Alfred North Whitehead (x-y) was a great British mathematician and philosopher who ended his career At Harvard University in America.

Whitehead said that "The negative judgment is the height of perception." This means that we sometimes know "what is", but often know "what is not".

For example, you drop an erring or button or whatever on the rug in your bedroom with the door closed. You don't know immediatley where it is and perhaps not later. But you know it couldn't have bopunced its way across the room and under the door -- that is, it's not outside the bedroom.

In mathematics, it's difficult to prove that a statement is always correct, but one