According to Paul van Geert, logistic growth is a function of three parameters: the present level, the rate of change, and a limit on the level that can be reached that depends on the available resources for the functioning of the process under consideration. Educational psychologists have also expanded the field into the study of student development. Some important theorists in this field include Arthur W. Chickering and William G. Perry. Arthur W. Chickering taught that students want to learn because of future success in career and life. Developing cognitive and critical think skills are important in preparing students all success. A way to bring self-confidence and competence is done by having a purpose in all their actions they take and what motivates them. Pascual-Leone, Case, and Halford attempt to explain development along the sequence of Piagetian stages and substages. Pascual-Leone aligned this sequence with a single line of development of mental power that goes from one to seven mental units. Case suggested that each of four main stages involves different kinds of mental structures and he specified the mental load of the successive levels or substages of complexity within each of the main stages. He said that there may be different central conceptual structures within each level of executive control structures that differ between each other in reference to the concepts and semantic relations involved. Halford attempted to specify the cognitive load of the mental structure that is typical of each of the main stages. Fischer stressed the importance of skill construction processes in building stage-like constructs and he emphasized the role of the environment and social support in skill construction. Commons offered a description of the successive levels of cognitive development while allowing for the explicit reference to the particularities of concepts and operations specific to each of the domains. Demetriou integrated into his theory the constructs of speed of processing and control of processing, and he formulated the functional shift model, which unifies Pascual-Leone's notion of underlying common dimension of capacity development with the notion of qualitative changes in mental structure as development progresses along this dimension. Dynamic systems theory can model how different processes interact dynamically when developmental hierarachies are built. It is suggested that fluid intelligence, that is the general mechanisms underlying learning, problem solving, and the handling of novelty, depends on these developmental processes. Changes in these very mechanisms seem able to explain, to a considerable extent, the changes in the quality of understanding and problem solving at successive age levels. An overarching definition of intelligence can be as follows: The more mentally efficient (that is, the faster and more focused on goal), capable (that is, the more information one can hold in mind at a given moment), foresighted (that is, the more clearly one can specify his goals and plan how to achieve them), and flexible (that is, the more one can introduce variations in the concepts and mental operations one already possesses) a person is, the more intelligent (both in regard to other individuals and in regard to a general developmental hierarchy) this person is.