to believe one's eyesMost commonly used with ne pas: ne pas en croire ses yeux means “not to believe one's eyes” or “can't believe one's eyes.” The possessive agrees with the subject: mes yeux, tes yeux, ses yeux, nos yeux, and so on. In the example, the exact term appears in the infinitive phrase “en croire ses yeux.”
FR: Il faut en croire ses yeux pour comprendre ce qui s'est passé.
EN: You have to believe your eyes to understand what happened.