Many organs in the body have a fairly specific main function, such as cleaning or pumping blood, producing bile, or digesting. Nothing is ever simple, of course, and all the organs of the body have highly complex, interconnected functional roles. The digestive system involves many different steps; the kidneys help regulate blood pressure; while the heart changes the way it pumps in a very complex and context-dependent manner. Experts in different areas of human biology have a wealth of knowledge about the morphology and physiology of organs, at multiple levels of description. For example, much is known about what cellular and molecular processes occur as the kidneys filter blood, or as food is digested. Knowledge about the functions of organs is not yet complete, but there is reasonable agreement about the overall picture—namely, which organs have what function.
But the brain seems different. There is much less agreement about what is its main function, and much less knowledge about how it fulfills the various functions attributed to it. Of course, everyone agrees that the brain subserves perception, decision-making, and action—and perhaps that it is the seat of consciousness, self and soul. There is a reasonable degree of knowledge about some aspects of the brain, such as the mechanism behind action potentials, and about what happens when neurons fire. But most would agree that it would be controversial or even preposterous to claim that there is one main function of the brain, on a par with the heart’s pumping of blood.
Yet there is an emerging view that claims that the brain has one overarching function. There is one thing the brain does, which translates convincingly to the numerous other functions the brain is engaged in. This chapter will introduce this idea and will show that, whereas it may be controversial, the idea is not preposterous. It will help us understand better all the things that the brain does, how it makes us who we are, and what we are.
The main version of the idea is labeled the free energy principle, and was proposed by Karl Friston (2010). It unifies and develops a number of different strands of thinking about the brain, about learning, perception and decision-making, and about basic biology. The principle says that biological organisms on average and over time act to minimize free energy. Free energy is the sum of prediction error, which bounds the surprise of the sensory input to the system. Put one way, it is the idea that brains are hypothesis-testing neural mechanisms, which sample the sensory input from the world to keep themselves within expected states. Generalizing greatly, one might say that, just as the heart pumps blood, the brain minimizes free energy.
Before moving on to introduce and defend this idea, it will be useful to explain why the analogy to the functions of other organs is apt. Once a function is identified it serves as a unifying, organizing principle for understanding what the organ does. For example, even though the heart acts very differently during rest and exercise, it still pumps blood. Similarly, even though the brain acts very differently during the awake state and during sleep it still minimizes free energy. Taking such a general approach therefore helps to provide a unified account of the brain.
Related to this, there is a type of objection that will have little bite on the organ-focused account of the brain. To see this, consider again the heart. The heart pumps blood, and this function is realized in part by the way the contraction of the heart muscle occurs—a process that depends on intricate ion flows across heart cell membranes. One should not object to the notion that the heart pumps blood by referring to the fact that what happens in the heart is an intricate cellular ion flow. This is so even though one might be able to understand much about the heart just by being told the cellular and molecular story. The story about the function and the story about a level of realization of that function are not in conflict with each other. Similarly, one cannot object to the free energy principle by pointing to facts about what the brain does (e.g., what happens as action potentials are generated, or as long-term potentiation is instantiated). The reason for this is that those low-level processes might be ways of realizing free energy minimization. At best such objections are calls for explanatory work of the sort “how can the generation of action potentials be realizations of free energy minimization?”
These two points together suggest that the functional, organ-based account of the brain is reductionist in two ways (familiar from discussions in philosophy of science). On the one hand it seeks to reduce all the different things the brain does to one principle, namely free energy minimization. This is a kind of theory reduction, or explanatory unification. It says that one theory explains many different things. On the other hand, it is consistent with a kind of metaphysical reduction where the overall function is in the end realized by a set of basic physical processes. Here, mental function is fully physical and fully explained by free energy minimization. It is interesting to note that no one would object to such a two-fold reductionism for the heart and other organs, yet it is controversial or even preposterous to do so for the organ that is the brain. For these reasons, it is useful to keep in mind the simple idea that the brain is also an organ. Much of the discussion in this chapter revolves around these two reductive aspects: how can the free energy principle explain everything? And can it provide the functional scaffolding that would allow realization by brain activity?