If the human brain was so simple
that we could understand it,
we would be so simple
that we couldn’t.

This quote by Emerson M. Pugh (although often misattributed) illustrates one of the greatest unknowns in Cognitive (Neuro-)Science. While understanding the brain may be the central objective of this field of research, the question of whether we will ever achieve this goal is unlikely to be answered anytime soon – this would require either doing so, or finding a conclusive proof that it is impossible. (Besides, there is so much still left to find out about the brain that, at this point, it hardly matters in practice whether a full understanding will ever be reached.) Thus, most of the discussion on this topic so far has been from a philosophical viewpoint. But before we have a look at some of the different approaches to this, it is necessary to analyze and redefine the question itself.

“Will” or “can”? And who is “we”?
Firstly, let’s clarify that we need to be asking whether humans can – in principle – understand the brain, not whether we will ever do so; as one depends only on the nature of the brain itself, the other on countless additional random factors: Even if we theoretically had the ability, we might go extinct before ever finding out. Speaking of it – in this last sentence, the word “we” referred to humanity as a whole, but who or what does it mean in terms of our question? Maybe the nature of the brain is such that it could not be grasped by a single mind, but instead by taking the mental capacities of multiple humans together, creating a cognitive process that goes beyond the individual? As this would not be very satisfying (for each individual), let’s take it to mean “any single person” for our purposes; such that we are asking if it is possibly achievable (given whatever time and resources necessary) for one mind to acquire and hold all the information necessary to understand the brain.

What does “understand” mean?
After using the term rather arbitrarily, we should think about what we actually want to achieve when we say “understand”. One might say, it means to know how and why the brain does everything it does, be able to predict and explain every little action and reaction, every process and outcome. This would mean nothing more or less than building (or being able to build) a complete model or replica that captures every aspect of the brain – neurons, chemical transmitters, electrical impulses and so on – down to the last molecule. Conversely, one might reverse-engineer the brain, understanding it through the process of simulation, as computer scientist and futurist Ray Kurzweil argues.
There is little reason to doubt that such models are theoretically achievable; with a big emphasis on theoretically, but, after all, we’ve taken major first steps into this direction: Scientists have identified and modeled the complete connectome (“wiring diagram”) of a simple worm species, C. elegans, and are currently working to achieve the same for rats and mice. In machine learning, artificial neural networks (ANNs), inspired by their neurobiological counterpart, are growing more and more influential and have led to new insights about the brain itself.
But then again, even though we could have a complete model of the inner workings of the brain, this may not lead to the understanding we are looking for. After all, coming back to the example of ANNs… we created them, we know their inputs and outputs – but not how to interpret their inner workings, even though we can trace them from beginning to end. That is to say, we cannot explain conceptually how and why a certain input is transformed into a certain output (although recent work is trying to close this gap).

In the end, to see whether such a “simple” model can be satisfactory, we last but not least need to examine what exactly we mean when we talk about this “brain” that we want to understand.

What is “the brain”?
The passage above (hopefully) illustrated that if we tried really hard, we should be able to achieve a complete understanding of the physical aspects of the brain. But what about other aspects, of which it is simply not known whether they are physical or not, such as thoughts, memories, subjective experience – in short, consciousness? Surely, when we use the general term “brain”, we want to know about those as well, as they are very closely associated.
Thus, our initial question might be stated more precisely as such: Could any single individual theoretically acquire a complete, conceptual understanding of every aspect of the human brain, including how consciousness arises from it? Assuming that the latter is the only aspect of which an understanding is not certainly theoretically possible – as it is uncertain whether it is physical or not – we see that the question is equivalent another one: Is it in principle possible to understand the relation between consciousness and the brain? This leads us to what is known as the mind-body problem in philosophy, which questions the relationship between the human mind and body.

Different philosophical theories and their implications
Theories on how mind and body are related are as old as the question itself and can be broadly divided into two categories, dualism and monism. The former can be traced back to René Descartes and states that mind and body are fundamentally different substances that merely interact in some way. Should this be true, then, even if we’d manage to understand the brain (the physical part), it wouldn’t bring us a single step closer to understanding the (non-physical) mind.
Monism, on the other hand, is the theory that mind and body are one and the same thing. If we accept this, or in particular its most popular form, physicalism, we believe that every mental process can be reduced to a physical process. Thus the answer to our main question as stated above would be “yes, it is possible” (assuming that indeed we are capable of understanding every physical aspect necessary).

The mind is physical, but we could never understand it?
In contrast to these classical theories, a more recent physicalist view postulates that while consciousness is indeed a fully physical phenomenon, we simply lack the ability to understand how it arises. The knowledge is “beyond the rim of human intellectual competence”, in the words of one of the theory’s most prominent advocates, British philosopher Colin McGinn. We are cognitively closed off from it due to the ways in which we think (or rather the ones in which we don’t).

An interesting argument for this position, which has been labelled “new mysterianism”, is based on Gödel’s incompleteness theorem, borrowing from the field of mathematics. The theorem states that every formal logic system entails truths that are impossible to demonstrate based on the system’s premises alone. The idea of this inherent limitation in developing a complete conception of itself is argued to apply to the brain as well.
However, naturally, there are also those who criticize new mysterianism and the concept of cognitive closure. They dismiss Gödel’s theorem as not applying to the brain, which they say is strictly speaking not a formal system of rules and symbols. It would be arrogant to claim that something is not understandable simply because one has failed to understand, they say, and that just because we don’t know something doesn’t mean we never will.
So is there still hope? It obviously depends on who you are asking, and new arguments and theories will continue to be brought forth in the field. In the end, it’s up to you to decide which philosophical view you subscribe to – just keep in mind that your choice has crucial implications for your personal stance on the question whether we could possibly ever understand the brain.