Thematic variation

The thematic systems of the clause are those where corresponding members of the contrasting classes (such as active My father wrote the letter and passive The letter was written by my father) are prototypically thematic variants. Thematic variants have the same propositional content, but differ in the way it is ‘packaged’ as a message. We select one rather than another from a pair or larger set of thematic variants depending on which part(s) of the message we wish to give prominence to, on what we regard the message as being primarily about, on what parts of it we assume the addressee already knows, on what contrasts, if any, we wish to make, and so on.

The general definition given above makes reference to prototypes because the corresponding clauses are not invariably thematic variants. For example, active The Head willingly made Kim convenor and passive Kim was willingly made convenor by the Head are not thematic variants because the passive, unlike the active, allows an interpretation where it is a matter of willingness on the part of Kim rather than the Head. But such cases are relatively exceptional: normally the switch from active to passive leaves the propositional meaning intact, and similarly with other thematic systems.

Not all thematic variation is attributable to differences between the terms of a grammatical system.