Nature, at its microphysical level, constitutes the subject matter of quantum theory, also known as quantum mechanics. Never in the history of physics has there been a theory that has changed so drastically the shape of science as quantum mechanics; nor has there been a scientific theory that has had such a profound impact on human thinking. Since its inception in the early part of the 20th century, quantum mechanics has played, and still does play, a significant role in philosophical thought both as a source of metaphysical ideas and as an important example of a scientific revolution. Thus, the advent of the quantum paradigm has gradually challenged the traditional philosophical substratum of science, the representational-visualizable description of micro-physical entities and phenomena, the commonly perceived part-whole relationship that is built into classical physics, the relationship of cause and effect, the unrestricted validity of deterministic laws, and even the nature of physical reality and its independence from the process of knowledge.
Quantum mechanics is a statistical theory of the microscopic structure of matter. Within the standard framework, the codification “statistical theory” should not be understood as referring to our incomplete knowledge of the real physical state of a system, or to our computational insufficiency arising because of the complexities of the microphysical situation. On the contrary, it should be considered as an aspect of the irreducibly probabilistic behavior of matter at the microscopic level of description, as a genuine feature of reality that breaks with the apparent determinism of the classical Newtonian world. Quantum features such as noncommutativity, nonlocality, non-separability, and the generalized phenomenon of quantum entanglement have been forcing us to revise radically the intuitive classical ideas about physical reality. According to our current understanding, for a consistent realist interpretation of quantum theory, the concept of realism must not be associated with ideas taken over from classical physics, such as atomism; localizability; separability; or similar philosophical presuppositions such as strict subject-object partition, mechanistic determinism, and ontological reductionism.
The Classical Conception of Nature
Classical physics is essentially atomistic in character. It portrays a view of the world in terms of analyzable, separately existing, but interacting self-contained parts. Classical physics is also reductionistic. It aims to explain the whole of forms of physical existence, of structures and relations of the natural world in terms of a salient set of elementary material objects linked by forces. Classical physics (and practically any experimental science) is further based on the Cartesian dualism of res cogitans (“thinking substance”) and res extensa (“extended substance”), proclaiming a radical separation of an objective, external world from the knowing subject that allows no possible intermediary.
In fact, the whole edifice of classical physics—be it point-like analytic, statistical, or field theoretic—is compatible with the following separability principle, which can be expressed schematically as follows:
Separability Principle: The states of any spatio-temporally separated subsystems S1, S2,…, SN of a compound system S are individually well-defined, and the states of the compound system are wholly and completely determined by them and their physical interactions, including their spatio-temporal relations.
The aforementioned separability principle delimits the fact, upon which the whole classical physics is implicitly founded, that any compound physical system of a classical universe can be conceived of as consisting of separable, individual parts interacting by means of forces that are encoded in the Hamiltonian function of the overall system (determining the total energy of the system), and that, if the full Hamiltonian is known, maximal knowledge of the values of the physical quantities pertaining to each one of these parts yields an exhaustive knowledge of the whole compound system.
The notion of separability has been viewed within the framework of classical physics as a principal condition of our conception of the world, a condition that characterizes all our thinking in acknowledging the physical identity of distant things, the independent self-autonomous existence of spatio-temporally separated systems. The primary implicit assumption pertaining to this view is a presumed absolute kinematic independence between the knowing subject and the object of knowledge, or equivalently, between the measuring system (as an extension of the knowing subject) and the system under measurement. The idealization of the kinematically independent behavior of a physical system is possible in classical physics because of both the Cartesian-product structure of phase space, namely, the state space of classical theories, and the absence of genuine indeterminism in the course of events or of an element of chance in the measurement process. During the act of measurement, a classical system conserves its identity. Successive measurements of physical quantities, such as position and momentum, which define the state of a classical system, can be performed to any degree of accuracy. The results combined can completely determine the state of the system before and after the measurement interaction, because its effect, if not eliminable, takes place continuously in the system’s state space and is therefore predictable in principle.
