Many anthropologists believed that societies develop according to one universal order of cultural evolution. This belief, called the Unilineal Evolution, explained cultural similarities and differences among societies by classifying them into three sequential stages of development: savagery, barbarism and civilization.
Franz Boas and his students did not believe in this theory and instead criticized it as based on insufficient evidence since there is no historical evidence to demonstrate its validity. He also criticized Unilineal Evolution for its method of gathering data because at that time many anthropologists relied on missionaries or traders for data collection and anthropologists themselves rarely went to the societies that they were analyzing. Boas argued that those anthropologists who did not really see and experience first hand those societies, organized their second-hand data in unsystematic manners to fit their preconceived ideas.
In answer to the theory of Unilineal Evolution, Boas and his students presented Historical Particularism. This theory claims that each society has its own unique historical development and must be understood based on its own historical context. Boas pioneered the idea of field research and he developed the method of participant observation as the basic strategy of collecting data. As an example, and based on his principle that cultural theories should be derived from concrete ethnographic data, Boas collected a vast amount of first-hand cultural data from Native American tribes in the United States. Using detailed ethnographic studies he argued that a society is understandable only in its own specific cultural context, especially its historical process. Boas believed that cultures form as the product of their environment, their psychology, and their history and should be understood on their own terms. He maintained that cultures could have similar traits for a variety of different reasons including simultaneously invention, adoption, or trade. This could be due to similar environments, cultural contact, or other forms of historical incidents.