The Passive Lexical Rule
Since the theory of HPSG has a strongly lexical orientation, the active-passive relationship is treated as a relationship between two verb forms, and a lexical rule is used to capture the generality of that relationship. I.e., a lexical rule is introduced that relates actives to passives. We call this rule 'Passive Lexical Rule'.
Before reading on, try to think of what the Passive Lexical Rule has to do. Hint: Remember the properties of the active - passive relation.
The Passive Lexical Rule has to meet the conditions of the active - passive relation and hence do the following (the order does not matter):
You might wonder why this list does not mention the property that the passive participle is usually preceded by a form of be. The reason is that we do not want to include this property in our rule. You will find out more about this in the chapter The Verb Be in Passive Sentences.
How can these properties be captured by a lexical rule? Try to think about it for a moment and formulate such a rule before you go on.
The following is a Passive Lexical Rule that satisfies the desiderata given above:
Passive Lexical Rule (adapted from SWB 2003)
We leave it as an exercise to explain how the above mentioned properties are captured by this rule (cf. Explaining the Passive Lexical Rule).
Several other points of explanation need to be made here:
1. It was mentioned at the end of the section Properties of the Active - Passive Relation that intransitive verbs normally do not have passive counterparts. That's why the Passive Lexical Rule applies only to lexemes belonging to a subtype of tv-lxm, which excludes strictly intransitive and prepositional intransitive verb lexemes.
2. The OUTPUT of this rule is of type part(iciple)-lxm. Since this is a subtype of const(ant)-lxm, passive participles undergo the Constant Lexeme Lexical Rule. This rule changes the type of the second member of the lexical sequence to type word. The type word, however, is constrained to satisfy the Argument Realization Principle. Hence, the OUTPUT of the Constant Lexeme Lexical Rule will be subject to the Argument Realization Principle.
3. The Passive Lexical Rule constrains its output, the passive verb, to be [FORM pass]. Since FORM is a head feature, it also shows up on the mother VP node due to the Head Feature Principle. That way, we can ensure that the passive verb be combines with the correct verb form, the passive participle, by constraining its lexical entry to select a VP[FORM pass] complement (cf. The Verb Be in Passive Sentences).
For the final step, go to the section The Verb Be in Passive Sentences to see how we deal with the fact that passive participles are usually preceded by a form of be.