Yet despite the progress of the natural sciences in understanding the natural world, they have remained sequestered from the other great branches of learning. The social sciences and humanities are generally thought to be too grounded in ineffable phenomena of mind and culture, too complex and holistic, and too dependent on historical circumstance to be consilient with the natural sciences.

That venerable perception, I believe, is about to change. The reason is that the natural sciences, doubling in information content every two decades or less, have now expanded to touch the material processes that generate mental and cultural phenomena. Two disciplines–the brain sciences and evolutionary biology–are now filling the ancient gap between dual epistemologies to serve as bridges between the great branches of learning.

The brain sciences are a conglomerate of research activities by neuroscientists, cognitive psychologists, and philosophers (“neurophilosophers”) bound together by their conviction that the mind is the brain at work and, as such, can be understood entirely as a biological phenomenon. For their part, evolutionary biologists address the origin of the mental process, which is also considered a biological process. In particular, they focus on the instinct-like emotional responses and learning biases that affect individual development and the evolution of culture.

The key and largely unsolved problem of the brain sciences is the neuron circuitry and neurotransmitter fluxes composing conscious thought. The most important entree to the problem is brain imaging, the monitoring of brain activity by the direct mapping of its metabolic patterns. The current method of choice in brain imaging is positron emission tomography (PET) scanning, which measures activity in different parts of the brain by the amount of their blood flow–hence the oxygen and energy being delivered to them. The patient is first injected with a small amount of rapidly decaying isotope of oxygen or another harmless radioactive material that emits elementary particles called positrons. The positrons interact with electrons in tissue reached by the isotope, resulting in radiation that can be picked up by a camera. As the patient experiences a sensation, or reflects upon a subject, or feels and emotion, blood flow increases within a tenth of a second in the activated part of the his brain, and the corresponding change is detected by the scanner.

An alternative method of brain imaging is functional magnetic resonance imaging (fMRI). Its precursor recording method is static magnetic resonance imaging (MRI), which is based on the response of molecules in body tissues to radio waves after the molecules have been forced into a certain orientation by a powerful magnet. The magnitude of the response rises according to the water content of the tissues, which in turn increases while blood (half of which is water) flows into the active areas. Researchers convert MRI to fMRI, which enables them to use it to monitor brain activity, by recording multiple images through time. The images are then viewed in rapid succession to create moving images in the manner of conventional cinematography. The fMRI method is more efficient in this respect than PET scanning, having been improved to record hundreds of images per minute.

As in all biological research, the overall evolution of brain scanning is toward ever deeper, finer, and faster probes of activity. Other methods directed toward these goals, based on different physical phenomena from those employed in PET and fMRI, have recently opened a new chapter in imaging technology. One method, still limited currently to experimental use in animals, is the application of voltage-sensitive dyes to the surface of the living brain. The electrical conduction of the nerve fibers literally light up the dyes in patterns that can be tracked by photodiode cameras. Images have been recorded in excess of a thousand per second, allowing more nearly continuous monitoring than PET and fMRI scanning.

As the twenty-first century opens, we can expect to witness the invention of even more sophisticated methods of brain imaging, as well as refinement of those already in use. With luck, scientists will eventually reach their ultimate goal of monitoring the activity of intact brains continuously and at the level of individual nerve fibers. In short, the mind as brain-at-work can be made visible.

Brain imaging and experimental brain surgery, together with analyses of localized brain trauma and endocrine and neurotransmitter mediation, have permitted a breakout from age-old subjective conceptions of mental activity. Researchers now speak confidently of a coming solution to the brain-mind problem.

Some students of the subject, however (including a few of the brain scientists themselves), consider that forecast overly optimistic. In their view, technical progress has been largely correlative and has contributed little to a deeper understanding of the conscious mind. They consider it the equivalent of mapping the communicative networks of a city, correlating its activity with ongoing social events, and then declaring the material basis of culture solved. Even if brain activity is mapped completely, they ask, where does that leave consciousness, and especially subjective experience? How to express joy in a summer rainbow with neurobiology? Perhaps these phenomena rise from undiscovered physicochemical phenomena or exist at a level of organization still beyond our comprehension. Or maybe, as a cosmic principle, the conscious mind is just too complicated and subtle ever to understand itself.

This view of the mind as mysterium tremendum is, in the opinion of most brain researchers, unjustifiably defeatist. It is the residue of mind-body dualism, the impulse to posit a master integrator–whether corporeal or ethereal– located somewhere in the brain and charged with integrating information from the neural circuits and making decisions. The perception weighs too lightly the alternative and more parsimonious hypothesis: That activity of the neural circuits is the mind, and as a consequence nothing more of fundamental aspect is needed to account for mental phenomena at the highest levels. In this view, the hundred million or so neurons, each with an average of thousands of connections to other neurons, are enough to symbolize the thick stream of finely graded information and emotional coloring we introspectively recognize as composing the conscious mind.

To envision the immense amount of information that can be encoded, consider the following hypothetical example supplied by neurobiologists. Suppose that the chemoreceptive brain were programmed to sort and retrieve information by vector coding. Suppose further that combined activities of nerve cells imposing the codes classify individual tastes into combinations of sweetness, saltiness, and sourness. The brain need only distinguish 10 degrees in each of these taste dimensions to discriminate 10x10x10 or 1000 substances.

A large part, if not the totality, of mental activity comprises scenarios built with such symbolic information. The scenarios are usually reconstructions of the here and now, during which the brain is flooded with fresh sensory information. Many others recreate the past as it is summoned from long-term memory banks. Still others construct alternative possible futures, or pure fantasy.