The Thomas theorem is a theory of sociology which was formulated in 1928 by William Isaac Thomas and Dorothy Swaine Thomas:
If men define situations as real, they are real in their consequences.[1]
In other words, the interpretation of a situation causes the action. This interpretation is not objective. Actions are affected by subjective perceptions of situations. Whether there even is an objectively correct interpretation is not important for the purposes of helping guide individuals' behavior.
The Thomas theorem is not a theorem in the mathematical sense.