Doing justice to quantum mechanics calls for a deeper examination of the relations between our experience, its objects, and its subjects than either third-person interpretations or the first-person singular interpretation of the QBist permit. The metaphysical space opened by Bohr's employment of the "Kantian wedge" between the objective world, about which we can communicate, and the world "in itself" allows quantum mechanics to unfold its metaphysical potential. This in turn makes it possible to go a long way towards bridging the epistemological gap between the empirical and transcendental conceptions of reality.
Doing justice to quantum mechanics calls for a deeper examination of the relations between our experience, its objects, and its subjects than either third-person interpretations or the first-person singular interpretation of the QBist permit. The metaphysical space opened by Bohr's employment of the "Kantian wedge" between the objective world, about which we can communicate, and the world "in itself" allows quantum mechanics to unfold its metaphysical potential. This in turn makes it possible to go a long way towards bridging the epistemological gap between the empirical and transcendental conceptions of reality.
Abstract Metaphysicians should pay attention to quantum mechanics, but not because it provides definitive answers to many metaphysical questions. The theory itself is remarkably silent on the nature of the physical world, and the various interpretations of the theory on offer present conflicting ontological pictures. Rather, quantum mechanics is essential to the metaphysician because it reshapes standard metaphysical debates and opens up unforeseen new metaphysical possibilities. Even if quantum mechanics provides few clear answers, there are good reasons to think that any adequate understanding of the quantum world will result in a radical reshaping of our classical worldview in some way or other. Whatever the world is like at the atomic scale, it is almost certainly not the swarm of particles pushed around by forces that is often presupposed. This book guides readers through the theory of quantum mechanics and its implications for metaphysics in a clear and accessible way. The theory and its various interpretations are presented with a minimum of technicality. The consequences of these interpretations for metaphysical debates concerning realism, indeterminacy, causation, determinism, holism, and individuality (among other topics) are explored in detail, stressing the novel form that the debates take given the empirical facts in the quantum domain. While quantum mechanics may not deliver unconditional pronouncements on these issues, the range of possibilities consistent with our knowledge of the empirical world is relatively small—and each possibility is metaphysically revisionary in some way.
After a summary of Bohr's views and their relation to Kant's theory of science, two fruitless lines of attack on the measurement problem are discussed: the way of the psi-ontologist and the way of the QBist. In the remainder of the paper the following results are obtained. (i) Because the testable correlations between outcomes of measurements of macroscopic positions are consistent with both the classical and the quantum laws, there is no conflict between the superposition principle and the existence of measurement outcomes. (ii) Intrinsically, each fundamental particle is numerically identical with every other fundamental particle. What presents itself here and now with these properties and what presents itself there and then with those properties is one and the same entity, herein called "Being." (iii) The distinction between a classical domain and a quantum domain is essentially a distinction between the manifested world and its manifestation. By entering into reflexive spatial relations, Being gives rise to (a) what looks like a multiplicity of relata if the reflexive quality of the relations is ignored, and (b) what looks like a substantial expanse if the spatial quality of the relations is reified. (iv) The reason why quantum mechanics is a calculus of correlations between measurement outcomes is that it concerns the progressive realization of distinguishable objects and distinguishable regions of space. (v) The key to the relation between quantum mechanics and experience is that Being does not simply manifest the world, Being manifests the world to itself. It is at once the single substance by which the world exists and the ultimate self or subject for which it exists. The question how we are related to this ultimate self or subject is discussed.
What is the proper metaphysics of quantum mechanics? In this dissertation, I approach the question from three different but related angles. First, I suggest that the quantum state can be understood intrinsically as relations holding among regions in ordinary space-time, from which we can recover the wave function uniquely up to an equivalence class (by representation and uniqueness theorems). The intrinsic account eliminates certain conventional elements (e.g. overall phase) in the representation of the quantum state. It also dispenses with first-order quantification over mathematical objects, which goes some way towards making the quantum world safe for a nominalistic metaphysics suggested in Field (1980, 2016). Second, I argue that the fundamental space of the quantum world is the low-dimensional physical space and not the high-dimensional space isomorphic to the ``configuration space.'' My arguments are based on considerations about dynamics, empirical adequacy, and symmetries of the quantum mechanics. Third, I show that, when we consider quantum mechanics in a time-asymmetric universe (with a large entropy gradient), we obtain new theoretical and conceptual possibilities. In such a model, we can use the low-entropy boundary condition known as the Past Hypothesis (Albert, 2000) to pin down a natural initial quantum state of the universe. However, the universal quantum state is not a pure state but a mixed state, represented by a density matrix that is the normalized projection onto the Past Hypothesis subspace. This particular choice has interesting consequences for Humean supervenience, statistical mechanical probabilities, and theoretical unity.
