The purpose of this paper is to lay the groundwork for the development of a scientific theory of complex human functioning. We first discuss the assumptions on which our thinking is based, then advance the argument that behavior, and human activity in general, may be more fully understood in light of current data on the structural organization of the central nervous system. The brain is organized as a modular, distributed, self-organizing system, which is in constant transaction with the environment. Because of its plasticity, structural and functional change occurs in the brain as a result of experience throughout life. It is our thesis that complex human behavior is organized in a similar manner - that is, human personality and behavior manifest themselves as modular systems. The insights provided by an understanding of the relationship of brain and behavior may enhance the capacity to explain both normal and pathological personality functioning.