There is a huge literature on the question of what makes an assertion such as
There is a car in my driveway
appropiate. In the philosophy I am familiar with, the debate is cast as being about the norm of assertion:
The quest for the norm of assertion is the quest for a norm (roughly, an imperative) such that a speech act counts as an assertion if and only if it is subject to said norm. Candidate norms of assertion take the form
: Assert that p only if F(p)
where F(p) is a function that takes the proposition to be asserted and outputs a specification of the circumstances in which such an assertion is warranted. There are various contenders for the role of F(p) . Let me discuss them briefly in turn.
: F(p) takes p to itself.
That is: one should assert only what is true (see Weiner 2005, 2007.) This looks like a natural demand on assertion; after all, assertions aim at the truth -- that is, it is very likely that they fulfill their function in language by mapping onto facts (see Millikan 1984, p. 108f). The standard complaint against Truth is that it is far too weak to be the central norm of assertion. For what if one truly asserts that p on the basis of extremely poor evidence, e.g., one asserts a lucky guess or a correctly believed product of wishful thinking? Surely, one may claim, the asserter in these instances would be subject to criticism despite satisfying Truth. (Lackey 2007, p. 604)
The obvious fix to the this problem is to demand that the asserter be justified in believing the asserted proposition. Thus, the justification norm,
: F(p) takes p to the asserter is justified in believing that p.
This is sometimes put in terms of the asserter having a reasonable/rational belief in p. Justification is also plausible: it seems reasonable to assume that the asserter has to be in the right kind of evidential relation to the proposition asserted -- and, indeed, we find assertions blameless when they are done in these circumstances. On the other hand, there are cases of objectionable assertions made in the appropriate evidential state. Lotteries seem to supply such examples: if I assert 'Your ticket did not win' merely because I know that only one in a milion does, I may have extremely good evidence -- one that makes you ticket not winning overwhelmingly likely, and my belief that it won't rational and reasonable -- but I still contravene the norm of assertion. Williamson summarises the idea:
I may believe on good evidence that your lottery ticket did not win; I am not warranted in asserting that it did not win. I may believe on good evidence that I shall not be knocked down by a bus tomorrow; I am not warranted in asserting that I shall not be knocked down by a bus tomorrow. Neither belief nor belief on good evidence warrants assertion. (Williamson 2000, p. 260)
So, if truth is not enough and justification is not enough, the obvious next place to look is knowledge.
: F(p) takes p to the asserter knows that p.
This is, probably, the mainstream position in this debate. Knowledge has been defended, e.g., by Williamson 2000, DeRose 2002 and many others since. This norm seems to solve the problems with Truth and Justification: in none of these cases does the asserter know the asserted proposition. It is also well positioned to explain the appeal of Truth and Justification: knowing the target proposition implies both that it is true and that the asserter is in a good evidential situation. But Knowledge has not passed unchallenged. Pressure coming from Truth defenders takes, not unexpectedly, the form of unknown but true propositions which it is proper to assert. Weiner 2005, for example, offers examples of assertions based on reasonably competent hunches. For example, 'This is the work of Professor Moriarty! It has the mark of his fiendish genius.' as said by Sherlock Holmes, based merely on the looks of the deed in question -- cf. Weiner 2005, p. 231 -- seems like a proper assertion, but it also seems that Sherlock doesn't know that Moriarty did it; he is just making an educated guess.
This is just a very quick introduction to the problem of the norm of assertion. If you ask me, I think the sensible position is pluralism about such norm: in a certain sense truth is enough to make an assertion appropriate; in another sense justification is enough; in yet another sense knowledge is enough. There are good arguments for this kind of pluralism, but their discussion would take us too far afield.
DeRose, Keith. 2002. “Assertion, Knowledge, and Context.” Philosophical Review 111: 167–203.
Lackey, Jennifer. 2007. “Norms of Assertion.” Noûs 41: 594–626.
Millikan, R. 1984. Language, Thought and Other Biological Categories. The MIT Press.
Weiner, Matthew. 2005. “Must We Know What We Say?.” Philosophical Review 114: 227–251.
———. 2007. “Norms of Assertion.” Philosophy Compass 2: 187–195.
Williamson, Timothy. 2000. Knowledge and Its Limits. Oxford University Press.