Heinz von Foerster is one of the founders of the science of cybernetics: he produced the first parallel computers and contributed to many other path-breaking developments. In these essays he discusses some of the fundamental principles that govern how we know the world and how we process the information from which we derive that knowledge. Included are path-breaking articles concerning the principles of computation in neural nets (1967), the definition of self-organizing systems (1960), the nature of cognition (1970).