Infra-Bayesian physicalism (IBP) is a mathematical formalization of computationalist metaphysics: the view that whether something exists is a matter of whether a particular computation is instantiated anywhere in the universe in any way. In this post, we cover the basics of IBP, both rigorously and informally, its relevance to agent foundations and the alignment problem, and present new definitions that remove the biggest limitation in the previous formulation of IBP: the monotonicity requirement. Having read the original post on IBP is not required to understand this post, though prior familiarity with infra-Bayesianism is useful for understanding the technical parts.

Why Physicalism?

A physicalist agent - that is, an agent that uses IBP - does not regard itself as inherently special. This is in contrast to a Cartesian agent. A precise definition of an IBP agent is highly technical and can be found in the original IBP post, but here are some ways in which physicalist agents and Cartesian agents differ:

To a Cartesian agent, actions and observations are fundamental. To a physicalist agent, computations are fundamental. A physicalist agent's rewards are determined by which computations are instantiated in the universe and to what extent; we will give a rigorous formalism for this later in the post.
Cartesian agents have a sharp Cartesian boundary between the agent and the environment. Physicalist agents do not.
A Cartesian agent is a fundamental part of its ontology. It models the world purely in terms of its actions and observations, so it naturally treats its actions observations as "more fundamental" than the laws of physics. A physicalist agent naturally views all of the universe on equal footing.

IBP naturally solves many problems in agent foundations:

The problem of privilege: A Cartesian agent using the Solomonoff prior will give unreasonably high credence to beliefs such as, "If every single pixel recorded by my camera is the exact same color, this will immediately cause the fine-structure constant to double". This is because, in relation to the agent's input channel, the Kolmogorov complexity of an otherwise-simple hypothesis that depends on a pixel's color is very low. Such issues are also relevant when a Cartesian agent is considering simulation hypotheses. By contrast, a physicalist agent will give low credence to such hypotheses since a description of t

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