'Ethics is probably the most difficult concept to define.' justify this statement.
One justification for the veracity of the quote is the simple truth that humans have been trying since before the Ancient Greeks to define ethics. As centuries have passed, more and more versions of ethics and the subject's implication for the way we live our lives have resulted. The difficulty in defining ethics comes from the varying views of human nature. If a person believes in absolute morality, that will affect his or her ethics (in business, medicine, relationships, etc.). For example, if I believe that lying is always wrong and never an option, then I will not "cover" for someone at work, and I will not call in sick when I'm not truly sick. If, however, someone believes in relative morality, that will also affect that person's ethical standards. That person might be willing to lie for a coworker because his relationship with that coworker is more important that telling the truth to his employer (that decision results from that person's ethics); similarly, that person might call in sick when he is not sick if it means that he will be able to spend more time with his partner or child, to further develop that important relationship.
Because so many people have different experiences and philosophies of life, ethics in most humans' eyes will never be universal.
There is an episode of the sitcom, "The Office," which might help here. I would examine the episode from Season 5 called, appropriately enough, "Business Ethics." Ethics in business is a challenge because there are other factors which help to confuse issues in making business decisions. The idea of "the right thing to do" is clouded by personal beliefs or value systems, as well as workplace climate, and even the financial repercussions of decisions. Finding decisions where all of these elements converge in a harmonious and symmetrical manner is challenging, if not outright impossible. Add to this the fact that all decisions in business have to follow a chain of command, but by definition ethical decisions are personal decisions made in accordance to one's own voice and understanding of issues, and there is a tone of dissonance that seems to be a distinct and real part of the concept of business ethics. These reasons help to make it so difficult to define and even more challenging in the business realm. If you have a chance to examine the episode mentioned, I think you can see how this plays out quite effectively.