 Kielhofner's main theoretical contribution was his idea that a complete understanding of the complex manner through which human occupation is initiated, pursued and completed necessarily requires systems approaches that rely upon research in the biological, psychological and social sciences.He was among the first theorists in his field to use general systems theory and later dynamical systems theory to describe the complexities of his model, which described the iterative, interactive relationships between a person's volition, habituation (roles and habits), performance capacity, and the social and physical environment.He was also the first theorist to clearly explain the importance and relevance of the temporal dimension to occupational therapy practice.