How to find the object of reference of a latin relative pronoun?

Question

In De Bello Gallico, book 1, chapter 1, it starts as follows:

Gallia est omnis divisa in partes tres, quarum unam incolunt Belgae, aliam Aquitani, tertiam qui ipsorum lingua Celtae, nostra Galli appellantur. Hi omnes lingua, institutis, legibus inter se differunt.

My doubt is concerning the object of reference of the relative pronoun qui. I can certainly understand the sentence and translate qui as those, but in my opinion this a case of relative pronoun and yet I cannot find its object of reference.

Is there any srategy to find the object of reference of a relative pronoun? Certainly there are cases where it is quite evident like in the case of quarum in this extract (partes tres), but it seems that in other cases it is not so clear.

Answer 1

Descriptively speaking, relative clauses can be classified into two types depending on having an external antecedent or not (e.g., please see https://en.wikipedia.org/wiki/Relative_clause ): cf. so-called "bound relative clauses" and so-called "free relative clauses", respectively.

The former are the "typical" ones, which, as you point out, have an antecedent in the main clause (e.g., (... in partes tres) quarum unam incolunt Belgae). The latter, the ones that seem to bother you, lack an explicit antecedent external to itself (e.g., (tertiam incolunt Ø) qui ipsorum lingua Celtae, nostra Galli appellantur).
NB: Ø is the empty slot that could be filled by the null nominative demonstrative ei.

Answer 2

Two ways:

1. By agreement in gender and number—but not case, since the pronoun's case in the subordinate clause may differ from that of its antecedent in the parent clause.

2. By sense—that is, by what interpretation makes sense in context.

You're running into trouble, though, because qui in that sentence has no antecedent. It refers to the as yet unnamed inhabitants of the third part of Gaul. It might help to see another sentence that works this way:

Qui desiderat pacem, praeparet bellum. (Vegetius, Epitoma Rei Militaris, III)

Who desires peace should prepare for war.

English has this construct, too, though today it sounds old-fashioned to use "who" in this way, without being preceded by another pronoun, like "one", to serve as an antecedent.

The pronoun together with its subordinate clause serves as a noun in the parent clause. The same principle is at work in the phrase "To whom it may concern". The whole clause "whom it may concern" is the object of "to".

Here are another couple examples from the same book by Vegetius (ch. 25, on what to do if you've been partially or wholly defeated):

Innumerabilibus hoc accidit bellis, et pro superioribus sunt habiti qui minime desperarunt. Nam in simili condicione fortior creditur quem aduersa non frangunt.

This [a comeback after suffering losses] has happened in innumerable wars, and [those] who least despaired were found victorious. For in like conditions [of partial defeat], whom adversities don't break is believed stronger.

Here's an example well known in both Latin and English:

Quem Iuppiter vult perdere, dementat prius.

Whom Jupiter wishes to destroy he first makes mad.

I've seen this construct often used to introduce new terminology, as in your sentence.