Pronoms toniques employés seuls

Question

Toi, tu partiras.

Lui n'a rien fait.

Eux ne partiront pas.

Moi, toi, nous, vous, elles (s) toniques, employés devant le verbe, sont repris par je, tu, etc. Au contraire, lui et eux peuvent se passer de il et ils. Pourquoi cette différence ?

Answer 1

EDIT: Je ne me rends compte que maintenant que la question était posée en français. Un peu tard pour traduire ma tartine à cette heure, mais j'y reviendrais peut-être plus tard.

There's a linguistic concept called the Person Hierarchy that notes that agreement or special behaviours tend to apply more often to the first person than to the second, and to the first and second together than to the third.

This is especially relevant to agreement between verbs and their arguments, and it's common cross-linguistically for verbs to only agree with first and second person arguments, or to only agree with third-person arguments if there's no first or second person argument in the same clause.

The root cause of this is that the participants in a conversation are always relevant to the current conversation, animate and human, whereas the third person might be an object, an animal, something totally new to the conversation, or indeterminate. In other words, the first and second persons are always topical, while the third might not be.

Verbal agreement is generally thought to come from pronouns that fused with the verb, and pronouns tend to be used more often for things that are topical. This means agreement will develop slower, and be more optional, for the third person than the two others (plural agreement is also slower to develop then singular).

As it happens, the French clitic pronouns are commonly thought to be in the middle of this transformation into agreement markers, via the doubling of topicalised elements with a pronoun. The spread of this transformation follows the person hierarchy, as well as two others: the animacy hierarchy (person > animate non-human > object) and the definiteness hierarchy (Personal Pronoun > Proper Name > Definite > Specific > Non-specific):

1. 1st and second person singular pronouns: Obligatory doubling

Moi, *(je) lui parle

2. Other personal pronouns: Optional doubling, near-systematic in casual speech

Elle, (elle) lui parle

3. Proper nouns, definite noun phrases referring to humans: Optional doubling, very frequent in casual speech

Ta sœur, (elle) lui parle

4. Definite noun phrases referring to animals and objects: Optional doubling, frequent in casual speech

La porte, sa couleur, elle lui plait pas

and so on until you get to the end of the scale:

5. Positive indefinite pronouns: Prohibited doubling in formal writing, rare doubling in casual speech

Est-ce que chacun (il) a de quoi manger ?

6. Negative indefinite pronouns: Prohibited doubling in formal writing, vanishingly rare doubling in casual speech:

Personne (*il) disait rien

7. The indefinite inanimate pronouns tout and rien are at the very bottom of the scale and don't admit any doubling in most dialects.

Tout (*il) est fini

To summarise, moi and toi show a special behaviour because they're special. As the most topical referent possible, they tend to be used as topics more often, which in French means they get doubled by a weak pronoun. As the most frequent doubled elements, they're the forerunners of the current evolution toward the obligatory cooccurrence of arguments and weak pronouns.

But the optional pronominal doubling with third person pronoun is an accident of history. If French was formalised and standardised today instead of 4 centuries ago, it's likely all the reprise of all the subject pronouns would be considered obligatory, since sentences like "Lui n'a rien fait" are only learned once in school, in imitation of the morphosyntax of an earlier stage of the language. It is but a snapshot of a fleeting state of the language, as French continues to progress down the animacy hierarchy.