Thomas’ Theorem is a sociological theorem authored by W.I. Thomas and D.S. Thomas in 1928. It states that “if men define situations as real, they are real in their consequences.” In short, this means that humans define and respond to situations as a result of their own experiences and thereby create the consequences that they expect.
An example of this might be how a person who firmly believes in the existence of and power of supernatural forces, i.e. voodoo or witchcraft, is much more likely to suffer from illnesses or injuries that they believe could be brought about by practitioners of those arts.