QBism may be the most significant contribution to the search for meaning in quantum mechanics since Bohr, even as Bohr's philosophy remains the most significant revision of Kant's theory of science. There are two ironies here. Bohr failed to realize the full extent of the affinity of his way of thinking with Kant's, and QBists fail to realize the full extent of their agreement with Bohr. While Bohr's discovery of contextuality updates Kant's transcendental philosophy in a way that leaves the central elements of the latter intact, Kant's insight into the roles that our cognitive faculties play in constructing physical theories can considerably alleviate the difficulties that Bohr's writings present to his readers. And while throwing a QBist searchlight on Bohr's writings can further alleviate these difficulties (as well as reveal the presence in them of the salient elements of QBist thought), Bohr's writings can in turn provide answers to important questions that QBism leaves unanswered (and also allay some of QBism's excesses). In the final sections I confront the two most impenetrable mysteries yet unearthed: making sense of quantum mechanics, and the dual mystery of making sense of (i) the existence of consciousness in a seemingly material world and (ii) the existence in consciousness of a seemingly material world. Here the relevant arguments are framed in the context of the philosophy of the Upanishads, according to which we (as Schrödinger put it) "are all really only various aspects of the One." There is no world that exists out of relation to consciousness, but there are different poises of consciousness. In particular, there is a poise of consciousness peculiar to the human species at this point in time, and there are poises of consciousness that are yet to evolve (and that may be essential to averting the calamities towards which humanity appears to be heading).
This chapter repeats for quantum mechanics the comparison carried out for relativity in Chapter 5. Its thesis is that the same three interpretative families, neo-Kantian, empiricist and scientific realist, reappear in the philosophical reception of quantum mechanics, but in a more radical form, because the dispute now reaches the meaning of the quantum state, of probability and of measurement. The chapter first identifies what the formalism established in Chapter 4 leaves open and treats the Copenhagen framework as a point of departure rather than a unified doctrine. It then reconstructs the neo-Kantian interpretation (the Kantian affinities of Bohr’s complementarity, Cassirer, Grete Hermann and contemporary transcendental perspectives), the empiricist interpretation (Heisenberg’s observability principle, instrumentalism, Schlick, Frank, Reichenbach and van Fraassen’s constructive empiricism) and, on equal terms, the realist interpretation: Einstein and the EPR argument, Popper’s propensities, the de Broglie–Bohm theory, Bell’s beables and local causality, realist modal interpretations, and the Everett and collapse programmes. The realist family is the most developed of the three in quantum mechanics, but it is also internally plural: it affirms that the theory refers to a mind-independent reality while disagreeing about whether that reality consists of particles, fields, propensities, branching worlds or definite and possible properties. On this basis the chapter compares the three interpretations of causality and of stationary action. Causality appears as a relativized condition of objective description, as a statistical relation without surplus content, or as an objective but non-local structure; stationary action appears as a coordination axiom of quantum dynamics, as an empirically equivalent formulation, or as a real dynamical structure that in Bohm’s theory directly guides motion. The result confirms the pattern of Chapter 5 and identifies the precise point, the relation between quantum possibility and actuality, from which the BBMT bridge of Chapter 7 will start.
Quantum mechanics is an exemplary case of something that calls for critical philosophical reflection. One philosophical path is to seek a logic or ontology for quantum theory, another is to understand quantum mechanics by examining how and why physicists were pushed into developing it, and still another is to construe laboratory measurements of quantum phenomena not as connected with beliefs about types of real or irreal objects but as actions that produce experiences, and the quantum formalism as a guide for anticipating future such experiences.