A theory is a mental abstraction that helps us explain things, ourselves, and events.
Theories come and go.
That's one reason most people in nonfiction fields approach dated books or materials with some degree of skepticism. Theories affect the men who know them. The Western man can never completely understand the Asian man because each has a unique theoretical interpretation of the events of the world. Theories give direction to the modern frontiers of pedantic knowledge. But the former can also lead to theory modification, providing the necessary adjustment of claims to keep it in vogue as a way of explaining reality.