What makes an opinion valid?

"Your opinion is only as good as the facts you use to back it up."

That's what my US government teacher always used to say back in high school to keep class discussions focused.  Given the dismal state of public discourse in the US today, this axiom seems more important than ever.  Yet things have spiraled so badly out of control that it also strikes me as sadly inadequate.  


I would like my readers to help me remold this statement to give it greater instructional value in injecting some meaning back into the vitriolic - and often pointless - debates which rage throughout our society today.  


Here's a first stab:


For an opinion to have validity, it must not only be backed up by facts, but also by a grasp of what constitutes a proven fact, and by an understanding of the facts in their proper context.  


I look forward to your thoughts on how to improve this statement.