W.I. Thomas, or William Isaac Thomas, was an American sociologist born in 1863 and known for his contributions to the field of sociology. He is best recognized for the Thomas Theorem, which states that if people define situations as real, they are real in their consequences. This idea emphasizes the importance of perception in social interactions. Thomas was also influential in the development of Chicago School sociology, which focused on urban sociology and the study of social behavior in cities. His work laid the groundwork for understanding how social environments shape individual actions and societal norms.