Joseph F. Rychlak

“The word theory is often interpreted as meaning something that is unfounded or speculative, not accurately tied down to reality. We say "That's only a theory," meaning that it is subject to doubt and challenge by alternative theories. And we also say "This is a fact," meaning it is not to be doubted or challenged by alternative theories. Though it may be all right to distinguish between theory and fact this way in common language, the greater fact of the matter is that in science we have learned to accept the likelihood that two or more theories can account for the same pattern of observed facts (often called laws). We can prove as facts both that light is a series of particles and that it is a series of waves. Given such alternative explanations, it is proper for us to call everything that we understand mentally a theory, thereby recognizing that theory and thought are two ways of describing the same thing. It is impossible to theorize without thinking, and thinking is always a theoretical process. What we think is factual today, that is, our current theory, may on new evidence, or simply throgh re-analysis of existing evidence, turn out to be an error tomorrow.”
― Joseph F. Rychlak, Introduction to Personality and Psychotherapy: A Theory-Construction Approach