What would you think if I said to you, "You are going to die"? Would you look at me funny for stating the obvious? After all, what's the point in informing you of something you already know? Every one of us is going to die. Clearly there's no point in telling you about it, right?
Of course not. You'd take it as a threat. Why? Because you know I'm not likely to remark upon your mortality for no reason, so you'd think a little harder. And you'd figure out that was suggesting that your death was going to be a bit earlier than you had planned on.
There's a natural process in conversation in which we assume that what a person says is relevant and useful, and when it seems not to be on the surface, we dig into it a little further to figure out what they could be getting at. Paul Grice was a British philosopher of language who looked at the natural assumptions used in conversations, and he developed the notion of "conversational maxims" — implicit rules that speakers follow — in order to determine what provokes listeners to dig for implicit meaning.
"Make your contribution such as it is required, at the stage at which it occurs, by the accepted purpose or direction of the talk exchange in which you are engaged."
This is Grice's cooperative principle, which forms a basic description of how people ordinarily act in conversations. From it, he derived his four conversational maxims; the explanations below follow his but are my own words:
- Maxim of Quantity
Contribute the right quantity of information. Say what's required, but don't give information that is unnecessary.
- Maxim of Quality
Don't tell lies, and don't make assertions that you don't have enough evidence for.
- Maxim of Relevance
Don't say things that have nothing to do with the topic of conversation. What you say should be on point.
- Maxim of Manner
Be clear with what you say, and be reasonably brief (or as the joke goes, eschew obfuscation and prolixity.) Your statements should be unambiguous and easy to understand.
Let me recount a little incident that happened to me to illustrate the idea. I was talking on the phone with my pal Jim-Bob. The conversation turned to mutual acquaintances in our circle, and I asked him about two particular people I didn't know very well. See, I'd heard some things about those two guys that made me wonder a little bit what they were like. I sez to Jim-Bob, "Jim-Bob," sez I, "what sort of folks are Joe-Bob and John-Bob? Are they good people?"
And Jim-Bob responded. "Well, I don't know either of 'em all that well," sez Jim-Bob. "But you know, every time I've dealt with Joe-Bob, he's been great." And the message was clear. On the surface, it would seem like Jim-Bob hadn't fully answered my question. After all, he didn't say anything about John-Bob. Isn't this a violation of the Maxim of Quantity? Of course not. The message about John-Bob was clear — according to Jim-Bob at least, John-Bob is not good people at all.
That's not to say that people always follow these maxims. But it's usually safe to assume that they are, unless you have prior reason to doubt it. And we engage in this process all day long. If you asked a friend if she wanted to go see a movie, and she said, "My parents are coming over for dinner tonight and the house is a complete mess," you'd know what she meant without any doubt. But on the surface, the response seems like a non sequitur. It's not a direct response to your question. It's only because you assume your friend is not deliberately saying something irrelevant that you examine her statement further and realize that she's explaining that she can't go because she's too busy cleaning.
Of course, all this happens without much awareness, at least most of the time. In fact, you do it constantly. "Gosh, it's chilly in here," is not an unneeded statement about someone's bodily comfort, it's a request that the window be closed. If George asks Charlotte whether Therese is coming to the party, and she says that Therese has been sick lately, it's not a pointless digression, it means she's doubtful. This is a concept from pragmatics, which is the area of linguistics that studies conversations and the meanings hidden in what people say; while semantics deals with meanings that are inherent in sentences or unambiguously derived from them, pragmatics looks at speech to find things that can be understood through inferences based on context and common knowledge.
Pay attention to a few of the conversations you have during the day and you'll see just how often questions aren't literally answered or seemingly irrelevant statements are made. You'll see obvious evidence that you're doing a good deal of processing, below the level of awareness, every time you talk to another person.
References
"Grice's Conversational Maxims" (http://www.ux1.eiu.edu/~cfbxb/class/1900/prag/grice.htm)