Consequently, classical physical quantities or properties are taken to obey a so-called possessed values principle, in the sense that the values of classical properties are considered as being possessed by the object itself independently of any measurement act. That is, the properties possessed by an object depend in no way on the relations obtaining between it and a possible experimental context used to bring about these properties. No qualitatively new elements of reality are produced by the interaction of a classical system with the measuring apparatus. The act of measurement in classical physics is passive; it simply reveals a fact that has already occurred. In other words, a substantial distinction between potential and actual existence is rendered obsolete in classical mechanics. Within the domain of the latter, all that is potentially possible is also actually realized in the course of time, independently of any measuring interventions. It should be noted, in this respect, that this is hardly the case in the quantum theory of the measurement process.
The Quantum Conception of Nature
The Generalized Phenomenon of Nonseparability
In contrast to classical physics, standard quantum mechanics systematically violates the conception of separability. From a formal point of view, the source of its defiance is due to the tensor-product structure of Hilbert-space quantum mechanics and the superposition principle of states, which incorporates a kind of objective indefiniteness for the numerical values of any physical quantity belonging to a superposed state. The generic phenomenon of quantum nonseparability, experimentally confirmed for the first time in the early 1980s, precludes in a novel way the possibility of defining individual micro-objects independently of the conditions under which their behavior is manifested. Even in the simplest possible case of a compound system S consisting of just two subsystems, S1 and S2, that have interacted at some time in the past, the compound system should be treated as a nonseparable, entangled system, however large is the distance among S1 and S2. In such a case, it is not permissible to consider subsystems S1 and S2 individually as distinct entities that enjoy intertemporal identity. The global character of their behavior precludes any description or any explanation in terms of individual systems, each with its own well-defined state or predetermined physical properties. Only the compound system S, as a whole, is assigned a well-defined (non-separable) pure state. Therefore, when a compound system such as S is in an entangled state, namely a superposition of pure states of tensor-product forms, maximal knowledge of the whole system does not allow maximal knowledge of its component parts, a circumstance with no precedence in classical physics. Erwin Schrödinger, one of the eminent founders of quantum theory, explicitly anticipated this counterintuitive state of affairs by acknowledging the phenomenon of nonseparability as the most characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought.
The generic phenomenon of quantum nonseparability casts severe doubts on the existence of isolated (sub)systems and the applicability of the notion of atomism, in the sense that the parts of a quantum whole no longer exist as precisely defined individual entities. The nonseparable behavior of an entangled quantum system precludes the possibility of describing its component subsystems in terms of well-defined pure states. In fact, whenever the pure entangled state of a compound system is decomposed in order to represent subsystems, the effect can extend only up to a representation in terms of incompletely specified statistical (reduced) states of those subsystems. Consequently, the state of the whole system cannot be determined by the states of its component parts, this being the case even when these parts occupy distinct regions of space however far apart, because at the quantum domain, in complete contrast to classical physics, it is only the compound state of the whole system that exhaustively specifies the probabilistic entangled correlations among the states of the parts. In this respect, the phenomenon of quantum nonseparability undeniably reveals the holistic character of entangled quantum systems. Quantum mechanics is the first—and right now the only—logically consistent, mathematically formulated, and empirically well-confirmed theory that incorporates as its basic feature that the “whole” is, in a nontrivial way, more than the sum of its “parts,” including their spatiotemporal relations and physical interactions. Contrary to the situation in classical physics, when considering an entangled compound system, “whole” and “parts” are dynamically related in such a way that their bidirectional reduction is, in principle, impossible. 
The Context Dependence of Objects
From a foundational viewpoint of quantum theory, quantum mechanical nonseparability and the sense of quantum holism arising out of it refer to a context-independent, and thus to an observer- or mind-independent, reality. The latter is operationally inaccessible. It pertains to the domain of entangled correlations and quantum superpositions obeying a non-Boolean logical structure. Here, the notion of an object whose aspects may result in intersubjective agreement enjoys no a priori meaning independent of the phenomenon into which it is embedded. In quantum mechanics, in order to be able to speak meaningfully about an object, to obtain any kind of description, or to refer to experimentally accessible facts, the underlying wholeness of nature should be decomposed into interacting but disentangled subsystems, namely, into “measured objects” and “detached observers” (measuring apparata) with no (or insignificantly) entangled correlations among them. This subject-object separation is sometimes metaphorically called the Bohr/Heisenberg cut.
