Understanding the human mind in biological terms has emerged as the central challenge for science in the 21st century. We want to understand the biological nature of perception, learning, memory, thought, consciousness and the limits of free will. That biologists would be in a position to explore these mental processes was unthinkable even a few decades ago. Until the middle of the 20th century, when I began my career as a neuroscientist, the idea that mind, the most complex set of processes in the universe, might yield its deepest secrets to biological analysis and perhaps do this on the molecular level could not be entertained seriously.