The mirror was often used as a symbol of the vanity of woman. The moralizing, however, was mostly hypocritical.

You painted a naked woman because you enjoyed looking at her, you put a mirror in her hand and you called the painting Vanity, thus morally condemning the woman whose nakedness you had depicted for your own pleasure.

The real function of the mirror was otherwise. It was to make the woman connive in treating herself as, first and foremost, a sight.