The presuppositions of the latter are automatically satisfied in classical physics, in conformity with the separability principle mentioned earlier. In a non-separable theory like quantum mechanics, however, the concept of the Bohr/Heisenberg cut acquires the status of a methodological regulative principle through which access to empirical reality is rendered possible. The innovation of the Bohr/Heisenberg cut, and the associated separation of a quantum object from its environment, is mandatory for the description of measurements. It is, in fact, necessary for the operational account of any directly observable pattern of empirical reality. The very possibility of devising and repeating a controllable experimental procedure presupposes the existence of such a subject-object separation. Without it, the concrete world of material facts and data would be ineligible; it would be conceived in a totally entangled manner. In this sense, a physical system may count as an experimental or a measuring device only if it is not holistically correlated or entangled with the object under measurement.
Consequently, any atomic fact or event that happens is raised at the observational level only in conjunction with the specification of an experimental arrangement—an experimental context that conforms to a Boolean domain of discourse—namely, to a set of observables co-measurable by that context. In other words, there cannot be well-defined events in quantum mechanics unless a specific set of co-measurable observables has been singled out for the system-experimental context whole, because in the quantum domain, one cannot assume, without falling into contradictions, that observed objects enjoy a separate, well-defined identity irrespective of any particular context. One cannot assign, in a consistent manner, definite sharp values to all quantum mechanical observables pertaining to a microscopic object, in particular to pairs of incompatible observables, independently of the measurement context actually specified. In terms of the structural component of quantum theory, this is due to functional relationship constraints that govern the algebra of quantum mechanical observables, as revealed by the Kochen-Specker theorem and its recent investigations. In view of them, it is not possible, not even in principle, to assign to a quantum system noncontextual properties corresponding to all of its observable quantities. In fact, any attempt to simultaneously attribute context-independent, sharp values to all observables of a quantum object forces the quantum statistical distribution of value assignment into the pattern of a classical distribution, thus leading, for instance, to contradictions of the so-called Greenberger-Horne-Zeilinger (GHZ) type.
This state of affairs reflects most clearly the unreliability of the commonsensical possessed values principle of classical physics, according to which the values of physical quantities are regarded as being possessed by an object independent of any measurement context. The classical-realist underpinning of such an assumption is conclusively shown to be incompatible with the structure of the algebra of quantum mechanical observables. In general, well-defined values of quantum observables can be regarded as pertaining to an object of our interest only within a framework involving the experimental conditions. The latter provide the necessary conditions whereby we make meaningful statements that the properties we attribute to quantum objects are part of physical reality. Consequently, the nature of quantum objects is a context-dependent issue, with the experimental procedure supplying the physical context for their being. Because of the genuinely non-separable structure of quantum mechanics, a quantum object can produce no informational content that may be subjected to experimental testing without the object itself being transformed into a contextual object. In this respect, the introduction of the experimental context operates as a formative factor on the basis of which a quantum object manifests itself. The classical idealization of sharply individuated objects possessing intrinsic properties and having an independent reality of their own breaks down in the quantum domain. Quantum mechanics describes physical reality in a substantially context-dependent manner.
