This quote from J.K. Rowling appears in Harry Potter and the Philosopher's Stone, spoken by Albus Dumbledore to Harry after he stares into the Mirror of Erised. The mirror shows the viewer's deepest desire, and Harry becomes lost in the vision of his deceased parents. Dumbledore warns that obsessive longing for what is lost or unattainable can prevent a person from living fully in the present. The line encapsulates a key theme in the series: the importance of facing reality with courage rather than retreating into fantasy.