"So my optimism is no mild and unreasoning satisfaction. A poet once said I must be happy because I did not see the bare, cold present, but lived in a beautiful dream. I do live in a beautiful dream; but that dream is the actual, the present,—not cold, but warm; not bare, but furnished with a thousand blessings. The very evil which the poet supposed would be a cruel disillusionment is necessary to the fullest knowledge of joy. Only by contact with evil could I have learned to feel by contrast the beauty of truth and love and goodness." Read the last sentence from the text:
"Only by contact with evil could I have learned to feel by contrast the beauty of truth and love and goodness."

Explain how Helen Keller develops this idea in the text. Use specific details to support your answer.