Why think there's any tension between calling something a theory and calling it factual?
Well, it turns out that some proficient users of English (philosophers, even) have drawn a sharp contrast between theory and fact.
Roy Wood Sellars (Wilfrid's father) was a prominent American philosopher (Ontario-born) at the University of Michigan. He was elected President of the American Philosophical Association's Western Division in 1923. He taught a logic course at Michigan and wrote a textbook based on the course. The text, Essentials of Logic, was first published in 1917. Sellars released a revised edition in 1925. I can find only the 1917 edition on-line but the following quotation is from the 1925 edition.
Just after characterizing the percept-concept distinction as a psychological one, Sellars says that for logic,
the more distinctive contrast is between fact and theory, between what we accept and what we dispute. ... [W]e should constantly seek to separate the facts of the case from theory which is added as an interpretation. Confusion and disputation arise where fact and theory are not sharply distinguished. ... Now, fact is that which is granted on the basis of good evidence; it is that which is considered indisputable. ... [F]act in this sense may be either concrete or abstract. The Law of Gravitation is a fact because it is an assertion which is not questioned. For some, evolution is another such fact; for others, it is not. A theory, on the other hand, is that which is tentative, but which would, if accepted, solve the problem. (Sellars, Essentials of Logic [Revised edition, 1925], pp. 28-29)So, where some would say that our physicists have a correct theory of gravitation, a theory that has been shown by the facts to be true, Sellars would say, instead, that the relevant physical formulations are fact, not theory. For me, evolution is fact, not theory, but it might, for you, be theory and not fact; I could try to change your mind by showing you good evidence for evolution, but that evidence better not support an interpretation (since that's just theory). I don't like this way of using the word 'theory', but I guess it can't be dismissed as a recent abuse of the term. (The 1917 edition of Sellars' book has a shorter statement of the fact-theory contrast [pp. 24-5], one that does not include evolution as an example.)
Back to opinion. As Sellars uses the word, theory can of course be distinguished from opinion; e.g., perhaps he would say that theories are more elaborate and are more geared towards solving specific problems than opinions need to be. Still, on Sellars' way of speaking, theories resemble opinions (as defined by Reid and Locke in the previous post) in at least one respect, since they, too, are more 'tentative', or less certain, beliefs; in Reid's terms, they're judgments with 'some mixture of doubt' (Reid, Essays on the Intellectual Powers of Man, Essay VI 'Of Judgment', Ch. I ['Of Judgment in General'], sec. iii, 1785).
Finally, note that in the above passage, Sellars puts forth an epistemological conception of fact. For him, facts aren't just truths; rather, they're truths 'granted on the basis of good evidence' (Sellars, 1925, p. 28).