Accordingly, quantum objects cannot be conceived of as things in themselves, as absolute bare particulars of reality, enjoying intrinsic individuality. Instead, they represent carriers of patterns or properties that arise in interaction with their experimental context/ environment or, more generally, with the rest of the world; the nature of their existence—in terms of state-property ascription—depends on the context into which they are embedded and on the subsequent abstraction of their entangled correlations with the chosen context of investigation. As Werner Heisenberg has frequently declared, what we observe is not nature itself, but nature exposed to our method of inquiry. Or, to put the same in purely physical terms, because of underlying quantum nonseparability, the world appears as a complex whole. However, once a particular question is put to nature and therefore a given context is specified, the oneness of the whole falls apart into apparent parts. Thus, whereas quantum nonseparability refers to an inner level of reality— a mind-independent reality that is operationally elusive—the introduction of a context is related to the outer level of reality—the contextual or empirical reality that results as an abstraction in the human perception through deliberate negligence of the all-pervasive entangled correlations between observed objects and their environments. The latter abstraction is dictated by the fact that, within the framework of quantum mechanics, it is solely the process of disentanglement that generates the perceptible separability and determinateness of the objects of empirical reality by means of an experimental intervention that suppresses (or sufficiently minimizes) the factually existing entangled correlations of the object concerned with its environment. It is then justified to say that the fulfillment of the disentanglement procedure provides a level of description to which one can associate a separable, albeit contextual, concept of reality whose elements are commonly experienced as distinct, well-localized objects having determinate properties.
Active Scientific Realism
The context dependence of quantum objects does not contradict a realistic view of the world as usually assumed. Scientific realism and quantum nonseparability are not incompatible, but the relationship between them points to the abandonment of the classical conception of physical reality and its traditional metaphysical presuppositions, most notably atomism and strict ontological reductionism. Because of the substantially context-dependent description of reality suggested by quantum mechanics, one ought to acknowledge that the patterns of nature emerge and become intelligible through an active participation of the knowing subject that, by itself, is not and cannot be perspective-independent. In this sense, the conceptualization of the nature of reality, as arising out of our most basic physical theory, calls for a kind of contextual realism that we call active scientific realism.
It is active because it indicates the contribution of rational thought to experience; it acknowledges the participatory role of the knowing subject in providing the context of discourse; as previously argued, the identification of a specific pattern as an object depends on the process of knowledge, on the abstraction or suppression of the entangled correlations between the object concerned and its external environment, and this may be done in different, physically nonisomorphic ways depending on the chosen context of investigation.
It is realism because, given a context, concrete objects (structures of reality) have well-defined properties independent of our knowledge of them. According to the conception of active scientific realism, the experienced or empirical reality is a functional category. Functional to the engaging role of the knowing subject, so that the empirical reality is perceived as not something given a priori, a “ready-made” truth dictated allegedly by an external point of reference, but as something affected by the subject’s action. Whereas in the old classical paradigm, scientific descriptions were regarded as being independent of the knowing subject and the process of knowledge, in the new quantum paradigm, epistemology, namely, the understanding of the process of knowing—has to be explicitly included in the description of natural phenomena. In other words, epistemology inevitably becomes now an integral part of the theory. The nonseparable structure of quantum theory, the experimentally well-confirmed holistic features arising out of it, and the subsequent context-dependent description of physical reality conflict with the rather comfortable view that the world’s contents are knowable in an intrinsic and absolute sense. Within the domain of quantum mechanics, knowledge of reality in itself—the real such as it truly is, independent of us—is impossible in principle.
Independent Reality and Contemporary Physics
It should be underlined, however, that even if our knowledge will never be knowledge of reality itself, independent of the way it is contextualized, there is no reason, from the viewpoint of natural philosophy, to reject as meaningless or redundant the notion of a mind-independent reality. On the contrary, we consider the latter as unassailable in any scientific discourse; we amply recognize its existence as being logically prior to experience and knowledge; we acknowledge its externality to the mind structure as being responsible for resisting human attempts in organizing and conceptually representing experience. But, significantly, in the quantum domain, the nature of this independent reality is left unspecified. Because of the generalized phenomenon of quantum nonseparability, we must conceive of independent reality as a highly entangled whole with the consequence that it is impossible to conceive of parts of this whole as individual entities, enjoying autonomous existence, each with its own well-defined state. Neither reality considered as a whole could be comprehended as the sum of its parts, because the whole, according to considerations mentioned earlier, cannot be reduced to its constituent parts in conjunction with the spatiotemporal relations among the parts. Therefore, quantum nonseparability seems to pose a novel limit on the ability of scientific cognizance in revealing the actual character of independent reality itself. Any detailed description of the latter necessarily results in irretrievable loss of information by dissecting the otherwise undissectable. From a fundamental viewpoint of quantum mechanics, any discussion concerning the nature of this indivisible whole is necessarily of an ontological, metaphysical kind, the only confirmatory element about it being the network of entangled interrelations that connect its events. In this respect, it can safely be asserted that reality thought of as a whole is not scientifically completely knowable, or, at best, in D’Espagnat’s expression, it is veiled. Hence, our knowledge claims to reality can be only partial, not total or complete, extending up to the structural features of reality that are approachable by the penetrating power of the theory itself and its future development.
