Décio Krause

The so-called Received View (RV) on quantum non-individuality states, basically, that quantum particles are not individuals. It has received an amount of criticism in the recent literature, most of it concerning the relation between the RV and the relation of identity. In this chapter we carefully characterise a family of concepts involved in clarifying the view, indicating how the very idea of failure of identity, commonly used to define the RV, may be understood. By doing so, we hope to dissipate some misunderstandings about the RV, which shall also be seen as evidence of its tenability.

This book provides new approaches to the debate on individuality and non-individuality in quantum mechanics, with chapters written by leading experts in the field. A significant issue in the philosophy of quantum mechanics, the choice between individuality and non-individuality, and the well-known metaphysical underdetermination between these options in quantum theory, receive here new treatment, including metaphysical, logical, and methodological analyses. Scholars and students engaged in the philosophy of quantum mechanics and the metaphysics of science, as well as philosophers concerned with the impact of quantum mechanics on their disciplines will find this book to not only new proposals and arguments for the various sides of the debate, but also new venues for exploring the subject further.

This chapter argues that the general philosophy of science should learn metaphilosophical lessons from the case of metaphysical underdetermination, as it occurs in non-relativistic quantum mechanics. Section presents the traditional discussion of metaphysical underdetermination regarding the individuality and non-individuality of quantum particles. Section discusses three reactions to it found in the literature: eliminativism about individuality; conservatism about individuality; eliminativism about objects. Section wraps it all up with metametaphysical considerations regarding the epistemology of metaphysics of science.

Inconsistencies! What do they mean? Can we support them? With this paper, we hope to contribute to the claim that we can tolerate inconsistencies in certain situations even without considering any logic that may enable us to do that, say some paraconsistent logic. We argue that in many cases where we apply reason we work in domains where inconsistencies appear and even so we neither get them out (but ‘support’ them) nor modify the underlying logic (such as classical logic) to avoid logical troubles. To make things more precise, we distinguish between inconsistency, anomaly, and contradiction. Our thesis is that we can reason sensibly even with classical logic in the presence of inconsistencies once (as we explain) we either ‘do not go there’ or make things so that the inconsistencies cannot be joined to arrive at a contradiction. Some sample cases are given to motivate the discussion.

This book provides new approaches to the debate on individuality and non-individuality in quantum mechanics, with chapters written by leading experts in the field. A significant issue in the philosophy of quantum mechanics, the choice between individuality and non-individuality, and the well-known metaphysical underdetermination between these options in quantum theory, receive here new treatment, including metaphysical, logical, and methodological analyses. Scholars and students engaged in the philosophy of quantum mechanics and the metaphysics of science, as well as philosophers concerned with the impact of quantum mechanics on their disciplines will find this book to not only new proposals and arguments for the various sides of the debate, but also new venues for exploring the subject further.

In this paper we argue that physical theories, including quantum mechanics, refer to some kind of ‘objects’, even if only implicitly. We raise questions about the logico-mathematical apparatuses commonly employed in such theories, bringing to light some metaphysical presuppositions underlying such apparatuses. We point out to some incongruities in the discourse holding that quantum objects would be entities of some ‘new kind’ while still adhering to the logico-mathematical framework we use to deal with classical objects. The use of such apparatus would hinder us from being in complete agreement with the ontological novelties the theories of quanta seem to advance. Thus, we join those who try to investigate a ‘logic of quantum mechanics’, but from a different point of view: looking for a formal foundation for a supposed new ontology. As a consequence of this move, we can revisit Einstein’s ideas on physical reality and propose that, by considering a new kind of object traditionally termed ‘non-individuals’, it is possible to sustain that they still obey some of Einstein’s conditions for ‘physical realities’, so that it will be possible to talk of a ‘principle of separability’ in a sense which is not in complete disagreement with quantum mechanics. So, Einstein’s departure from quantum mechanics might be softened at least concerning a form of his realism, which sees separated physical objects as distinct ‘physical realities’.

