Once one steps back and considers how language is action, it becomes apparent than any utterance can *do* something beyond the literal meaning associated with the string of words. Consider how the same message is conveyed (indirectly) via the following:
(25) I'm cold.
(26) Can you close the window?
(27) I'd be much obliged if you closed the window.
(28) You might want to close the window.
(29) May I ask you to close the window?
(30) Do me a favour with the window.
(31) Did you forget the window?
Indirect speech acts are those for which the intended force differs from that conventionally associated with the utterance form. You can use a paraphrase to access the illocutionary force conventionally associated with an utterance. For example, you can add "I'm informing you that ..." to (25) to see that the conventional force associated with an assertion like (25) is an act of informing. However, the intended illocutionary force is an act of requesting. A parallel difference holds for (26): The paraphrase might be something like "I'm asking whether you're able to close the window", revealing that the conventional illocutionary force is an act of questioning (seeking information), whereas the intended illocutionary force is again an act of requesting. Needless to say, there's lots of inference involved.
How do indirect speech acts succeed? Well, taking an example like (26), the literal meaning and literal force of the utterance must be computed by the participants (="Are you physically able to close the window?"). There are specific principles or inference rules that will derive the indirect force from the literal one to establish the intended meaning ("You close the window"). In the case of (26), it's the reasoning that the speaker is unlikely to be checking whether the hearer is physically capable of closing the window (because that is probably evident), and lacking a real concern about physical ability and taking into account the expectation that the speaker is cooperative, one can infer the intended request.
KEY POINT: Speech acts are Gricean.
Content adapted from S. Kaufmann (2008) teaching materials, Northwestern University.