What Counts as Physical? Why Single-Aspect Monisms Must Be False

This paper proposes a definition of the physical in terms of in-world efficient causation: a physical difference is a difference within a world that produces another difference within that world according to laws or regularities. I argue that this distinction clarifies the limits of physical explanation and exposes problems for physicalism and other single-aspect monisms. Through cases involving a video-game "glitch," phenomenal redness, and quantum measurement, I distinguish law-governed transitions within a world from differences whose sufficient causes cannot be located within that world. I then argue that consciousness and personal identity introduce differences not captured by a complete description of in-world physical relations. The resulting framework preserves a meaningful distinction between physical and mental causation while motivating a dual-aspect idealist ontology.