The function f(x) appears to be selecting for words that are positive, optimistic, and related to nature. It returns values close to 1 for words such as "happy", "joyful", "peaceful", "inspiring", "tree", "flower", "bird", and "mountain", and values close to 0 for words such as "sad", "angry", "fearful", "hopeless", "car", "city", "computer", and "phone".