This project explores the free energy principle (FEP), a highly controversial neuroscientific theory that claims that all forms of brain function can be reduced to a single type of process. The proponents of the FEP claim that if their theory is correct, it has the potential to 1) unify explanation in neuroscience, and 2) identify a fundamental ontological feature that neural operations, especially those that subserve cognition, evolved to exhibit. These unificatory and evolutionary claims are highly attractive, as they address two major issues surrounding the nature of theory in cognitive neuroscience. However, the extreme reductionism of the FEP has raised concerns about its explanatory validity, in particular that it comes across as an unfalsifiable “theory of everything.” In order to secure its place as a plausible theory of brain function and ultimately cognition, the FEP must first pass empirical and theoretical muster. This calls for two things: first, that its claims be verified by empirical testing, and second, that its claims be whittled down into more manageable premises detailing how it can bear out its unificatory and evolutionary promises.