As a cognitive neuropsychologist, Tim Shallice considers the general question of what can be learned about the operation of the normal cognitive system from the study of the cognitive difficulties arising from neurological damage and disease. He distinguishes two types of theories of normal function - primarily modular and primary non-modular - and argues that the problems of making valid inferences about normal function from studies of brain-damaged subjects are more severe for the latter. He first analyzes five well-researched areas in which some modularity can be assumed: short-term memory, reading, writing, visual perception, and the relation between input and output language processing. His aim is to introduce the methods about normal function mirror ones derived directly from studies of normal subjects and indeed at times preceded them. He then more theoretically examines these inferences, from group studies and individual case studies to modular and non-modular systems. Finally, he considers five areas where theories of normal function are relatively undeveloped and neuropsychology provides counterintuitive phenomena and guides to theory-building: the organization of semantic systems, visual attention, concentration and will, episodic memory, and consciousness.