The overall theme of the "Mirror" is one of self-reflection. The mirror offers an honest, unbiased analysis of what it sees: "I am silver and exact. I have no preconceptions." The mirror doesn't judge but only reveals what it sees, unchallenged by biases or preconceived ideas that might otherwise influence its perceptions. As a result, it forces the woman to see the reality of not only her physical appearance but deeper issues that lie within her. The mirror reflects the woman's image, but more importantly it forces her to engage in deep self-inspection.
The Struggle of Aging
The woman returns to the mirror -- in the symbolic form of a lake -- to re-examine her appearance. She wants to know the truth that only the mirror provides. The woman realizes she's aging, and it upsets her: "She rewards me with tears and an agitation of hands." The woman understands that candles and the moon don't offer an authentic, realistic view of who she is -- only the mirror can do that. Nonetheless, she's saddened by the reality that she's no longer a young woman and has been replaced by an old woman. "In me she has drowned a young girl, and in me an old woman rises toward her..." The woman struggles with the loss of her beauty and innocence as she's aged. She's devastated and appalled by what she sees.
Humanlike Characteristics of the Mirror
Plath gives the mirror humanlike traits, such as a heart, even though it doesn't offer judgment. These traits help the reader understand why the woman is emotionally connected to it. The mirror has looked at the opposite pink, speckled wall so long that it says, "I think it is part of my heart." Even though the mirror isn't a living organic object, it notices the opposing wall, the darkness and the faces that stand before it. The mirror understands how significant it is to the woman and says, "I am important to her." Its humanlike traits give the woman a reason for trusting in it for self-reflection -- "Searching my reaches for what she really is."
Because the last line of Plath's poem ends with such a drastic and unsightly parallel to describe the woman's aging reflection, the reader is left to believe the woman feels bitter resentment about her condition. The mirror says her reflection, "Rises toward her day after day, like a terrible fish." The woman is unhappy about her physical appearance and likely her inner self too. The time-consuming process of self-reflection has led to a frustrating and disappointing level of self-awareness. She doesn't see herself as a beautiful, thriving woman with age and wisdom on her side, but as a detestable fish.