If I were to give a simple formula or recipe for distinguishing between what I consider to be admissible plans for social reform and inadmissible Utopian blueprints, I might say:
Work for the elimination of concrete evils rather than for the realization of abstract goods. Do not aim at establishing happiness by political means. Rather aim at the elimination of concrete miseries. Or, in more practical terms: fight for the elimination of poverty by direct means‐‐for example, by making sure that everybody has a minimum income. Or fight against epidemics and disease by erecting hospitals and schools of medicine. Fight illiteracy as you fight criminality. But do all this by direct means. Choose what you consider the most urgent evil of the society in which you live, and try patiently to convince people that we can get rid of it.
But do not try to realize these aims indirectly by designing and working for a distant ideal of a society which is wholly good. However deeply you may feel indebted to its inspiring vision, do not think that you are obliged to work for its realization, or that it is your mission to open the eyes of others to its beauty. Do not allow your dreams of a beautiful world to lure you away from the claims of men who suffer here and now. Our fellow men have a claim to our help; no generation must be sacrificed for the sake of future generations, for the sake of an ideal of happiness that may never be realized. In brief, it is my thesis that human misery is the most urgent problem of a rational public policy and that happiness is not such a problem. The attainment of happiness should be left to our private endeavours.
It is a fact, and not a very strange fact, that it is not so very difficult to reach agreement by discussion on what are the most intolerable evils of our society, and on what are the most urgent social reforms. Such an agreement can be reached much more easily than an agreement concerning some ideal form of social life. For the evils are with us here and now. They can be experienced, and are being experienced every day, by many people who have been and are being made miserable by poverty, unemployment, national oppression, war and disease. Those of us who do not suffer from these miseries meet every day others who can describe them to us. This is what makes the evils concrete. This is why we can get somewhere in arguing about them; why we can profit here from the attitude of reasonableness. We can learn by listening to concrete claims, by patiently trying to assess them as impartially as we can, and by considering ways of meeting them without creating worse evils.
Karl Popper, Conjectures and Refutations, p. 361