Liver scientists have a hypothesis – that the biological functioning of the liver, from the organism level down to the cellular level – is actually responsible for all the functions that appear to correlate with the liver. This is not a controversial hypothesis.

If, for example, those with severe liver disease, such as cirrhosis, lose the ability to make certain clotting factors, to regulate their blood glucose, to produce bile and digest fats, to metabolize drugs, and detoxify the blood, and to eliminate certain metabolic products from the blood, such as bilirubin, then those are all things that the liver actually does. In other words, the reason for the correlation is that liver function is the ultimate biological causation of all these things. Other parts of the body may be involved, but the liver is in the loop somewhere.

This hypothesis holds up very well. Liver disease reliably correlates with these downstream effects, and curing the liver or replacing it in severe cases reverses these effects. We can also culture liver cells in the lab and measure many aspects of their biological function, such as producing enzymes and metabolizing drugs. In many cases we have identified genes for liver enzymes, and can see that they are active in liver cells and not active in non-liver cells (mostly, there are always layers of complexity). We can even identify in some cases genetic mutations that produce altered function of those enzymes and produce specific diseases.

In fact the scientific hypothesis that the biological activity of the liver causes everything we see as liver function is so well established that it is taken as a given, and not questioned by any serious scientist. Over decades, countless labs and researchers doing countless studies have all relied upon this assumption, and it has worked out well.

This all may seem so obvious that it is silly to bother to point it out. But what if there were those who claimed that, in actual fact, some of what we think of as liver function is actually a manifestation of liver fairies. These are mystical entities that live in the liver. They are invisible and undetectable, but they carry out some of the functions we think of as liver function. The only reason that these functions correlate with the liver is because that is where the liver fairies live. They become unhappy when their home is not healthy, and stop doing some of their functions.

Defenders of the liver fairy hypothesis point out that we do not understand 100% of liver function. Whatever we currently do not fully understand – that is what the liver fairies are doing. Also, much of what we assume is liver function is a matter of correlation only. This does not prove causation, and so we cannot rule out by correlational evidence the alternate causational hypothesis of liver fairies. And further – you cannot prove that liver fairies do not exist.

I would counter these arguments first by pointing out that this is a massive argument from ignorance, or “fairy of the gaps” argument. Science does not progress by simply assuming a supernatural cause for anything not currently understood. Further, such supernatural (or paranatural if you think they represent natural laws we do not yet know or understand) explanations do not generate any predictions. They are not testable. At best proponents can say that the liver fairy hypothesis predicts that our biological understanding will remain incomplete. But this would be true in any case, because scientific knowledge is never done. A more appropriate criterion is whether or not the biological hypothesis produces results – does it advance our understanding by making testable predictions that are later validated.

Even when proponents of any “fairy of the gaps” hypothesis do make predictions, when those predictions fail they simply move the goalposts. “We will never discover how the liver does X because X is a function of liver fairies.” But when we do discover how the liver does X, they say – “Well, we will never discover how it does Y.” Notice they never admit they were wrong about X.

I would also counter that the evidence is not correlational only. You can actively do stuff to the liver, whether damaging it or healing it, and this will predictably alter liver function. The arrow of causation does not go the other way – you cannot cause cirrhosis of the liver by injecting someone with bilirubin.

Finally, scientists do not have to prove that liver fairies do not exist, just as they do not have to prove the infinite number of possible paranormal hypotheses are not true. That is not how science works. We would need specific evidence for liver fairies, and this would have to make some sort of testable predictions. Otherwise liver fairies are worse than false – they are irrelevant. They are “not even wrong.” They are a completely unnecessary step that ads nothing to our knowledge of how the body works, or to anything else.

You have probably anticipated by this point where I am going will all of this. No one, as far as I know, has proposed the existence of liver fairies. This is because liver function is not integral to anyone’s belief system (again, as far as I know). But many people have proposed the exact same arguments for brain fairies, otherwise known as dualism. You can transpose all of the arguments I laid out above, for and against, by just changing liver to brain and liver function to mental function or mind. The arguments are the same, and they are just as vacuous.

I anticipate some will argue that the analogy is not apt because liver function is physical, while mental function is not. But this is irrelevant, and also not true. Saying mental function is not physical is assuming your conclusion – the question is whether or not mental function is entirely physical. By all evidence it is – at least it is the abstract conception of what the brain does. It is not a thing, it is an activity. It is like saying that soccer is not physical. Sure, the soccer ball, field, nets, and players are physical, but the abstract concept of the game of soccer is not. Soccer is the activity, it is an idea, but the substrate is physical.

The same is true for mental function – it is what the biological brain does. The problem that some people have with this idea, however, is results from the fact that the brain evolved to create the seamless illusion of mental function. We are not aware of all the mechanistic aspects of brain function because it conspires to hid those mechanisms from our awareness. But we do see them none-the-less – every time we experience an optical illusion, hallucination, false memory, misperception, or other hiccup of the brain. We tend to brush or laugh off these experiences, but collectively they are another window into the mechanistic aspect of our mind.

There are no brain fairies. It is an unnecessary hypothesis that is not even wrong. The mind is what the brain does.