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Does this 3lb of tissue govern our actions without our even knowing?
Are we governed by unconscious processes? Neuroscience believes so - but isn't the human condition more complicated than that? Two experts offer different views

David Eagleman, neuroscientist at Baylor College of Medicine in Texas and bestselling author

It is clear at this point that we are irrevocably tied to the 3lb of strange computational material found within our skulls. The brain is utterly alien to us, and yet our personalities, hopes, fears and aspirations all depend on the integrity of this biological tissue. How do we know this? Because when the brain changes, we change. Our personality, decision-making, risk-aversion, the capacity to see colours or name animals - all these can change, in very specific ways, when the brain is altered by tumours, strokes, drugs, disease or trauma. As much as we like to think about the body and mind living separate existences, the mental is not separable from the physical.

This clarifies some aspects of our existence while deepening the mystery and the awe of others.

For example, take the vast, unconscious, automated processes that run under the hood of conscious awareness. We have discovered that the large majority of the brain's activity takes place at this low level: the conscious part - the "me" that flickers to life when you wake up in the morning - is only a tiny bit of the operations. This understanding has given us a better understanding of the complex multiplicity that makes a person. A person is not a single entity of a single mind: a human is built of several parts, all of which compete to steer the ship of state. As a consequence, people are nuanced, complicated, contradictory. We act in ways that are sometimes difficult to detect by simple introspection. To know ourselves increasingly requires careful studies of the neural substrate of which we are composed.

Raymond Tallis, former professor of geriatric medicine at Manchester University and author

Yes, of course, everything about us, from the simplest sensation to the most elaborately constructed sense of self, requires a brain in some kind of working order. Remove your brain and bang goes your IQ. It does not follow that our brains are pretty well the whole story of us, nor that the best way to understand ourselves is to stare at "the neural substrate of which we are composed".

This is because we are not stand-alone brains. We are part of community of minds, a human world, that is remote in many respects from what can be observed in brains. Even if that community ultimately originated from brains, this was the work of trillions of brains over hundreds of thousands of years: individual, present-day brains are merely the entrance ticket to the drama of social life, not the drama itself. Trying to understand the community of minds in which we participate by imaging neural tissue is like trying to hear the whispering of woods by applying a stethoscope to an acorn.

Of course brain activity is automated and, as you say, runs "under the hood of conscious awareness", but this doesn't mean that we are automatons or that we are largely unconscious of the reasons we do things. If, as you put it in Incognito, "the conscious you is the smallest bit-player in the brain" to the point that even our most important and personal decisions - such as choice of spouse, where to live, or occupation - are directed by brain mechanisms of which we are unaware, how would you have become sufficiently aware of this unawareness to write about it in your book Incognito (which incidentally shows little evidence of having been written by an automaton)?

David Eagleman

The uses of neuroscience depend on the question being asked. Inquiries about economies, customs, or religious wars require an examination of what transpires between minds, not just within them. Indeed, brains and culture operate in a feedback loop, each influencing the other.

Nonetheless, culture does leave its signature in the circuitry of the individual brain. If you were to examine an acorn by itself, it could tell you a great deal about its surroundings - from moisture to microbes to the sunlight conditions of the larger forest. By analogy, an individual brain reflects its culture. Our opinions on normality, custom, dress codes and local superstitions are absorbed into our neural circuitry from the social forest around us. To a surprising extent, one can glimpse a culture by studying a brain. Moral attitudes toward cows, pigs, crosses and burkas can be read from the physiological responses of brains in different cultures.

Beyond culture, there are fruitful questions to be asked about individual experience. Your experience of being human - from thoughts to actions to pathologies to sensations - can be studied in your individual brain with some benefit. With such study, we can come to understand how we see the world, why we argue with ourselves, how we fall prey to cognitive illusions, and the unconscious data-streams of information that influence our opinions.

How did I become aware enough about unawareness to write about it in Incognito? It was an unlikely feat that required millennia of scientific observation by my predecessors. An understanding of the limitations of consciousness is difficult to achieve simply by consulting our intuition. It is revealed only by study.

To be clear, this limitation does not make us equivalent to automatons. But it does give a richer understanding of the wellspring of our ideas, moral intuitions, biases and beliefs. Sometimes these internal drives are genetically embedded, other times they are culturally instructed - but in all cases their mark ends up written into the fabric of the brain.

Raymond Tallis

Some of what you have just said sounds like common sense and a retreat from the radical thesis advanced in Incognito. There you put unconscious brain mechanisms in the driving seat - which is why your book has attracted such attention - and argue that important life decisions are strongly influenced by "the covert machinery of the unconscious".

You cite startling studies that show how choice of marital partner, place to live, and occupation are shaped by an implicit egoism built into the brain, such that if your name begins with D you are more likely to marry a person, live in a town, and pursue an occupation beginning with "D". I think the stats are wobbly - there are a disproportionate number of lawyers as well as dentists called Dennis - but this illustrates your position.

