Albert Einstein said, "The most incomprehensible fact about nature is that it is comprehensible." Indeed, physical phenomena are comprehensible. Weather changes, chemical reactions and molecular structures appear quite complex, but their underlying principles are amazingly simple. All physical phenomena are governed by four fundamental forces - gravity, electromagnetism, strong and weak nuclear forces. Recently, these forces can further be unified by a single theory -- the string theory.

What about the brain? Is its ability of comprehension comprehensible? In the seventeenth century, Isaac Newton had discovered the gravitational law and Robert Boyle had formulated the equation for gas properties, but human beings were still quite ignorant about physiological systems. William Harvey, a pioneer in blood circulation, told Robert Boyle that it was the valves in the veins which guided him to the next phase of his thinking.

We now know that thoughts are not controlled by blood vessels and sorrow has nothing to do with a "broken heart". They come from our brain -- the only thing which can try to understand itself. With a size smaller than a microwave oven, our brain contains more than 10 billions of neurons. Each neuron is connected to many (up to thousands) other neurons. The junction between neurons is called a synapse. Brain functions are the results of signal transmission through trillions of synapses.

Since neurons are made up of the same atoms as found in the physical world, it might be possible to explain brain functions in terms of physical laws. In the past few decades, significant progress has been made toward this goal. The formation of short-term memory through learning has been well understood. However, conversion from short-term to long-term memory is still a mystery. In Chapters 9 and 10, we shall present a possible mechanism which involves microtubules.