Directions: The short passage below is followed by a question based on its content. Answer the question on the basis of what is stated or implied in the passage.
If we are right in what we have been saying, most of the greatest men, other than artists, have been actuated in their important activities by motives unconnected with sex. If such activities are to persist, and are, in their humbler forms, to become common, it is necessary that sex should not over-shadow the remainder of a man's emotional and passionate nature. The desire to understand the world and the desire to reform it are the two great engines of progress, without which human society would stand still or retrogress. It may be that too complete a happiness would cause the impulses to knowledge and reform to fade.
What is the theme of the passage?