But now you row back from your radical position and suggest that the brain doesn't call so many shots and is just one player.

That which is acted out in the public space (maintained by conscious human beings) that we call "culture" is at least as important. Once this is granted, then brain science will have a more modest role in explaining why we do things, and an even smaller one in framing social policy. It will tell us little about our "moral attitudes towards ... crosses and burkas". Our moral attitude to anything depends upon many things we are conscious of (which is why it is so variable) as well as things we are not. A burka or a cross isn't just a stimulus triggering automated responses, even ones conditioned by culture. Think of the (very conscious) argument about the law governing wearing these items in public.

Even when you concede in Incognito that "consciousness is the long-term planner", you still can't let go of the idea of the largely unconscious brain being in charge. This is because you want to privilege brain science. Your case is assisted by personifying the brain, as when you say things like "the brain cares about social interaction".

But we haven't finished with self-contradictions yet. May we talk about umwelt and illusions?

David Eagleman

We should probably agree that there is no contradiction between the fact that the unconscious brain can be in the driver's seat, and also influenced both from the inside (genetic) and outside (the larger society). Of course culture is important, and neurobiology should never aim to divert funding away from social research. But this is like advising an author of a book about planets that he should have written about galaxies instead. My interest in Incognito is to understand individual human experience - our cognitive illusions, where our ideas come from, how come we can move our arm with no sense of the musculature, how we effortlessly recognise a friend's face better than the best computer programs, why we can argue with ourselves, why it is difficult to keep a secret.

As an example of these individual experiences, I'm glad you brought up illusions and the umwelt. Visual illusions reveal that perceptions generated by the brain do not necessarily correlate with reality. Hallucinations, dreams, and delusions illustrate the same point.

And the story goes even deeper. We don't have a strong grasp of what reality "out there" even is, because we detect such an unbearably small slice of it. That small slice is called the umwelt.

Each organism presumably assumes its umwelt to be the entirety of objective reality. Until a child learns that honeybees enjoy ultraviolet signals and rattlesnakes see infrared, it is not obvious that plenty of information is riding on channels to which we have no natural access. In fact, the part of the electromagnetic spectrum visible to us is less than a ten-trillionth of it. Our sensorium is enough to get by in our ecosystem, but no better.

The concept of the umwelt neatly captures the idea of limited knowledge, of unobtainable information, and of unimagined possibilities. I think it's a good starting point for our intuitions about our own experience.

Raymond Tallis

If the unconscious brain and its response to "influences" such as genes and society really is "in the driver's seat", there is no difference between driving and passenger seats. Incognito presents us as more helpless, ignorant and zombie-like than is compatible with the kinds of lives we actually live and, what's more, with doing brain science.

While you are not as imperialistic on behalf of brain science as many of your contemporaries, you still maintain that examining neural tissue (the acorn) can tell us more about ourselves as social beings and the society we inhabit and create together (the whispering woods) than is, or in my opinion ever will be, the case.

Ultimately, your position is either a truism (we are sometimes deceived about the nature of reality and why we do things) or self-contradictory (we are usually or always deceived about the nature of reality and why we do things). Yes, there are illusions, dreams, delusions and hallucinations; but we could not recognise them for what they are unless the vast majority of our experiences were not illusions, delusions, dreams or hallucinations.

Your claim that we are sealed off from most of reality is even more clearly self-contradictory. Because our awareness is mediated through the evolved brain, you argue, our mental life is "built to range over a certain territory". We are therefore confined to an umwelt - "an unbearably small slice of reality". If this were the whole story of our condition, how would we know it? We would have as little inkling of the limitations of our consciousness as other animals do of their own. Or have researcher bees demonstrated to their gob-smacked hive-mates that rattlesnakes see infrared?

Knowledge transcends immediate experience and corrects some of our intuitions about ourselves. But this knowledge is a part - a huge part - of our conscious (repeat, conscious) mental life. Without it, we could not do the weekly shopping - never mind engage in a correspondence such as this.

David Eagleman

It is not contradictory to recognise that we are sealed off from most of reality, and that we can discover more of it by a process of careful experimentation. That is the endeavour of science. For example, you cannot see, hear or touch radio waves, but you can build machines to translate the waves into the biologically delimited language in which you can understand them. You can build such machines only because science reaches beyond what we know to discover new realms.

Neuroscience is uncovering a bracing view of what's happening below the radar of our conscious awareness, but that makes your life no more "helpless, ignorant, and zombie-like" than whatever your life is now. If you were to read a cardiology book to learn how your heart pumps, would you feel less alive and more despondently mechanical? I wouldn't. Understanding the details of our own biological processes does not diminish the awe, it enhances it. Like flowers, brains are more beautiful when you can glimpse the vast, intricate, exotic mechanisms behind them.