Determine the valence intensity of the following tweet, considering the mental state of the author. Assign a real number between 0 and 1, where 0 denotes extremely negative and 1 denotes extremely positive.
Tweet: What terrible thing to see and have no vision-\n\nHelen Keller-\n\n-Begin with the end in mind-\n\nStephen Covey-\n\n #whereareugoing
0.426