The cause of much misery throughout history has been the idea that human beings, either individually or as the collective human race, are perfectible. Everyone dreams of a perfect society, and can describe what it might be like, but some people believe that it is possible in this world. Others recognize that humans, although they may be very good, are never quite perfect, and that there will be no perfect society, and no perfect human beings, in this life. As for the next life ... well, we'll leave that discussion for another time!
But, to get specific (which is, after all, what it takes to get a good grade on your essay test!), we see Metternich saying that the twenty-five years of bloodshed and chaos (ten years of French Revolution and fifteen years of Napoleonic rule) which dominated European history was caused largely by "presumption; the natural effect of the rapid progression of the human mind towards the perfecting of so many things." What he's saying is this: our minds constantly turn toward the idea of perfection - we image the perfect weather, the perfect music, the perfect school, the perfect car, the perfect vacation, etc. But we are carried away by passion, which makes us forget that the world, and the human beings in it, are good, but not quite perfect, and that they are essentially imperfect, i.e. by nature imperfect, and thus cannot be ultimately perfected. We can always make the world a better place, but we can never make it a perfect place. So Metternich concludes, "one must not dream of reformation while agitated by passion; wisdom directs that at such moments we should limit ourselves to maintaining."
Edmund Burke had a similar view: given the reality of the world's imperfection, the practical way of organizing human society will be found "in compromises sometimes between good and evil." Human societies face different problems, and we cannot fix each one perfectly: "it is better that the whole should be imperfectly and anomalously answered," i.e., on average we can do a good job taking care of the problems which face society, but not a perfect job.
