 15.5 million new chemicals per year), experimental chemical knowledge can increase indefinitely without reaching a state of being complete. It is an open question to what extent theoretical approaches can compensate for the incompleteness on the experimental level.

Chemistry differs from other sciences in that its theoretical concepts need to serve different methodological goals. Besides the traditional goals of accurately describing, explaining, and predicting phenomena, theoretical concepts in chemistry also fulfill purposes of classification and synthesis. By 2004 the chemical classification system had distinguished 78.3 million different substances and ordered them by classes and subclasses. And beyond mere prediction of phenomena, theoretical concepts provide experimental guidelines for producing millions of new substances and reactions per year. For all three methodological goals, the main theoretical approach has been chemical-structure theory, which emerged in mid-nineteenth century and has been influenced and diversified by many different developments since, including quantum chemistry and spectroscopic instrumentation. Apart from this theory, a multitude of other theoretical concepts and models have been developed for particular substance classes and phenomena and for various purposes.

The main methodological issue in current philosophy of chemistry is to bring order to this complex picture without imposing upon chemistry methodologies tailored to other disciplines. Several case studies have shown that received approaches, for instance, Karl Popper's view that science makes progress by falsifying theories, are rather useless in chemistry. There is some agreement that chemists favor methodological pluralism and pragmatic application of models, rather than methodological universalism and the ideal of a single axiomatic theory. A study on scientific realism has suggested that entity realism, rather than theory realism, is a more appropriate methodological ideal in chemistry. The received methodological focus on methods of justification has been widened to include methods for research, that is, for developing new knowledge. Many detailed studies on the different kinds and uses of models in chemistry, from theoretical chemistry to chemical engineering, have been undertaken. Besides the impact of quantum mechanics (see the next section), the impact of spectroscopic instrumentation on theoretical concepts since the mid-twentieth century has received considerable attention, in fact, so much attention that interest in the "instrumental revolution" has replaced the older focus on the eighteenth-century "chemical revolution" by Antoine-Laurent Lavoisier and others. The methodological integration of both chemical analysis and synthesis, which form the major experimental activity of chemists, has overcome received distinctions between science and technology. Studies on the formal sign-language system of chemistry, consisting of structural formulas and reaction mechanisms, have illuminated its multipurpose theoretical capacity, but further studies are required to understand changes stemming from various theoretical and experimental developments.

Reducibility to Physics

Whether chemistry is reducible to physics is a question that could come up only in the mid-nineteenth century, when modern physics emerged as its own discipline, because the former meaning of "physics" (natural science or natural philosophy) included chemistry as a branch. Before then, mechanical (physical) approaches were among several competing approaches within theoretical chemistry, though not very successful. The question became meaningful only with the development of quantum mechanics and its application to chemistry since the late 1920s. Following a speech by Paul Dirac in 1929, many quantum physicists and philosophers of physics have taken for granted that the whole of chemistry would be reducible to quantum mechanics, and so would be part of physics.

Wary of making such bold claims, philosophers have carefully distinguished between different meanings of "reduction." An ontological reduction claims that the supposed objects of chemistry are actually nothing other than the objects of quantum mechanics and that quantum-mechanical laws govern their relations. In its strong, eliminative version, an ontological reduction states that there are no chemical objects proper. Antireductionists argue that theoretical entities are determined by their corresponding theories, and that theoretical entities of different theories cannot be identified. For instance, from the different meanings of the term "electron" in quantum electrodynamics and in chemical-reaction mechanisms, they conclude that the term "electron" has different references, which rules out an ontological reduction. An epistemological or theoretical reduction claims that all theories, laws, and fundamental concepts of chemistry can be derived from first-principle quantum mechanics as a more basic and more comprehensive theory. This claim has prompted many detailed studies (see below). Methodological reductionism, while acknowledging the current failure of epistemological reduction, recommends applying quantum-mechanical methods to all chemical problems, because that would be the most successful approach in the long run (approximate reductionism). But the mere promise of future success is not convincing unless accompanied by a comparative assessment of different methods. By modifying the popular notion that the whole is nothing but the sum of its parts, philosophers have developed two further versions of reductionism. Emergentism acknowledges that new properties of wholes (say of water) emerge when the parts (say oxygen and hydrogen) are combined, but it does not deny that the properties of the whole can be explained or derived from the relations between the parts (epistemological reductionism). Supervenience, in a simple version, means that, although epistemological reductionism might be wrong, the properties of a whole asymmetrically depend on the properties of the parts, so that every change of the properties of the whole is based on changes of the properties of the parts or the relations between the parts, but not the other way round. When these terms are applied to the reduction of chemistry to quantum mechanics, that is, to chemical entities as wholes and quantum-mechanical entities as parts, emergentism and supervenience presuppose elements of epistemological or ontological reductionism. Thus, criticism of these positions applies accordingly. For instance, if one denies that chemical electrons are the same as quantum-electrodynamic electrons or, more generally, that quantum-mechanical entities are proper parts of chemical wholes, one ends up rejecting supervenience altogether.