Any scientific theory (here we consider physical theories only) has an underlying logic, even if it is not totally made explicit. The role of the underlying logic of a theory T is mainly to guide the proofs and the accepted consequences of the theory’s principles, mainly when described by its axioms. In this sense, the theorems of the underlying logic are also theorems of the theory. In most cases, if pressed, the scientist will say that the underlying logic of most physical theories is classical logic or some fragment of a set theory suitable for accommodating the theory’s mathematical and logical concepts. We argue that no physical theory and no philosophical discussion based on the theory should dismiss its underlying logic, so the arguments advanced by some philosophers of physics in that certain entities (the considered case deals with quantum entities) can be only weakly discerned or be just ‘relational’ and that they cannot be absolutely identified by a monadic property, are fallacious once one remains within a ‘standard’ logico-mathematical framework, grounded on classical logic. We also discuss the (to us) unjustifiable claim that some properties (such as those that came from logic) would be ‘illegitimate’ for discerning these entities. Thus, this paper may interest logicians, physicists, and philosophers.

Some philosophers of physics have addressed criticisms of the so-called Received View (RV) of non-individual quantum objects, also called the orthodox view. Dennis Dieks made a very good resume of these criticisms in Dieks (in: Non-reflexive logics, non-individuals, and the philosophy of quantum mechanics: essays in honor of the philosophy of Décio Krause, Synthese Library, Springer, 2023) and Bigaj (Identity and indiscernibility in quantum mechanics. New directions in the philosophy of science, Palgrave Macmillan, 2022) has a more detailed account. In considering (mainly) these works and with some additional mentions, I hope to dissipate some misunderstandings about the RV and clarify what is happening with such a view. According to Dieks, the RV doesn’t fit the practice of physics since, in some situations, the physicist assumes that quantum objects can be treated individually, imitating standard objects (individuals) of classical physics. Dieks also proposes an Alternative View (AV), generally called the heterodox view, which would give a view of the fundamental ontology of quantum physics and which would be more by the way physicists usually proceed. I think that the AV cannot be viewed as the fundamental ontological theory despite being suitable for practical purposes. Furthermore, I think that it does not conflate the RV but is complementary to it, substituting it when quantum objects are sufficiently apart and can be treated in conformity with the classical way. From my point of view, in the practice of physics, one can adopt AV. Still, the RV is more adequate when we are looking for logical and foundational analyses, at least when the supposed metaphysics comprises non-individuals.

The purpose of this paper is to argue that neither mathematics nor logic can be applied ‘directly’ to reality, but to our rational representations (or reconstructions) of it, and this is extended to scientific theories in general. The difference to other approaches (e.g., Nancy Cartwright’s, Bueno & Colyvan’s or Hughes’) is that I call attention to something more than what is involved in such a process, namely, metamathematics. A general schema of ‘elaboration’ of theories, which I suppose cope with most of them, is presented and discussed. A case study is outlined, the quantum case, whose anchored description, in my opinion, demands a different metamathematics and a different logic.

Schrödinger logics are logical systems in which the principle of identity is not true in general. The intuitive motivation for these logics is both Erwin Schrödinger's thesis (which has been advanced by other authors) that identity lacks sense for elementary particles of modern physics, and the way which physicists deal with this concept; normally, they understandidentity as meaningindistinguishability (agreemment with respect to attributes). Observing that these concepts are equivalent in classical logic and mathematics, which underly the usual physical theories, we present a higher-order logical system in which these concepts are systematically separated. A ‘classical’ semantics for the system is presented and some philosophical related questions are mentioned. One of the main characteristics of our system is that Leibniz' Principle of the Identity of Indiscernibles cannot be derived. This fact is in accordance with some authors who maintain that quantum mechanics violates this principle. Furthermore, our system may be viewed as a way of making sense some of Schrödinger's logical intuitions about the nature of elementary particles.

This book provides an interdisciplinary perspective on one of the most fascinating and important open questions in science: What is quantum mechanics talking about? Quantum theory is perhaps our best confirmed physical theory. However, despite its great empirical effectiveness and the subsequent technological developments that it gave rise to in the 20th century, from the interpretation of the periodic table of elements to CD players, holograms and quantum state teleportation, it stands even today without a universally accepted interpretation. The novelty of the book comes from the multiple viewpoints and subjects investigated by a group of researchers from Europe and North and South America.

