What is consciousness?

The mind is as much a part of the physical world as the body. And we might eventually be able to read each other’s thoughts 

To different people, consciousness is different things. To some it is the sense of self, of being a particular person with a unique history. To others it is the delicate shade of feeling blue, of sensing the passage of time, or remembering a painful event. I experience consciousness at its most intense when rock climbing. I feel the cool granite under my hands, the sun warming my back, the deep blue Californian sky above me and the siren song of the void beneath me. Common to all is the aspect of feeling, of experience. To be conscious of anything means experiencing something, sensing something. The mystery at the heart of the mind-body problem as perceived in western philosophy is how such subjective attributes arise from an objective description of the world in terms of atoms and force fields, molecules, nerve cells, organs and the other material entities that make up the universe.

Body and soul

To most people the answer is obvious: humans have souls. These do not belong to the physical realm but are spiritual in nature. It is the soul that experiences events and that causes any action to occur. However, following the 18th-century Enlightenment scholars of the natural world started to emphasize the importance of empirically testable explanations rather than faith-based ones. Thus, scientists, by and large, find such dualistic doctrines of soul versus body deeply unsatisfactory for many reasons – they can’t be tested objectively, they fail to explain how the soul and the body interact, or how or when the soul enters the body, or whether animals have souls.

Brain and mind

Instead, the scientific world-view is based on the interplay of blind physical forces and energies that gave birth, over aeons, to the cosmos, to our planet and to the first self-replicating, molecular organisms. Shaped by the action of natural selection in a relentless competitive environment, these assemblies evolved to the complex and highly differentiated creatures that populate Earth today. Animals possess highly specialized organs – brains – that sense the environment, represent, store and manipulate information about it, and that control the movements of their bodies. These nervous systems also produce something more ineffable: the sentient mind that experiences the sights and sounds of life. And how this takes place is a deep mystery. Consciousness is a property of highly evolved organisms and must, therefore, have one or more functions that enable the organism to better survive the daily struggle for existence.

It is likely that at least some higher animals share some aspects of consciousness with humans. Although not all creatures talk, introspect or think about the inevitability of death and taxes, we all feel pain and enjoy life, we are all nature’s children. At least the higher mammals have brains that are very like ours, some (like a mouse’s) smaller and some (like a dolphin’s or a whale’s) larger than ours. Indeed, brains are so alike that only a handful of experts can tell a grain of brain matter from a mouse from a grain of human brain.

The relationship between the conscious, subjective mind and the objective, physical brain is intimate and close. The link’s exact nature is uncertain, but there is no doubt that any thought, percept, memory or action goes hand in hand with a brain process. All evidence points to the conclusion that without bio-electrical activity in brain tissue, no consciousness can occur. Put succinctly: no brain, never mind.

Students of the brain look to the specialized junctions that connect individual neurons (synapses), and to electrical pulses (the primary means of communication among neurons), as the elements from which mind emerges. Understanding how this happens remains a key challenge for the 21st century.

For its size, the brain is the most complex system in the known universe. The average human brain contains perhaps 20 billion to 100 billion highly adaptive and networked neurons with trillions of synapses. Even the brains of much simpler organisms, such as the fruit fly Drosophila, are currently beyond our ability to probe and understand thoroughly.


Most scholars of the mind assert that consciousness is a holistic, Gestalt property that emerges from the entire brain. They believe that it is silly to try to identify particular architectural, neuronal or synaptic attributes that can explain any one conscious sensation. However, the late Francis Crick and I disagree.

Molecular biology is so replete with examples of sophisticated molecular machines serving a specific purpose that the same is likely to be true of the brain. Specialized neuronal circuits or gadgets probably underlie consciousness. We know already that not all parts of the brain contribute equally to conscious sensation. Electrophysiological evidence from humans and monkeys shows that exuberant electrical activity in parts of the cerebral cortex does not necessarily give rise to conscious perception. It is likely that any one conscious sensation is generated by a coalition of active neurons that dominate the cortical symphony for a fraction of a second and suppress other coalitions of neurons. The representational content of this coalition, this group of nerve cells, expressed by a specific pattern of electrical pulses across this assembly, is what the brain experiences as sensation. As this coalition is suppressed and supplanted by another group of active neurons, the content of consciousness shifts, in a never-ending dance until sleep intervenes.

The sub-conscious mind

To what extent are we conscious of what is going on in our brains? Friedrich Nietzsche and Sigmund Freud popularized the notion of the unconscious as a realm of the mind controlling human behaviour but itself not accessible to conscious introspection or knowledge. While many of Freud’s ideas – including penis envy, the Oedipus complex, the Id and other fanciful creations – lack objective standing, science has provided credible evidence for the existence of sensory-motor behaviours that function in the absence of consciousness.

