Bertrand Russell on knowledge for the Encyclopedia Brittanica:
[A]t first sight it might be thought that knowledge might be defined as belief which is in agreement with the facts. The trouble is that no one knows what a belief is, no one knows what a fact is, and no one knows what sort of agreement between them would make a belief true.
But that wonderful quote is misleading if left there. In fact it introduces Russell’s careful exploration and explanation of those terms. Crucially: “We are thus driven to the view that, if a belief is to be something causally important, it must be defined as a characteristic of behaviour.”