To see what it is, we need some background on speech acts.
Here are some other things that we might perform:
admitting, advising, announcing, assuring, authorizing, censuring, committing, complimenting, conceding, confessing, congratulating, defining, denying, granting, hypothesizing, inquiring, insisting, judging, pardoning, pleading, pledging, predicting, proposing, reporting, reprehending, thanking, urging, vowing, warning.
Many things the policeman said are true.
According to Strawson’s theory, John is thereby endorsing or confirming several things that the policeman said. But John has actually done no such thing. There is not even one thing that the policeman said that John has confirmed.
If that is right, then the following pairs of sentences ought to have the same meaning, but they don’t (and they might even differ in truth value):
Each pronoun here has an antecedent expression, which is a noun or noun phrase (hence the name 'pronoun'). These are all so-called lazy uses of pronouns - we could equally well have used the antecedent expression again:
In these cases we cannot just replace the pronoun with its antecedent expression - doing so would change the meaning of the sentence:
John believes that snow is white and Mary believes that snow is white too.
According to prosententialists, 'that is true' here is a prosentence, whose antecedent is the sentence 'snow is white':
No exercise this week - time to catch up/tidy up.