Mirrors in art carry a variety of different meanings and associations.
The oracle of Apollo at Delphi demanded of the ancient Greek ‘know thyself,’ and mirrors have often been used as symbols of wisdom and self-knowledge.
But Apollo also required ‘nothing in excess,’ and the mirror can just as easily imply vanity, an unhealthy amount of self-regard. The peril of over admiring one’s mirror image is reflected in the ancient Greek myth of Narcissus, the boy who having fallen in love with his reflection.
In Christian art the mirror came to represent the eternal purity of the Virgin Mary:
“She is called a mirror because of her representation of things, for as all things are reflected from a mirror, so in the blessed Virgin, as in the mirror of God, ought all to see their impurities and spots, and purify them and correct them: for the proud, beholding her humility see their blemishes, the avaricious see theirs in her poverty, the lovers of pleasures, theirs in her virginity.” (Jacobus de Voragine)