From Jason Stanley’s Knowledge and Certainty:
If knowing a proposition requires that proposition to be true, we would expect (7) to sound like an assertion of a trivial conceptual truth and (8) to sound like an assertion of an obvious falsity:
(7) Everything anyone knows is true.
(8) There is something someone knows that isn’t true.
(7) is obviously true, and (8) obviously false. Similarly, if knowing a proposition requires believing that proposition, then we should expect (9) to be a trivial truth and (10) to be obviously false:
(9) Everything someone knows she believes.
(10) There is something someone knows that she doesn’t believe.
Finally, if knowing a proposition requires having evidence for that proposition,
we would expect (11) to sound like a trivial truth and (12) to sound obviously false:
(11) If someone knows something, she has a reason to believe it.
(12) There is something someone knows that she doesn’t have any reason
An assertion of (11) certainly seems true, and (12) seems false. If it is intuitively obvious that knowledge requires subjective certainty, we should expect (13) and (14) to seem like banal truths and (15) to seem obviously false:
(13) I’m certain of everything I know.
(14) Everyone is certain of everything she knows.
(15) There are some things I know, of which I’m only fairly certain.
However, (13) and (14), unlike (7), (9), and (11), do not sound like banal truths. An utterance of (15) also does not share the obvious sense of falsity of (8), (10), and (12). Similarly, if knowledge requires epistemic certainty, we should expect (16) to be a banal truth, on a par with (7) and (9), and we should expect (17) to seem clearly false, on a par with (8), (10), and (12):
(16) Everything I know is certain to be true.
(17) There are some things I know, which are only fairly certain to be
But (16) does not seem like a banal truth, and (17) seems perfectly in order.