Here is a nice refutation of the everyone knows argument kindly submitted by Jim Smith.
The argument "everyone knows" to substantiate a belief is not one that will withstand logical examination. Here are a few examples of what "everyone knows" that are observably false.
At one time, over 99% of the people on earth "knew" the earth was flat. This is in spite of the fact that the curvature of the earth is observable from any seashore or mountain top. No matter where you go on earth, the curvature, to the eye, remains the same. This shows that it is not a local phenomenon but a consistent feature of the globe. Yet, "everyone knew" the earth was flat.
As recently as 1900, at least 99% of the population would tell you that, "Man will never fly". Even more recently many would have said, "If God wanted man to fly, he would have given us wings". Nonetheless, we do fly despite what "everyone knows".
Even in 1960, most people would not believe that men would ever walk on the moon. Even after the fact, many swore it was a stunt filmed in a Hollywood studio. Because "everyone knows" that is just "Buck Rogers stuff".
Even more recently, something as ubiquitous today as cell phones were a fantasy from the Dick Tracy comic strip. Because "everyone knows" you can't have a phone without wires that will send voice, text, and pictures.
The truth is, just because a lot of people say a thing is true, does not make it true. As Abraham Lincoln once said, "If I am right, everyone will know it soon enough. If I am wrong, a hundred angels swearing I am right will not change it."
This brings me to Rule seven in my list of universal truths. "Beliefs, no matter how sincerely held, do not alter facts." Even if millions of people share those beliefs, it does not change the facts. The one thing that never can change is that very few people ever bother to consider facts when forming their beliefs. Mark Twain remarked, "Common sense is the sum total of all prejudices acquired by the age of 16."