Back in 1928, the sociologists W. I. Thomas and D. S. Thomas conceived a theorem, now named after them, that states, “If men define situations as real, they are real in their consequences”.
If we believe that there is a monster in the forest and we change our route to avoid it, then our belief is real in its consequence.
If we define some emotions, such as sadness or anger, as negative and we avoid them , then our belief is real in its consequence, we have fewer tools to understand ourselves, others and the impact of our relationships.