There is much more to knowledge of reality than sensory experience. Human discourse is full of concepts, including knowledge and reality, that are not directly tied to what we can see, touch, taste, smell, or hear. Philosophers, psychologists, and now neuroscientists attempt to figure out the nature of such concepts. For Plato, concepts were abstract entities he called the forms, existing in some heavenly realm graspable by souls. In contemporary cognitive science, concepts are mental representations, which the previous chapter implies are brain representations. A major current research problem is to figure out how patterns of neural firing play all the roles needed to explain the many cognitive uses of concepts.