[MayeulC]

It goes a bit further, and allows to contradict any negative sentence (some example stretch a bit, more context would typically be given):

    C'est impossible. - Si!
    Je ne peux pas - Si! (I can't - yes, you can!)

English used to have a system with four words for that purpose [1].

An interesting fact is that some early Gallo-Romance languages were named after the "yes" word in that language: langue d'{oc, oïl}. One can recognise oïl as a precursor for oui [2,3]

[1] https://en.wikipedia.org/wiki/Yes_and_no#The_Early_English_f... (cases with three forms such as French are detailled bellow).

[2] https://en.m.wikipedia.org/wiki/Langues_d%27o%C3%AFl

[3] https://en.m.wikipedia.org/wiki/Gallo-Romance_languages

[empath75]

Latin had no word for yes, so the various Romance languages adapted ‘hoc ille’ or ‘hoc’ (‘this is it’, or just ‘this’) or ‘sic’ — ‘like that’

[webreac]

I thought it was volo (I want) or nolo (I do not want) that was used for yes or no.

[OJFord]

I thought it was repetition of the verb, i.e. I agree with you, but only if the question was 'do you want ...'.

I think it's the same in Mandarin, but the question structure also includes both forms, e.g. 'this cake you want not want?'--'want'.

[empath75]

I obviously didn't mean that the Romans had no way to assert agreement with something. I said they didn't have a word that means 'yes'.

[OJFord]

Understood, and agreed.

Perhaps I misread though, I thought you were saying the verb 'want' is used in all cases, I was just saying my understanding is that it would be used when that was the verb in the question, but for 'do you go' similarly 'go' is used in the answer.

[Rondom]

The following Louis de Funes movie scene demonstrates the use of si quite well: https://m.youtube.com/watch?v=GioEH34fhxE

English dub: https://m.youtube.com/watch?v=me9ft6HeaMQ