Developmental systems theories are an important part of modern research of human development. These theories see individuals as a part of a developmental system. The system is composed of the individual and also of the environment. The theories emphasize the holistic principle in research and reject any form of reductionism. Developmental systems theories include dynamic systems theory, developmental contextualism, person–context interaction theory, and holistic, developmental, systems-oriented perspective. Unlike the dynamical systems theory, which is a mathematical construct, the dynamic systems theory of Esther Thelen and Linda Smith is primarily nonmathematical and comprises qualitative theoretical propositions in behavioral biology and is the broadest and most encompassing of all development theories.