I am open to the option that my brain has the saved the wrong definition on opinion, but
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http://dictionary.reference.com/browse/opinion noun 1. a belief or judgment that rests on grounds insufficient to produce complete certainty. 2. a personal view, attitude, or appraisal. 3. the formal expression of a professional judgment: to ask for a second medical opinion. 4. Law. the formal statement by a judge or court of the reasoning and the principles of law used in reaching a decision of a case. 5. a judgment or estimate of a person or thing with respect to character, merit, etc.: to forfeit someone's good opinion. 6. a favorable estimate; esteem: I haven't much of an opinion of him.
1 a : a view, judgment, or appraisal formed in the mind about a particular matter b : approval, esteem 2 a : belief stronger than impression and less strong than positive knowledge b : a generally held view 3 a : a formal expression of judgment or advice by an expert b : the formal expression (as by a judge, court, or referee) of the legal reasons and principles upon which a legal decision is based
I can not find a source that would support the claim that it would be a requirement to be not verifiable for an opinion.