Abstract
From religion to philosophy to science, all human systems of definition are formed by human brains. The nature and limits of the human brain are the nature and limits of those systems. This essay shows how the human brain works normally then unusually, and what this reveals about the limits of human knowledge. There are many conditions and instances where the brain processes information unusually, including mental disorders, physical events, and drug use. This essay focuses on the neurological events called mystical experiences.