The purpose of theory is to predict the future.
Theory is validated by how well it explains the past.
The predictive nature of theory is dependent upon how well the future reflects the past data selected for validation. The more the future differs from the past, the greater uncertainty surrounds the prediction.

A theory has only the alternative of
being right or wrong. A model has a
third possibility - it
may be right but irrelevant.
•Manfred Eigen
http://www.quotemeonit.com/theory.html

Why is typing people so very difficult even with these wonderful theories?