Nonreflexive quantum mechanics is a formulation of quantum theory based on a non- classical logic termed nonreflexive logic. In these logics, the standard notion of identity, as encapsulated in classical logic and set theories, does not hold in full. The basic aim of this kind of approach to quantum mechanics is to take seriously the claim made by some authors according to whom quantum particles are non-individuals in some sense, and also to take into account the fact that they may be absolutely indistinguishable. The nonreflexive formulation of quantum theory assumes these features of the objects already at the level of the underlying logic, so that no use is required of symmetrization postulates or other mathematical devices that serve to pretend that the objects are indiscernible. Here, we present the ideas of the development of nonreflexive quantum mechanics and discuss some philosophical motivations and consequences of it.

In this paper we deal with two applications of the square of opposition to controversial issues in the philosophy of quantum mechanics. The first one concerns the kind of opposition represented by states in superposition. A superposition of “spin up” and “spin down” for a given spatial direction, for instance, is sometimes said to originate particular kinds of opposition such as contradictoriness. The second application concerns the problem of identical particles. Identity and indiscernibility are entangled in discussions of this problem in such a way that a proper conceptual treatment of those issues through the square seems profitable.

This paper is a continuation of the authors' attempts to deal with the notion of indistinguishability (or indiscernibility) from a logical point of view. Now we introduce a two-sorted first-order modal logic to enable us to deal with objects of two different species. The intended interpretation is that objects of one of the species obey the rules of standard S5, while the objects of the other species obey only the rules of a weaker notion of indiscernibility. Quantum mechanics motivates the development. The basic idea is that in the ‘actual’ world things may be indiscernible but in another accessible world they may be distinguished in some way. That is, indistinguishability needs not be seen as a necessary relation. Contrariwise, things might be distinguished in the ‘actual’ world, but they may be indiscernible in another world. So, while two quantum systems may be entangled in the actual world, in some accessible world, due to a measurement, they can be discerned, and on the other hand, two initially separated quantum systems may enter in a state of superposition, losing their individualities. Two semantics are sketched for our system. The first is constructed within a standard set theory (the ZFC system is assumed at the metamathematics). The second one is constructed within the theory of quasi-sets, which we believe suits better the purposes of our logic and the mathematical treatment of certain situations in quantum mechanics. Some further philosophically related topics are considered.

In this paper we consider the phenomenon of superpositions in quantum mechanics and suggest a way to deal with the idea in a logical setting from a syntactical point of view, that is, as subsumed in the language of the formalism, and not semantically. We restrict the discussion to the propositional level only. Then, after presenting the motivations and a possible world semantics, the formalism is outlined and we also consider within this schema the claim that superpositions may involve contradictions, as in the case of the Schrödinger's cat, which is both alive and dead. We argue that this claim is a misreading of the quantum case. Finally, we sketch a new form of quantum logic that involves three kinds of negations and present the relationships among them. The paper is a first approach to the subject, introducing some main guidelines to be developed by a `syntactical' logical approach to quantum superpositions.

We discuss the idea that superpositions in quantum mechanics may involve contradictions or contradictory properties. A state of superposition such as the one comprised in the famous Schrödinger’s cat, for instance, is sometimes said to attribute contradictory properties to the cat: being dead and alive at the same time. If that were the case, we would be facing a revolution in logic and science, since we would have one of our greatest scientific achievements showing that real contradictions exist.We analyze that claim by employing the traditional square of opposition.We suggest that it is difficult to make sense of the idea of contradiction in the case of quantum superpositions. From a metaphysical point of view the suggestion also faces obstacles, and we present some of them.

The notion of an individual and the related issues on individuation are topics that appear in the philosophical discussion ever since the antiquity. The idea of an individual thing is intuitively clear: an individual is something of a specific kind that is a unity, having its own identity, and being so that it is possible at least in principle to discern it from any other individual, even of similar species. But when we try to leave the intuitive realm and push this idea to a logical analysis, we find a cluster of problems that are difficult to overcome within standard logico-mathematical contexts. In this work, we shall be concerned with some aspects of this intuitive concept of an individual and on some related facts about individuation taken from recent discussions that arose ever since the inception of quantum theory, pushing the discussion to a “logical” view, which in our opinion is still lacking in the usual debates on such issues. In the final part of the paper, we propose a metaphysics where the notion of identity is substituted, for some objects, by a weaker notion of indiscernibility, and we try to justify such a move. In most of the uses of the expression “quantum theory”, we shall not make explicit the distinction between the non-relativistic and the relativistic approaches, although they of course are quite different, for we think that the problems as we shall present them appear in both versions. But, as the text goes, the context will distinguish between them and these questions will become clear to the reader.

