Is Certainty Necessary for Knowledge?

The answer is “no.” Consider the argument 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 to believe.

“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
true.

“But (16) does not seem like a banal truth, and (17) seems perfectly in order.”

From page 4 (37). Here’s more:

“If one proposition obviously entails another, it will feel redundant to follow an assertion of one with an assertion of the other. So, redundant conjunctions provide evidence of entailments. In the case of the relation between knowledge and truth, we clearly see such evidence of entailment, as is witnessed by the oddity of (22):

“(22) I know that Bill came to the party. In fact, he did.

“We also see similar evidence in the case of the relation between knowledge and belief, as (23) is just as odd as (22):

“(23) I know that Bill came to the party. In fact, I believe he did.

“The reason (22) and (23) are odd is that knowing entails truth and belief. No new information is conveyed by assertions of the second sentences in (22) and (23). So, the utterances seem pointless, and the discourses odd.

“In contrast, we see no such evidence of entailments in the case of subjective and epistemic certainty. In both cases, there is no similar sense of redundancy:

“(24) I know that Bill came to the party. In fact, I’m certain that he did.
“(25) I know that Bill came to the party. In fact, it’s certain that he did.

“The discourses in (24) and (25), in contrast to those in (22) and (23), do not seem odd at all. The assertions of the second sentences seem to add new information to the information expressed by “I know that Bill came to the party”. If knowledge entailed subjective and epistemic certainty, this fact would be a mystery. The discourses in (24) and (25) would be just as odd as the discourses in (22) and (23). If knowledge does not entail subjective and epistemic certainty, this fact is explicable.”

From page 6-7 (38-39).

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