I suspect the kind of opinion the quote is talking about is as defined here; a belief or judgment that rests on grounds insufficient to produce complete certainty.
Adherents of rule #1, however, will never have grounds sufficient to produce complete certainty, in which case, “fire is hot” is an opinion by that definition.
However, a a pure counterexample rather than a mere logical knot:
could you please provide alternative examples of conventional opinions that were also held in the past?
Sure. “Murdering your brother out of jealousy is wrong.” That’s a fairly conventional opinion, no?
Adherents of rule #1, however, will never have grounds sufficient to produce complete certainty, in which case, “fire is hot” is an opinion by that definition.
However, a a pure counterexample rather than a mere logical knot:
Sure. “Murdering your brother out of jealousy is wrong.” That’s a fairly conventional opinion, no?