Many parts of the brain perform complex yet routine tasks without direct conscious input or control. They can deal automatically with certain simple but commonly encountered situations, which is why Crick and I have called them “zombie agents”. One can become conscious of the actions of one’s own zombie, but usually only after the fact.

Zombie agents control the eyes, the hands and other limbs. Much anecdotal evidence and psychophysical research emphasize rapid and effortless behaviours that antedate consciousness. This is particularly true of many highly practised and ritualized sports – rock-climbing, fencing, skiing or dancing. Mastery of any of these requires a surrendering of the conscious mind, a letting-go of the aim so ardently pursued, for the body and its senses to take over.

The hallmarks of a zombie agent are stereotypical, limited sensory-motor behaviour and immediate, rapid action. Its existence raises two larger questions. First, why aren’t we just big bundles of zombie agents? Why bother with consciousness, which wastes valuable time? I think the answer has to do with the fact that consciousness allows the system to plan a future course of action, permitting access to an infinite behavioural repertoire and to memory (“have I encountered this situation before?”). Second, what is the difference between the neuronal pathways governing unconscious behaviour (our zombie agents) and the activity patterns that give rise to specific conscious perceptions? Are they based on activity in different types of neurons segregated according to brain areas? Might the difference be the type of neural activity involved?

All these and more questions are being examined by some of the 40,000 scientists studying the brain. A new science is being born that seeks to explain subjective feelings within the natural order. Previously, the field was left to philosophers and mystics who could not agree on an understanding of the existence, let alone the nature, of consciousness.

There is no doubt that science will eventually characterize the material basis of consciousness. Given what we know today, there is little reason to believe that this will require fundamental new laws, though this always remains a possibility. It is likely that some immensely complex, ever-shifting and adaptive patterns of synaptic and neuronal activities in brains mediate conscious feelings, giving rise to subjective states.

Understanding the roots of consciousness will have immense consequences that can be put to good or bad uses. Many neurological diseases affect consciousness. Think of the black hopelessness of depression, the sociopathology of autism, the failure of memory in Alzheimer’s, the psychotic paranoia of schizophrenia and the self-destructive behaviour of drug addiction. Knowing the true nature of consciousness will enable science to develop therapeutic interventions, drugs and other means to cure and prevent these diseases.

Another practical development will be a conscious-o-meter – a sort of tricorder familiar from StarTrek that indicates whether a scanned subject – animal, newborn baby or aphasic patient unable to speak – is conscious or not.

Researchers are developing Brain-Machine Interfaces (BMIs) that are sensitive to local electrical activity in parts of the motor and sensory-motor cortex. Such neuroprosthetic devices are being implanted in experimental animals and paralyzed patients so they can control a wheelchair or a television by the power of thought.

Mind reading

In the future, it might be possible to read somebody’s mind by the use of sophisticated technologies that probe large number of individual neurons at high spatial and temporal resolution. (It is unclear whether this could ever be done from outside the skull, or whether mind reading will require electro-biological interfaces with the brain. In this way, we could know the innermost thoughts and sensations of another individual far more directly and intimately than any psychoanalyst. If such a device would also allow sensations to be generated in another brain, so that this individual could experience them as real (a limited form of this is occasionally done by neurosurgeons during certain types of brain operations), then true mind-to-mind communication would become possible (by means of some artifact, of course).

However, there will likely remain practical limits to experiencing another’s consciousness, for it is, to a significant extent, conditional on their previous experience as encoded in the patterns of synapses that make up the fabric of the vast neuronal networks of the brain. Only if these can be read out non-destructively will we be in a position to put ourselves into somebody else’s state of mind, including all of their memories. Only if we can scan all synapses and related neuronal elements could we truly download somebody into another medium, brain or computer. It is not clear whether there will be a sort of Heisenberg uncertainty principle for brains that would prevent this sort of perfect knowledge. I suspect that there is – that we can’t scan the entire brain in sufficient resolution and speed to copy accurately another person’s mind.

Finally, a complete and causal theory of consciousness should specify which organisms, biological or not, and under what conditions, will enjoy conscious states. What sort of architecture enables consciousness? Does being conscious require having a body? Answers to these questions might make it possible to design computers or robots that are sentient, that feel the pain and pleasure of experiences like you or me.

These and many more unknown consequences of our increasing understanding of the mind will play out over the coming decades. They will also have profound consequences for ethics, including a new conception of what it means to be conscious, that might radically contradict the traditional images people have made of themselves throughout the ages. How individuals and society will deal with this bounty of powerful knowledge remains to be seen.

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