Sigmund Freud's theory of psychosexual development, introduced in his 1905 work "Three Essay on the Theory of Sexuality," posits that personality forms through a series of childhood stages. In these stages, pleasure-seeking energies become focused on distinct "erogenous zones" of the body. The five proposed stages are the oral, anal, phallic, latent, and genital, each associated with a different primary source of pleasure. Freud believed that experiencing frustration or insufficient gratification during any stage could lead to a "fixation," potentially resulting in anxiety or neurosis that persists into adulthood. Conversely, adequate satisfaction at each stage contributes to the development of a healthy personality. For instance, the oral stage (birth to one year) centers on the mouth for gratification from feeding and exploration.