In this paper we consider the notions of structure and models within the semantic approach to theories. To highlight the role of the mathematics used to build the structures which will be taken as the models of theories, we review the notion of mathematical structure and of the models of scientific theories. Then, we analyse a case-study and argue that if a certain metaphysical view of quantum objects is adopted, namely, that which sees them as non-individuals, then there would be strong reasons to ask for a different mathematical framework for describing the structures that would be the models of the corresponding theory. In departing from the standard frameworks, we hope to bring to the scene, within the scope of the semantic approach, the importance of paying attention to some fundamental concepts usually only superficially touched by philosophers of science.

In this basically expository paper we discuss the role oflogic and mathematics in researches concerning the ontology of scientific theories, and we consider the particular case of quantum mechanics. We argue that systems of logic in general, and classical logic in particular, may contribute substantially with the ontology of any theory that has this logic in its base. In the case of quantum mechanics, however, from the point of view of philosophical discussions conceming identity and individuality, those contributions may not be welcome for a specific interpretation, and an altemative system of logic perhaps could be used instead of a classical system. In this sense, we argue that the logic and ontology of a scientific theory may be seen as mutually inftuencing each other. On the one hand, logic contributes to shape the general features of the ontology of a theory; on the other hand, the theory also puts constraints on the possible understanding of ontology and, respectively, on possible systems of logic that may be the underlying logic ofthe theory.

In this work, we focus on a very specific case study: assuming that quantum theories deal with “particles” of some kind, what kind of entity can such particles be? One possible answer, the one we shall examine here, is that they are not the usual kind of object found in daily life: individuals. Rather, we follow a suggestion by Erwin Schrödinger, according to which quantum mechanics poses a revolutionary kind of entity: non-individuals. While physics, as a scientific field, is not concerned with whether entities posited by a specific physical theory are individuals or not, answering this question is part of the quest for a better understanding of physical reality. Here lies, in large measure, the relevance of ontology.

It is supposed that any scientific theory (here we consider physical theories only) has an underlying logic, even if it is not made explicit. The role of the underlying logic of a theory T is mainly to guide the proofs and the accepted consequences of the theory’s principles, usually described by its axioms. In this sense, the theorems of the underlying logic are also theorems of the theory. In most cases, if pressed, the scientist will say that the underlying logic of most physical theories is classical logic or some fragment of a set theory suitable for accommodating the theory’s mathematical and logical concepts. We argue that no physical theory and no philosophical discussion on the basis of the theory should dismiss its underlying logic, so the arguments advanced by some philosophers of physics in that certain entities (the considered case deals with quantum entities) can be only weakly discerned or be just ‘relationals’ and that they cannot be absolutely identified by a monadic property, are fallacious if one remains within a ‘standard’ logico-mathematical framework, grounded on classical logic. We also discuss the claim that some properties (as those that came from logic) would be ‘illegitimate’ for discerning these entities is not justifiable. Thus, this paper may be interesting for logicians, physicists, and philosophers.

El tema de esta contribución me fue sugerido por el Profesor Michel Paty. Sabiendo que estoy muy interesado en la cuestión de la indiscernibilidad (o indistinguibilidad, palabra que aquí usaré como sinónima) de las partículas elementales y de sus consecuencias lógicas y filosóficas, él me hizo la excelente sugerencia de que escribiera esta pequeña nota sobre el papel desempeñado por Einstein en la consolidación de ese concepto, y también para hablar un poco de sus consecuencias lógicas y filosóficas para las bases de la física (y de la lógica) presente. En este artículo hago una revisión de algunos de los temas de naturaleza lógica relacionados con esa problemática, sin entrar en los detalles técnicos de la física, los cuales remito a los textos indicados en las referencias.