Assertion has both a pragmatic and a semantic aspect. First and foremost, assertion is a speech act. As such, assertion or asserting is a different type of speech act from commanding, asking a question, or expressing a wish. But if we consider the language system in abstraction from the uses to which it is put by speakers, we can distinguish among different types of sentence. We can distinguish among the grammatical moods: indicative or declarative, imperative, interrogative, and optative, among others. The mood distinctions belong on the side of semantics, on the side of linguistic meaning. Linguistic meaning is the meaning a sentence type has in virtue of the conventions of the language system to which it belongs. Speech acts, however, involve the tokening of sentence types.
So on the pragmatic side we have the distinctions among speech acts, and on the semantic side, the distinctions among moods. One question that arises is whether the speech acts map neatly onto the moods. When I make an assertion, must I use an indicative sentence? Or can I make an assertion using a non-indicative sentence? And can I utter an indicative sentence and not make an assertion? Can I make assertions using interrogative sentences? Can I make assertions using imperative sentences? Can one ask a question using an optative sentence? Here are five theses that seem true. Examples follow.
T1. One can make an assertion using a non-indicative sentence.
T2. One can utter an indicative sentence and not make an assertion.
T3. One can utter an interrogative sentence and be taken by one's audience to be making an assertion.
T4. One can utter an imperative sentence and thereby make an assertion.
T5. One can utter an optative sentence and thereby ask a question.
Ad (T1). Suppose A sincerely asks, 'Does God exist?' and B replies, 'Is there an angry unicorn on the far side of the moon?' It seems that B has answered A's question, and has done so by making an assertion, an assertion more straightforwardly put by an utterance of the indicative sentence, 'God does not exist.' And yet B has made his assertion by uttering an interrogative sentence. This appears to be evidence that one can make an assertion by using a non-indicative sentence. An interrogative form of words can be used to make an assertion.
Ad (T2). Suppose Johnny is picking his nose in public, and Mommy says to Johnny, 'We don't do that.' Mommy utters an indicative sentence, and yet does not make an assertion; she issues a command. A second example. 'Obama sucks' is an indicative sentence. But a tokening of this sentence type will not typically express a proposition or convey an assertion; it will typically be used to express dislike or contempt.
Ad (T3). I bought a hat recently at a swap meet for $20 and I asked the lady who sold it to me, 'Do you know how much this hat would cost at a retail outfitter's?' Her reply: 'How much?' The lady took me to be making an assertion that would normally be couched in some such indicative sentence as, 'I know how much this hat would cost elsewhere.' And yet, I was asking a genuine question. (I need to write a separate post on rhetorical questions.)
Ad (T4). Davidson gives this example: 'Notice that Joan is wearing her purple hat again.' The sentence is in the imperative mood, but is used to make an assertion. An example of my own. I say something you disagree with, and you reply, 'Get out of here.' You are not commanding me to leave, but denying my assertion with a counter-assertion clothed in an imperative form of words.
Ad (T5). Another example from Davidson: 'I'd like to know your phone number.' This grammatically optative sentence might be used to ask a question that would be put more directly as 'What is your phone number?'
This linguistic data seems to support the view that we must distinguish between speaker's meaning and sentence meaning, which is a species of linguistic meaning. Sometimes they coincide, but often they come apart, as in the above examples. What a speaker means or intends by his speech act often diverges from what sentences mean when considered in abstraction from speakers and their intentions.
REFERENCE: Donald Davidson, "Moods and Performances" in Inquiries into Truth and Interpretation, Oxford, 1984, pp. 109-123.