Carl Rogers has defined five dimensions for the model of a fully functioning person. Arguably, one of the most difficult of these to achieve is "trust feelings." That is, an individual does not deny, avoid, or suppress their instincts or intuitions. The individual trusts that their instincts are there to benefit them or signal something important to them in some way. Although they may not always be correct, a fully functioning person knows how to reason with them in a proper way.

In Winnicott's theory of the psychology of self, there is a constant tension between a person’s true self and false self. The true self can be best defined as a person’s raw being—who they are in their body and what they actually experience. The false self is how a person relates to surroundings. Sometimes the false self will come out in order to protect the true self. In the most severe cases the false self will completely replace the true self, creating a sense of disconnection from actual life experiences.

According to Erickson’s theory person who successfully completes a psychosocial development stage achieves ego quality—or ego strength. If a stage was managed poorly, then it will result in insecurity. 

Rene Descartes, a French philosopher who contributed greatly to the study of learning (i.e. the process by which we come to have knowledge), suggested that some of our knowledge is innate. In other words, human beings are born with ideas about certain concepts, such as infinity, perfection, or divine deities. In learning, this is referred to as the concept of nativism.

Darwin’s theory of evolution suggests that for all species—including humans—the mind (i.e. the capacity to learn) has evolved along with species’ adaption to environments. Evolutionary theory suggests that all species are related through a common evolutionary history. These theories have provided validation to the idea that animals can be studied and the findings of this research may be generalized to human beings.

Language learning is used as a prime example of the theory of biological preparedness because (a) infants appear to be born with innate language learning skills, (b) children in every culture share the developmental experience of learning language, and (c) brain scans indicate that specific areas of the brain are dedicated to language. 

Locke suggested the idea of empiricism, and believed that we experience the world through our senses, process the knowledge in our minds, and learn through our capacity to understand cause and effect. This was based on the writings of Aristotle, who wrote that all learning occurs through associations that we make about objects that are contiguous in time or space. 

Wilhelm Wundt would be the most likely to agree that nature and genetics are more influential than external or environmental factors in determining a person's traits. Wundt also utilized and expanded upon other concepts associated with Plato. For example, Wundt used the idea of mental imagery to further psychology and discovery through experimentation. Aristotle, Skinner, Watson, and Thorndike, on the other hand, believed that environment and experiences were more influential in determining a person's traits. 

The ego acts as a mediator between the id and the superego. The id is very impulsive, operating on the pleasure principle and seeking immediate gratification. The superego represents the ideal in every situation and operates on the perfection principle. It also is the part of our personality with morals and standards; thus, it produces feelings of guilt and pride. The ego is mediates between the id and the superego and is involved with the prefrontal cortex in planning. By planning, the ego is able to accomplish goals and maximize pleasurable results. Unlike the id, the ego can delay gratification using secondary processes. 

The oral psychosocial stage lasts from approximately 0-1 year. The psychosocial stages increase in age starting from oral and then progressing to anal, phallic, latency, and settling at the genital level. Freud proposed that throughout the duration of life, libido—sexual desire—shifts from one part of the body to another. Freud proposed that problems arise when one of these psychosocial stages was not satisfied in an individual. For example, if an adult is addicted to smoking cigarettes, then Freud might say that as a child he or she was punished for sucking on his or her thumb. As a result, this patient never satisfied the oral psychosocial stage; therefore, this patient seeks oral gratification from smoking.