[§ 529]. The reason why the relative must agree with its antecedent in both number and gender, whilst it need not agree with it in case, is found in the following observations.
1. All sentences containing a relative contain two verbs—John who (1) trusts me (2) comes here.
2. Two verbs express two actions—(1) trust (2) come.
3. Whilst, however, the actions are two in number, the
person or thing which does, or suffers them is single—John.
4. He (she or it) is single ex vi termini. The relative expresses the identity between the subjects (or objects) of the two actions. Thus who=John, or is another name for John.
5. Things and persons that are one and the same, are of one and the same gender. The John who trusts is necessarily of the same gender with the John who comes.
6. Things and persons that are one and the same, are of one and the same number. The number of Johns who trust, is the same as the number of Johns who come. Both these elements of concord are immutable.
7. But a third element of concord is not immutable. The person or thing that is an agent in the one part of the sentence, may be the object of an action in the other. The John whom I trust may trust me also. Hence
a. I trust John—John the object.