The Introductory Paragraph.

An opinion is something that cannot be proven. How that is useful, I have no idea.

From wordnetweb.princeton.edu/perl/webwn:

• a personal belief or judgment that is not founded on proof or certainty; "my opinion differs from yours"; "I am not of your persuasion"; "what are your thoughts on Haiti?"
• a message expressing a belief about something; the expression of a belief that is held with confidence but not substantiated by positive knowledge or proof; "his opinions appeared frequently on the editorial page"
• public opinion: a belief or sentiment shared by most people; the voice of the people; "he asked for a poll of public opinion"
• the legal document stating the reasons for a judicial decision; "opinions are usually written by a single judge"
• the reason for a court's judgment (as opposed to the decision itself)
• impression: a vague idea in which some confidence is placed; "his impression of her was favorable"; "what are your feelings about the crisis?"; "it strengthened my belief in his sincerity"; "I had a feeling that she was lying"

You just failed.

@kaede-kit "a belief or judgment that rests on grounds insufficient to produce complete certainty."

-Random House Dictionary http://dictionary.reference.com/browse/opinion

I'd make the distinction between not being based on certainty and not being based on complete certainty.

If you can't understand how opinions are useful you have my sincerest sympathies since performing every day activities like choosing between two brands of cereal must extremely difficult for you.