Isn't So
It isn't what we don't know that gives us trouble, it's what we know that ain't so. - Will Rogers
It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so. - Mark Twain
I honestly believe it is better to know nothing than to know what ain't so. - Josh Billings, 1874
And how is not this the most reprehensible ignorance, to think that one knows what one does not know. - Socrates, "Plato"
I appear to be wiser than he, because I do not fancy I know what I do not know - Socrates, "Apology"