1. Conversational Implicature.

The linguistic philosopher Paul Grice coined the term implicature to explain meanings which are implied, rather than explicitly stated. For example, if I say, "Some people believe in ghosts", I am implying that if only some people do, then some people don't believe in them. This is a simple case; it gets more complicated if, as often happens, understanding the implicature depends on the context of utterance. This would be the case if I say to a friend, "Are you coming to the theatre tonight?" and he replied, "It's Val's staff Christmas party." This would seem to be a completely irrelevant remark if I didn't know that Val is his wife and that they always go to each other's staff parties.
Grice derived his ideas from what he called the Co-operative Principle. This is based on the notion that when people are talking to each other they will normally co-operate, and will also assume that the other person is doing the same. For this reason, I would assume that my friend was being co-operative when he told me about his wife's party, and would infer that he was telling me that he wasn't coming to the theatre, rather than that he was changing the subject without answering my question. 
Building on these ideas, Grice further postulated four Maxims of Conversation, as follows:
The Maxim of Quantity. Make your contribution as informative as is required at that point in the conversation, and no more so than is required.
Example: Recently, someone said to me about the local team, "City are doing well this season". The implicature was that they were not top of the Division: if they had been, the speaker would have said so. 
The Maxim of Quality. Do not say that which you believe to be false, or for which you lack adequate evidence.
Example: If I say to someone travelling to India, "Don't drink the tap water", the implicature is that I believe or have evidence that to do so would be harmful.
This can also work with questions. If you ask me, "What is conversational implicature?" I will assume that the question is sincere and that it carries the implicature that you don't know what it is, that you want to know, and that you think that I can tell you.
The Maxim of Relation. Be relevant.
Example: The example of Val's party fits here.
The Maxim of Manner. Avoid obscurity and ambiguity; relate things in order. 
Example: "I got up and had breakfast" carries the implicature that I did those things in that order.

All these maxims can, of course, be flouted. Deliberate lies, rhetorical questions, tautology and even metaphors could be regarded as flouting one or more of them; and how often do we try to change the subject with a tempting red herring if we don't like the way the conversation is going? The co-operative principle doesn't hold good in all conversations all the time; but it does explain how we generally manage to understand what people mean, even if it's not exactly what they say.

2. Speech Acts.

It was the British philosopher J.L.Austin whose name was originally associated with speech acts. His theories rest on the fact that language can sometimes be used actually to accomplish an action. For example, when a Christian child is baptised, the priest will say the words "I baptise you…", and, as a result of those words being spoken, the child is baptised. Similarly, if a VIP is naming a ship at its launch, he or she will say something like "I name this ship The Flying Dustbin", and that then becomes the name of the ship. The verbs in these utterances (baptise, name) are called performatives, because they effectively perform an action.

However, certain conditions have to be in place for a performative to have the power to make things happen. The words "I now pronounce you husband and wife" as part of a marriage ceremony will make the couple married if the person saying them is licensed to perform marriages; if not, they have no power to make the couple legally married. In technical terms, performatives have illocutionary force only if they're spoken by someone with the right to say them in a way that accomplishes something: otherwise, they have no effect.

The conditions that make speech acts real are called felicity conditions. The most important of these are that the person speaking has the power or authority needed to make the words effective, and that the outcome isn't something that would have happened anyway, even if the words hadn't been spoken. 

Not all speech acts are explicit. If I go into my local newsagent's shop and say "I'm cancelling my papers for next week", that is an explicit speech act, as a result of which no papers will plop through my letter box for a week. If, on the other hand, when I'm paying my paper bill I say to the newsagent "I'm going to be away next week" there is the implicature that I don't want to have any newspapers delivered. This, assuming that the co-operative principle is observed, will have exactly the same effect, and this makes it an implicit or indirect speech act.

© C D Selwyn-Jones 2004