Scientific progress depends on the pursuit of good explanations, which are defined as theories that are difficult to vary while still accounting for the observed phenomena. Unlike bad explanations—such as myths or superstitions that can be easily modified to fit any new data—robust theories are deeply connected to the specific details of the reality they describe. The author argues that testability alone is insufficient for advancement, as even a refuted theory offers no path toward truth if its core components are arbitrary and replaceable. True rational inquiry begins with a problem, characterized by a conflict between existing ideas or expectations, and seeks a resolution through creative conjecture. This scientific frame of mind rejects instrumentalism and relativism, insisting that the physical world is real and that our explanations must accurately map its underlying mechanisms. Ultimately, knowledge grows not just by rejecting errors, but by seeking hard-to-vary accounts that truly explain why things happen as they do.