William Isaac Thomas

Born in 1863, he became one of the key early figures in American sociology and taught at the University of Chicago, where his work influenced the development of the Chicago school. His research often focused on social behavior, personality, immigration, and the way people interpret the situations around them.

He is widely associated with what became known as the Thomas theorem: the insight that when people define situations as real, those definitions shape what happens next. He also collaborated on major studies of immigrants and social life, including The Polish Peasant in Europe and America, a work that became important in sociological research.

Remembered for helping move sociology toward close observation of everyday life, he left a lasting mark on both sociology and social psychology before his death in 1947.