Mais c’est impossible ! Si, c’est possible : je suis un vampire

Question

Bonjour, Is “si” used because “impossible” is the negative of “possible”? The grammar lesson for using “si” only has examples using the “ne  ...” construction. Would this be the case for other similar words with a direct negative opposite?

Cécile Janty-Davies · Accepted Answer

Hi Gareth,
impossible = pas possible 
So it is a negative meaning.
The 'si' in effect contradicts the first statement.
Vous n'avez aucun DVD en anglais ? ---> Si, j'en ai quelques -uns
Hope this helps!

Chris Wirt · Answer

Si is not used to introduce a conditional clause. Here, it is used as a contradictory Yes, when the previous speaker had implied a negative reply.

Tu ne vas pas autravail? — Si, j’y vais!

You don’t go to work? — Yes (of course) I go there. (Literal translation)

Gareth Jones · Answer

Thanks Chris. I did understand the “Si” was used as a contradictory “yes”, but I have never seen it used with anything but “ne ...pas” as in your example and the examples in the grammar lesson. It seems that it is used if negation is implied?