To a great extent, Cordelia understood her father's condition as the natural consequence of the first scene in the first act. I think that Cordelia understands clearly that she recognized how her father is being duped by the protestations and inauthentic claims of her sisters. She sees this play out perfectly. Her vision is lucid. In many respects when it is revealed that she has read the letters from Kent with a sense of sadness and melancholy in Act IV, scene 3, it is almost a completion of this vision. Cordelia understands clearly from these letters that her father has been mistreated and deceived. In Act IV, scene 3, we understand that Cordelia has fully learned of her father's condition. It is a realization in which she has fought back tears shed at the threshold of revelation. It is in this that it becomes understood that "the stars" are the only explanation why one child is so different from another. It is at this scene in which it becomes clear that Cordelia has paid attention to her father's plight through both the letters that detailed it and also through her own sense of empathy in which she understands how her father has become victim to his own lack of vision.