The Role of Mind in the Physical World
The term reality in the quantum realm cannot be considered to be determined by what physical objects really are in themselves. As already noted, this state of affairs is intimately associated with the fact that, in contrast to classical physics, values of quantum mechanical quantities cannot be attributed to a quantum object as intrinsic properties. The assigned values cannot be said to belong to the observed object alone regardless of the overall experimental context that is relevant in any particular situation. Hence, the quantum mechanical framework seems to allow only a detailed description of reality that is co-determined by the specification of a measurement context. What contemporary physics, especially quantum mechanics, can be expected to describe is not how mind-independent reality is, as classical physics may permit one to presume. Within the domain of quantum mechanics, the role of the knowing/intentional subject, that is, the experimenter/observer, is indelibly marked. According to the physicomathematical structure of standard quantum mechanics, the mere freedom, on the part of the experimenter, to select the experimental context, to choose the question asked of nature and specify the timing of its asking, allows him or her to exert an indelible influence on a quantum system that is not, in general, fixed by the prior part of the physical world alone. Thus, by way of a generalized example, if A, B, and C represent observables (physical quantities) of the same quantum system, so that the corresponding Operator A commutes with Operators B and C ([A, B] = 0 = [A, C]),but not, however, the Operators B and C with each other ([B, C] 0), then the result of a measurement of A depends on whether the experimenter had previously subjected the system in a measurement of the observable B, a measurement of the observable C, or none of them. Hence, the value of the observable A depends upon the set of mutually commuting observables with which one may consider it; that is, the value of A depends upon the set of measurements one may select to perform. In other words, the value of the observable A cannot be thought of as pre-fixed, as being independent of the experimental context actually chosen. This may simply be viewed as a lemma of the Kochen and Specker theorem noted earlier, whereas analogous, still more vivid statements to this consequence can be made through recent investigations of established effects in quantum theory such as the quantum Zeno effect and the so-called delayed choice experiments.
Thus, quantum mechanics undeniably reveals a sense in which the knowing subject participates in bringing reality into being. Indeed, the conception of active scientific realism—put forward here as an interpretive, regulative hypothesis to physical science— attempts to incorporate the human factor into our understanding of reality within which human knowledge forms an indispensable part. It aims at appropriately expressing within physics the sense in which this is a “participatory universe,” just to recall John Wheeler’s phrase. It acknowledges the subject-object inherent engagement that is built into quantum theory by appropriately integrating the subjective human increments into an objectively existing physical reality. There is an ever-growing recognition that the Cartesian ideal of an absolute dichotomy between the human mind and the external world, allowing no intermediate medium, is fundamentally flawed. It leads to an alleged construction of decontaminated-by-human-qualities knowledge—a supposedly perspective-free view of the world—that is insufficient to really understand reality. In contradistinction, active scientific realism attempts to convey that the actual relation of the knowing subject to a world is not of absolute dichotomy, nor of independence or externality, but of active participancy. Thus, in coming to know the physical world, we also come to know ourselves as knowers and consequently understand how our perspective affects or contributes to our conceptualization about the world, because in contemporary physics, such an effect is unavoidable. And conversely, by knowing how we contribute to the knowledge claims about the world, we identify more securely the informational content the world itself contributes. In this view, human subjectivity and scientific objectivity, within the respective limits of their appropriateness, are no longer contraries; they are jointly achieved.
