William Isaac Thomas (August 13, 1863 – December 5, 1947) was an American sociologist, understood today as a key figure behind the theory of symbolic interactionism. Collaborating with Polish sociologist Florian Znaniecki, Thomas developed and influenced the use of empirical methodologies in sociological research and contributed theories to the sociology of migration. Thomas went on to formulate a fundamental principle of sociology, known as the Thomas theorem (aka Thomas dictum), whereby he would contend that "if men define situations as real, they are real in their consequences." This microsociological concept served as a theoretical foundation for the field of symbolic interactionism which was developed by Thomas's younger peers—primarily at the University of Chicago.