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From Lets Get Technical by Bartholomew Martin
Linguists, rhetoricians, and philosophers employ specialized terms when evaluating human speech acts. "Locution" refers to the literal meaning of a speech act. "Illocution" refers to the effect the speaker wants to achieve in making the utterance, while "perlocution" refers to the actual effect of the utterance upon the audience. The latter – the "perlocution" of a speech act, the way it is received by audience – is often affected by what is known as "extra-locutionary" factors.
For example, if a man wants to motivate his son to get up early and work, he might utter the cliché, "you know, son, the early bird gets the worm." The man is not so much concerned about the locution of his utterance – most likely he cares very little about the actual eating habits of birds. Instead, what is significant is his illocution; that is, he intends to provide ample motivation to get his son’s tail out of bed. If the man’s son responds with an indifferent shrug of his shoulders, coupled with a "yeah, maybe," that is the perlocution, the actual effect of the father’s utterance upon his audience. This perlocution could have been influenced by numerous extra-locutionary factors: perhaps the son was particularly tired that day, or maybe he just had enough of his father’s pathetic attempts to motivate him.