Although the predictive processing (PP) framework is quite controversial (see discussions, in this chapter we do not seek to defend it from objections. Rather, assuming such a framework is generally correct for the brain, we are going to explore the hypothesis that it might also be correct for the so-called second brain, i.e., the gut neuro-immune-endocrine network and its surroundings, especially gut microorganisms. It is well established that the gut can influence moods and many other psychological phenomena, as we shall see below, but whether it can be modeled by the PP framework is an open empirical question that has not been explored. In what follows, we will pursue this possibility.