Opinion is a slippery concept. Especially when you try to distinguish it from fact.
Now I know what you’re thinking. As a matter of fact, I did just fine on that section in third grade. “Babe Ruth has the highest slugging percentage of all time” is a fact; “Babe Ruth is the greatest baseball player of all time” is an opinion. Clear enough. But which quality, exactly, distinguishes them? Does ‘opinion’ just mean ‘more vague than fact’?
As far as I can tell, people distinguish opinion from fact in 3 mutually distinct ways, all of which are listed in the dictionary:
(One might hope that these last two will correspond in practice; that people will believe things with less certainty the more disagreement exists on it. But realistically, this is neither necessary nor necessarily desirable. We do value the quality of sticking to one’s guns in the face of opposition, after all – at least in principle – and even dignify it with the name “integrity”.)
A word that equivocates among these three meanings is not a very useful word. For one thing, only the first definition distinguishes itself unambiguously from fact; the other two require drawing an arbitrary line somewhere on the scale of certainty or consensus, lest we call all propositions opinions.
My own preference is to jettison the word ‘opinion’ entirely; to teach third-graders the fact-value distinction in its place, and also epistemic humility in general to account for the other two definitions. (Note that I have called this my preference rather than leaving the meaning ambiguous with opinion!) I can, in fact, think of no situation where the word serves to clarify rather than obfuscate. Whether as a cudgel (“That’s just, like, your opinion, man”) or as a shield (“This is just my opinion, but…”), there is all the difference in the world between claiming a proposition to be a value, an uncertain belief, or a controversial belief.
Values are meaningful. Certainty is meaningful. Consensus is meaningful. Opinion? Time to toss it from the lexicon.