The problem of aligning humans and artificial intelligences can be understood in terms of minimizing externalities between them. However, economics cannot define externality because it contradicts the rationality assumption. This paper applies the homeostatic principles, from anatomical homeostasis to its disorder – cancer, to define externality. Drawing upon the perspective of cancer as a problem of scaling cellular collectives, this paper shows how to redefine both externality and rationality in terms of cognitive light cones (which demarcate the scale of goals any agent can pursue). We propose that cognitive light cones are constructed out of interoceptive signals for the purpose of anatomical homeostasis. We show that externalities can be understood in terms of anatomical homeostasis and derive some important implications for AI alignment, including the possibility of using market mechanisms enable the mutual co-construction of alignment between artificial intelligences and humans.