"... if we can always save any theory in the face of conflicting evidence, then we can never know any theory to be true. If you know A you cannot believe or reasonably assert not-A, no matter what the evidence. Since, given the Quine-Duhem thesis, you can always believe or assert not-A, you can never know A. To put the point intuitively, the world never forces acceptance or rejection of theories. Within very wide limits we can believe what we want." (P 214b)

In other words:

According to the Quine-Duhem thesis, no hypothesis can be conclusively refuted. So we can consistently claim not-A (e.g., the earth does not revolve around the sun). So we can not know A (viz., the earth does revolve around the sun).