There is no permanent absolute justice, so it is very difficult to be justice or to have justness. Therefore, design or to design a thing is very difficult. This is just the crucial difference between design and art. Art clearly reflects the era and get the hearts of people, and it persist personal justice, it means artists only follow own mind. On the other hand, since design is a solution it needs to get a social justice as well as personal justice, it means designers solve simultaneous equations which have tremendous amounts of unknowns. I think the essence of design is summarized in here.