مَا أَنَا حَاسِبٌ، بَلْ كَاتِبٌ
I am not an accountant; rather, I am a writer.
The speaker rejects the attribution of accounting and then uses bal to affirm writing instead. It removes a mistaken description and asserts the one intended.
Syntactically, “mā” negates the first predication; “bal” turns to a replacement description for the same subject. When the listener confidently held the first judgment, this type of wording is especially suited to correcting a reversed belief.