Viewing the World From Within
In light of the preceding considerations, the common philosophical assumption concerning the feasibility of a panoptical, Archimedean point of view is rendered illusory in quantum mechanics. In contrast to an immutable and universal “view from nowhere” of the classical paradigm, quantum mechanics acknowledges in an essential way a perspectival character of knowledge. Although possible in classical physics, in quantum mechanics, we can no longer display the whole of nature in one view. Access to the non-Boolean quantum world is gained only by adopting a particular Boolean perspective, by specifying a certain Boolean context that breaks the wholeness (the underlying nonseparability) of nature. Consequently, the description and communication of results of experiments in relation to the non-Boolean quantum world presuppose the impossibility of a perspective-independent account, because one must, at the outset, single out an experimental context (determined by a set of co-measurable observables for the context-cum-quantum system whole) and in terms of which the definite result of a measurement can be realized.
Be sure that there is only one external reality, but every description of it presupposes—according to quantum mechanics—the adoption of a particular point of view. There is no such thing as a “from nowhere” perspective or a universal viewpoint. A complete knowledge of the world as a whole would have to provide an explanation of the perception process or the pattern recognition mechanisms of the knowing subject, because this is part of the world. It would have to include within a hypothetically posited ultimate theory an explanation of the conditions for observation, description, and communication to which we ourselves, as cognizant subjects, are already subjected. We cannot transcend them. This would be like attempting to produce a map of the globe that contained itself as an element. The usage of this metaphor is meant to convey the conceptually deep fact—reminiscent of Godel’s famous undecidability theorem for axiom-system in mathematics—that a logically consistent theory cannot generally describe its universe as its own object. In particular, the scientific language of our hypothetical universal ultimate theory would have to be semantically closed, and hence engender antinomies or paradoxes especially in relation to self-referential descriptions, as in the case of von Neumann’s account of quantum measurement that leads to an infinite regress of observing observers.
Be that as it may, the assumption of a “view from nowhere” appeared realizable prior to quantum mechanics, because in classical physics, the validity of separability and unrestricted causality led to the purely reductionist presumption that one could consistently analyze a compound system into parts and consequently deduce the nature of the whole from its parts. Because the part could be treated as a closed system separate from the whole, the whole could ultimately be described—by applying the conservation laws of energy, momentum, and angular momentum as the sum of its parts and their physical interactions, and hence the knowing subject would achieve its knowledge of physical reality from the outside of physical systems.
In the quantum theoretical framework, that picture is no longer valid. Because of the genuinely nonseparable structure of quantum mechanics, any compound entangled system is characterized by properties that, in a clearly specifiable sense, refer to the whole system but are not reducible to, implied by, or derived from any combination of the local properties of its parts. Hence, the legendary notion of the classical paradigm that wholes are completely describable or explicable in terms of their constituent parts is flagrantly violated. In quantum mechanics, as already adumbrated, the whole is simply not equivalent or reducible to the sum of its parts, including the spatiotemporal relations and physical interactions among the parts.
When all is said and done, the present situation in physics suggests that the natural scientist as a conscious being may operate within a mild form of the reductionist paradigm in trying to analyze complex objects in terms of parts, with the absolute certainty, however, that during the process, the nature of the whole will not be disclosed. The value of the reductionistic concept as a working hypothesis or as a methodological tool of analysis and research is not jeopardized at this point, but ontologically, it can no longer be regarded as a true code of the actual character of the physical world and its contents. Quantum mechanical nonseparability strongly suggests that the functioning of the physical world cannot just be reduced to that of its constituents, thought of as a collection of interacting but separately existing localized objects. Any coherent conceptualization of the physical world that is compatible with the predictions of quantum mechanics requires us to view the world as a complex network of relations or processes among events that may interchange or overlap or coalesce and thereby determine the texture of a unified dynamic whole. Although the latter can hardly be fully know-able, an enlightenment of its actual character may be given by the penetrating power of the theory itself and its future development. In this respect, it is rather safe to conjecture that the conception of quantum nonseparability will be an integral part of the next conceptual revolution in physics and may even be used as a regulative constructive hypothesis guiding the search for our deeper understanding of nature.
References:
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