Matthew Pusey: Is QBism 80% complete, or 20%?

The speaker begins by expressing gratitude to the conference organizers for the invitation and acknowledges the liberty taken to present a talk that diverges from typical conference contributions. The speaker hints at a focus on the information-theoretic interpretation of cubism, a subject they find both enlightening and contentious. The talk's evolution is outlined, beginning with an initial understanding of cubism from David Merman's talk and evolving through further research, leading to the recognition of multiple interpretations of cubism. The speaker also introduces a revised title reflecting this complexity and sets the stage for a discussion on the implications of cubism for our understanding of quantum theory.

The speaker delves into the nuances of cubism, an epistemic view of quantum theory, and its implications for understanding the quantum state. They reference David Wallace's work on decoherence and its role in addressing foundational questions in quantum mechanics. The speaker argues for the necessity of an interpretation of quantum mechanics, challenging the idea that quantum theory doesn't require one. They also discuss the realist versus epistemic interpretations of quantum states, highlighting the inconsistencies that arise when applying quantum theory to both microscopic and macroscopic systems.

The speaker continues the discussion on the nature of quantum states, measurement outcomes, and the subjective interpretation of these elements within the cubist framework. They explore the idea that quantum states represent an agent's beliefs about future experiences and question the notion of 'lambdas'—hypothetical variables that could predict measurement outcomes better than quantum mechanics allows. The speaker also addresses the subjective nature of the Hamiltonian, the rule used to update the quantum state, and the implications of these concepts for the interpretation of quantum mechanics.

The speaker reflects on the philosophical implications of viewing quantum mechanics through the lens of cubism, particularly the subjective nature of measurement outcomes. They discuss the potential problems with accepting a simplistic realist interpretation of quantum states, considering the phenomena of quantum entanglement and the arguments against the existence of 'lambdas'. The speaker also touches on the idea that measurement outcomes may not be as objective as traditionally thought, hinting at a deeper complexity in the nature of reality as suggested by quantum mechanics.

The speaker examines Bell's theorem and its implications for the objectivity of measurement outcomes in quantum mechanics. They present a thought experiment involving entangled states and the violation of CHSH inequality, suggesting that the outcomes of measurements may not be as objective as previously believed. The speaker also discusses the potential impact of this perspective on the interpretation of quantum mechanics and the philosophical stance one might take regarding the nature of reality.

The speaker brings in the philosophy of mind to discuss the concept of agency in the context of quantum theory and cubism. They challenge the notion that agents are physical systems that use quantum theory to structure beliefs about future experiences and question the solipsistic interpretation of cubism. The speaker also explores the idea that quantum theory might be more than just a tool for agents, hinting at a richer understanding of the world that extends beyond the quantum level.

The speaker argues for a richer interpretation of the quantum world, suggesting that cubism offers more than a minimalist view of quantum theory as a tool for agents. They reference the works of Chris Fuchs and the idea of a 'Jamesian' world, which posits a more complex and nuanced understanding of reality. The speaker also addresses the potential for further exploration and discovery within this framework, indicating that there is much more to learn about the nature of the world beyond the quantum level.

The speaker concludes by emphasizing the diversity of interpretations within cubism, likening it to the many varieties of basil in a garden. They suggest that cubism is not a single, unified theory but rather a collection of perspectives that offer different insights into the nature of quantum theory and reality. The speaker encourages an open-minded approach to these various interpretations, acknowledging the complexity and richness of the quantum world.