Modern philosophers do not say much about friendship, but ancient philosophers thought it was essential to ethics and the good life. They also did not think men and women could have friendships, with which most people might disagree today. For Aristotle, friendship was a necessary part of a healthy, well functioning life, the good life. Ken points out that we owe something to our friends; what happens if the obligations conflict with ones morality? There is clearly a philosophical concern behind friendship. John asks why living a life by yourself is not simply sufficient, why are friends needed? For our guest Martha Nussbaum, friendship is about sharing the things we care about. She says that one can understand the things one cares about better when that person shares it with others, where we can benefit someone for his/her own sake.

All this sharing is nice, but Ken asks about friends we keep, but with whom we have large disagreements, over politics, or religion for instance. Clearly there is a difference here. Martha notes that there are different bases for friendships, some are for pleasure, shared enterprise, or deeper goals, things that make your life meaningful.

There is also an ethical significance behind friendship regarding what you owe your friends or they owe you. For Martha, the most important factor is giving time to another person, The American legal system also regards the secrets shared in a relationship like a marriage as crucial for the relationship. There is also a role of empathy and sympathy for friends.

Is friendship a genetic leftover? Ken believes we are coalition-forming creatures, and that humans morally flourish by having another as a mirror, who also has concern for you.

• Roving Philosophical Reporter (Seek to 4:22): Rujun Shen looks at friendships in kindergarten and what brings children together.