PRINCETON - Fifty years from now, when Malia and Sasha are grandmothers, their father's presidency might seem most consequential because of a small sum - $100 million - for studying something small. "As humans," Barack Obama said when announcing the initiative to study the brain, "we can identify galaxies light-years away ... but we still haven't unlocked the mystery of the three pounds of matter that sits between our ears."

Actually, understanding the brain will be a resounding success without unlocking the essential mystery, which is: How does matter become conscious of itself? Or should we say, how does it become - or acquire - consciousness? Just trying to describe this subject takes scientists onto intellectual terrain long occupied by philosophers. Those whose field is the philosophy of mind will learn from scientists such as Princeton's David Tank, a leader of the BRAIN Initiative, which aims at understanding how brain regions and cells work together, moment to moment, throughout our lives.

If, as is said, a physicist is an atom's way of knowing about atoms, then a neuroscientist like Tank is a brain cell's way of knowing about brain cells. Each of us has about 100 billion of those, each of which communicates with an average of 10,000 other nerve cells. The goal of neuroscientists is to discover how these neural conversations give rise to a thought, a memory or a decision. And to understand how the brain functions, from which we may understand disorders such as autism, schizophrenia and epilepsy.

Biological causes have been determined for only about 3 percent of the disorders listed in the Diagnostic and Statistical Manual of Mental Disorders. With "mapping," scientists may at last establish connections between neurotransmitters and particular mental disorders. This might influence how pharmaceutical companies direct their research. And treatments of post-traumatic stress disorders might benefit from learning how the mind erases disturbing memories.

Much brain activity - much thinking - is not, Tank notes, the result of external stimuli. So, is the brain conversing with - acting upon - itself? This internal conversation is at the core of who - and what - we are.

New technologies enable scientists to watch the brain in action, monitoring neural activity as it thinks. Even a decades-old technology, functional magnetic resonance imaging (fMRI) reveals, Tank says, "what parts of the brain are active in particular computations and behaviors." In 50 years, fMRI images will seem as crude as Magellan's maps. We will understand thought processes with instantaneous cellular resolution, and hence the essence of what brains do, and what derails them.

Development of the transistor, progenitor of the Digital Age, required only advances in materials science. There is, Tank says, "no comparable base of knowledge for the brain" because there is no mechanistic understanding of how the brain works. Pharmacology is groping for therapeutic effects because drugs target particular receptors the workings of which are not understood. To the brain, small pills can be sledgehammers. Understanding brain dynamics will enable ever more precise chemical and other interventions.

Ethicists - and courts, and poets - will be warily watching what is learned about the neural basis of choices, habits, love and other important things.

Do we have bodies or are we bodies? What will become of the field of psychology as explorations of brain anatomy advance our understanding of how brain architecture influences, or even determines, behavior? "The devil made me do it" is no longer an exculpation. But what about "My brain circuitry made me do it"? Someday debates about free will may be resolved by understanding that we are responsible for our actions because we have "ownership" of three especially intricate pounds of matter.

George Will's email: georgewill@washpost.com.