After Hall and Lindzey (1957) introduced the theory-by-theory approach to the field, a dilemma arose. Hall and Lindzey organized the discipline beautifully, but left it divided theory-by-theory, with little apparent means of bridging their differences.
A decade later, Salvatore Maddi (1968) proposed a potential solution to the problem. He believed that the grand theories, those of Freud, Jung, Rogers, and others, could be compared with each other, and then the differences among them settled through research. To compare the theories, Maddi suggested a meta-theory -- a theory about personality theories. That meta-theory stated that the grand theories all address three areas of personality: its core, development, and periphery. Core statements, according to Maddi, concern the biological, fixed bases of personality. Developmental statements describe the processes which draw out the individual's core so that the person is effective in meeting environmental demands. Finally, through development, an individual takes on specific attributes or becomes a given personality type. The attributes and types are personality's periphery.
Using Carl Rogers' humanistic psychological theory as an example, Maddi might summarize it as: A person is born with an innate tendency for healthy growth (core). That growth tendency can be promoted through support and love from the environment (development), and if so promoted, will result in a fully-functional person (periphery).