Recent criticism has focused on epistemological reductionism by pointing out the technical limits of quantum mechanics with regard to particular chemical concepts, laws, and problems. Two quantum chemists, Guy Woolley and Hans Primas, have shown that the concept of molecular structure, which is central to most chemical theories, cannot be derived from first-principle quantum mechanics, because molecular structures cannot be represented by quantum-mechanical observables. Eric Scerri has argued that current quantum-mechanical approaches cannot calculate the exact electronic configuration of atoms, which was formerly considereda successful reduction of the chemical law that underlies the periodic system of elements. Jaap van Brakel has pointed out that successful applications of quantum mechanics to chemical problems frequently include model assumptions and concepts taken from chemistry. Joachim Schummer has argued that quantum-mechanical approaches are nearly absent and useless in areas that chemists are mainly concerned with: chemical reactions, synthesis, and classification.

Criticism of the reduction of chemistry to quantum mechanics, as the lowest level in the standard hierarchy of reductions, also challenges microreductionism as a general position and thus contributes to general philosophy. In the most detailed philosophical study on various forms of reductionism, Jaap van Brakel has used the case of chemistry to argue for a kind of pragmatism in which the "manifest image" of common sense and the empirical sciences is epistemologically primary over the "scientific image" of microphysics. Nikos Psarros presupposes a rejection of reductionism in his extensive project of seeking the cultural foundation of chemical concepts, laws, and theories in prescientific cultural practices, norms, and values. For many others, including Joachim Schummer, rejecting reductionism supports a pragmatist and pluralist position that clearly distinguishes between fields of research where quantum-mechanical approaches are strong and even indispensable and those where they are poor or even useless compared to other approaches. Once reductionism has lost its function of securing the unity of the sciences, new relationships between chemistry and other disciplines could become subject to philosophical and historical investigations, including studies of such multidisciplinary fields as atmospheric science, biomedical science, materials science, and nanotechnology.

Further Topics

Current philosophy of chemistry reaches far beyond ontological, epistemological, and methodological issues. On the one hand, there are strong trends in historical research. Pertinent classical works on chemistry by such philosophers as Aristotle, Immanuel Kant, Georg Wilhelm Friedrich Hegel, Pierre Duhem, Ernst Cassirer, and Gaston Bachelard have been rediscovered, and these have allowed reinterpretations of the history of philosophy of science. Philosophical works by chemists of the past, such as Benjamin C. Brodie, Wilhelm Ostwald, Frantisek Wald, Edward F. Caldin, Fritz Paneth, and Michael Polanyi, have also been rediscovered. Historians and philosophers of chemistry have explored the development of many fundamental concepts in chemistry, such as chemical substance, element, atom, the periodic system of elements, molecular structure, chemical bond, chemical reaction, affinity, and aromaticity. In addition, important historical developments in chemistry have been philosophically scrutinized, such as the transitions from alchemy to modern chemistry and from phlogistic to antiphlogistic chemistry; the emergence of physical chemistry, quantum chemistry, and biochemistry; and the development of molecular-model building and instrumentalization.

On the other hand, philosophers of chemistry have also applied theoretical insights to practical problems, discovered a wider spectrum of philosophical perspectives on chemistry, and engaged in contemporary issues. Epistemological and ontological studies have found useful applications in chemistry education and information management. Beyond the traditional scope of philosophy of science, perspectives on chemistry from philosophy of technology, language, culture, and literature, and from metaphysics, aesthetics, ethics, sociology, and public understanding of science have all been exploited. For instance, studies on the role of visualization and aesthetics in chemical research have been undertaken to understand the heuristics and dynamics of research in a broader cultural context beyond traditional epistemic and technological goals. Philosophers and historians have investigated the historical roots and the cultural value conflicts underlying the widespread chemophobic attitude of society and the peculiar opposition of natural versus chemical. In addition to taking up general professional ethics, philosophers have challenged the legitimacy of chemical-weapon research, questioned the alleged moral neutrality of synthesizing new substances for scientific purposes, discussed the scope of moral responsibility of chemists for their synthetic products, and developed moral frameworks for assessing chemical-research practice. Finally, with the rise of nanotechnology, in which chemistry is particularly involved, philosophers of chemistry have taken a leading role in discussing the societal and ethical implications of this nanotechnology of the ultra-small.

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Joachim Schummer (2005)