The objective of this paper is to offer an analysis of several key elements within Niels Bohr’s transcendental interpretation of quantum mechanics. After some stage setting, I will demonstrate that a transcendental perspective on Bohr offers several advantages over alternative interpretations. Specifically, I will argue that some of his most contentious claims become more plausible when viewed through a transcendental lens. However, despite these strengths, Bohr’s approach faces challenges. Following an evaluation of what I consider to be the primary weakness in his framework, the final section of the paper will explore potential avenues for enhancing the viability of the Bohrian project, with a specific focus on the role of phenomenology as a potential solution.

We quantify the difference between classical and quantum counterfactual effects, where an output distribution is somehow changed by the removal of signal (”blocking”) at some point. We show that there is a counterfactual gain in quantum counterfactual communication, which quantifies the effect it has above and beyond any classical counterfactual effect, and that this counterfactual gain comes from coherences.

This counterfactual gain contains a term proportional to a Kirkwood-Dirac quasiprobability term—when this is positive or zero, this counterfactual gain can only distribute probability more equi- tably over the set of outputs; however, if this Kirkwood-Dirac term is negative, blocking can cause output probability to focus on a specific outcome. We show that this difference between quantum and classical counterfactual effects results from the measurement backaction caused by this blocking.

We show that we cannot explain quantum counterfactual effects simply by removing detection events. We link this to attempts to argue from counterfactuals in quantum mechanics (e.g. when forming noncontextual and Bell inequalities), and show that this backaction effect forms a natural explanation for the violation of the statistical, or measurement, independence assumption used to form these inequalities. See J.R. Hance et al, Quant. Sci. Technol. 9, 045015 (2024) for more details.

In recent years, philosophers of physics—QBists in particular—have turned increasingly to the phenomenological tradition to make sense of quantum mechanics. And the work of Merleau-Ponty especially has caught their eye. Appealing to his later ontology of “flesh”, QBists describe agents who are constitutively embodied and embroiled within an external world and who are constitutively available to public scrutiny.

In this paper, I trace the provenance of Merleau-Ponty’s ideas back to Hegel’s account of mutual recognition. Not only are there striking analogies to be found, but it is clear that Merleau-Ponty derived these analogous elements (at least in part) directly from his engagements with Hegel’s thought. This analysis: (i) reveals a new (and surprising) extension of Hegel’s influence into contemporary philosophy of physics; (ii) sheds light on the philosophical relationship between Hegel and Merleau-Ponty; (iii) opens up fresh conceptual resources to QBists (relating in particular to their response to Wigner’s “friend” paradox); and (iv) generates a deep irony concerning the distinction between the so-called “analytic” and “continental” traditions in philosophy. An additional aim of the paper is to provide a clear statement of the explanatory relationship between QBism and phenomenology.

One of the most surprising findings of quantum mechanics is wave-particle duality. It suggests for example that both mechanical point particles and photons have state vectors and evolve both according to the Schroedinger equation. However, having a closer look at the standard quantum physics of these systems, we see that we do not really use the same formalism to describe them. For example, while the velocity of point particles depends on the shape of their wave packets, the same does not apply to photons which always move at the speed of light.

In this contribution, we, discuss a possible way of altering standard quantum mechanics by including features of a recent local photon model in its formalism. The aim of phase space quantum mechanics is to increase wave particle duality. At the same time, we address some issues related to the consistency between quantum and classical mechanics. Taking a philosophical point of view, one would expect that classical mechanics emerges from quantum mechanics when only expectation values are considered. Currently, this is only true in some cases. Moreover, phase space quantum mechanics might help us to eventually overcome the current gap between quantum physics and relativity.

We use the framework of Empirical Models (EM) and Hidden Variables Models (HVM) to analyze the locality and stochasticity properties of relativistic quantum theories, such as Quantum Field Theory (QFT). First, we present the standard definition of properties such as determinism, No Signaling, Locality, and Contextuality for HVM and for EM and their relations.

Then, we show that if no other conditions are added, there are only two types of EMs: An EM is either classical, in which case it is strongly deterministic, local, and non-contextual. Or else an EM is non-classical, in which case it is weakly deterministic, non-local and contextual. Consequently, we define a criteria for an HVM to be Lorentz invariant and prove that it implies No Signaling. As a result, we show that a relativistic quantum theory must be genuinely stochastic i.e., it cannot have a deterministic (strong or weak) HVM.

Finally, we discuss Bell’s definition of locality and show that it is equivalent to non-contextuality, and moreover we argue that Bell’s justification for this definition tacitly assumes non-contextuality. We propose an alternative definition of locality for contextual and relativistic theories that accounts for correlations that result from common history and renders QFT a local theory.

I propose a solution to a neglected conflict characterizing Bohr’s philosophy of quantum theory, namely between what I call the ’distinction thesis’ and the ’nonseparability thesis’ between quantum systems and classical apparatuses (Howard [1] p.163). Here is a quotation attesting to the former thesis: ”The essentially new feature in the analysis of quantum phenomena is the introduction of a fundamental distinction between the measuring apparatus and the objects under investigation.” ([2, pp. 3-4, my emphasis]. Here is one proving the latter: “The quantum postulate implies that any observation of atomic phenomena will involve an interaction with the agency of observation not to be neglected. Accordingly, an independent reality in the ordinary physical sense can neither be ascribed to the phenomena nor the agencies of observation” ([3] p. 580, my emphasis).

Many scholars have focused exclusively on the former thesis by concluding that Bohr’s approach to quantum mechanics is hopeless since it brings into play the irremediably vague distinction between the micro (the quantum) and the macro (the classical) realms. Howard, who correctly focussed on the latter thesis, has controversially claimed that the way out of the conflict is to appeal to decoherence [1].

By discussing the two-screen thought experiment proposed by Einstein, I argue instead that in virtue of the nonseparability thesis, Bohr regarded any classical system (measuring apparatuses included) as a quantum system. Consequently, by discussing Zinkernagel [4], I claim that Bohr is a quantum fundamentalist, since the distinction thesis is advanced only to fulfill the epistemic and pragmatic aim to get definite and communicable outcomes. Even though he regarded the measurement interaction as an (irreversible) physical process, Bohr did not need to invoke a collapse postulate, since treating a macroscopic apparatus as quantum or classical depends contextually on the experimenter’s variable aim: ”...for every particular case it is a question of convenience at what point the concept of observation involving the quantum postulate with its inherent ’irrationality’ is brought in” [3, p.580].

Finally, I will show that as a consequence of this pragmatic solution to the above conflict, Bohr adopted a non-constructive, principle theory approach [5] to the measurement problem: ”... the neglect of the atomic constitution of the measuring instruments themselves, ... is equally characteristic of the applications of relativity and quantum theory.” [6, p.236].