This series starts on BBC Radio 4 this week, and don't forget the additional material online, including this below from the introduction to my book The Meaning of Friendship...

What exactly is friendship? What is its nature, its rules, its promise? How can one differentiate between its many forms? How does it compare to, and mix with, the connections shared between lovers and within families? If at least a kind of friendship is elastic enough to survive the relational stresses and strains of our flexible ways of life, is that friendship also strong enough to bear the burden of the human need to belong, to be connected, to be loved?

These questions are trickier to answer than it might first seem because friendship is hugely diverse. Although it is relatively easy to come up with definitions that account for part of it, it is much harder to find one that does not exclude any of its facets. Aristotle, whose writing on friendship still sets the philosophical agenda to this day, found as much 2,500 years ago. Friendship, he proposed, is at the very least a relationship of goodwill between individuals who reciprocate that goodwill. A reasonable starter for ten. However, as soon as he tried to expand it, the definition seemed to unravel.

He looked around him and saw three broad groupings of relationships people called friendship. The first group are friends primarily because they are useful to each other – like the friendship between an employee and a boss, or a doctor and a patient, or a politician and an ally; they share goodwill because they get something out of the relationship. The second group are friends primarily because some pleasure is enjoyed by being together; it may be the football, the shopping, the gossip or sexual intimacy, but the friendship thrives insofar, and possibly only insofar, as the thing that gives the pleasure continues to exist between them. Aristotle noted that these first two groups are therefore like each other because if you take the utility or the pleasure away, then the chances are the friendship will fade.

This, though, is not true of the third group. These are people who love each other because of who they are in themselves. It may be their depth of character, their innate goodness, their intensity of passion or their simple joie de vivre, but once established on such a basis these friendships are ones that tend to last. Undoubtedly much will be given and much taken too but the friendship itself is independent of external factors and immensely more valuable than the friendships that fall into the first two groups.

The meaning of friendshipThat there are better or higher friendships – different people may call them soul friends, close or old friends, or best friends – as opposed to instrumental and casual friendships, or mere friendliness, is surely right. But to say that great friendship is defined solely by its goodwill seems to miss its essence. Goodwill exists in these best kinds of friendship, but, unlike the lesser types, best friendship – arguably the quintessential sort – is based on something far more profound. In other words, a definitional approach to friendship has its limits.

This ambiguity as to what friendship is reflects, then, the ambiguity that appears to be part and parcel of friendship in life. Try listing some of the friends you have – your partner, oldest friend, mates or girlfriends, one or two family members, work colleagues, neighbours, friends from online chat rooms, family friends, a boss perhaps, therapist, teacher, personal trainer – whoever you might at some time think of as a friend. A look at such a list puts your friends in front of you, as it were, and highlights the vast differences. For example, the friendship with your partner will in certain key respects be unlike that of your oldest friend, though you may be very close to both. Conversely, although friendship is for the most part a far less strong tie than say the connection to family, you may feel less close to members of your family in terms of friendship than others with whom you have no genetic or legal bond. Then again, lovers might make you blush and families can make you scream, but friendship – even soul friendship – is usually cool in comparison.

As you continue further down the list to the friends who are in many ways little more than acquaintances, associates or individuals for whom you have merely a sense of friendliness, it is obvious that friendship stretches from a love you could scarcely do without to an affection that you’d barely miss if it ended. Some people would say there is some minimal quality which means that it makes sense to call all of them friends perhaps Aristotle’s goodwill. Others would disagree: they are the sort who say they have a handful of friends and that others are people they only know. In other words, the ambiguity of friendship extends to the very possibility of prolific and profound friendship-making.

Personally, I think that Aristotle is on to something in his belief that the closest kind of friendship is only possible with a handful of individuals, such is the investment of time and self that it takes. ‘Host not many but host not none’, was his formula. He would argue that less is more and it is easy to substitute mere networking for the friendships it is supposed to yield. He actually went so far as to express a fear of having too many friends, ‘polyphilia’ as it might be called. There is an expression attributed to Aristotle that captures the concern: ‘Oh my friends, there is no friend.’

One of the things I think the philosophy of friendship tells us is that life produces personal relationships of many types, but out of these connections good friendship may or may not grow. Certain associations or institutions like work or marriage can foster friendship but those same associations or institutions need not necessarily be characterised by deep friendship themselves; friendship emerges, as it were, from below up. It is a fluid concept.

Another dimension to the ambiguity of friendship is its apparent open-endedness. Unlike institutions of belonging such as marriage which is supported and shaped by social norms, or work where individuals have contractually defined roles, friendship has no predetermined instructions for assembly or project for growth. People have to create their friendships mostly out of who they are, their interests and needs, without any universally applicable framework. On the one hand, this is a potential weakness, because a friendship may ‘go nowhere’ or ‘run out of steam’. On the other, it is a potential strength because there is also a freedom in this that is crucial to friendship’s appeal: it is part of the reason for the diversity within the family of relationships called friendship.

In summary, then, it seems that it is not possible to say unequivocally what friendship is. Sometimes it is intense, sometimes it is thin. Sometimes it appears to embrace many, sometimes only a few. This might seem to be a bit of a blow if the question is what is friendship. However, far from ambiguity automatically leading to philosophical impasse, an exploration of the very ambiguities of friendship is actually a very good way forward. After all, is not mistaking relationships for what they are not – that is being blind to their ambiguity – arguably the greatest cause of disappointment and failure? A married couple may assume they are friends in some deep sense when really they only have goodwill for each other because of the kids; unless they realise that, when the kids leave home, the marriage may falter too. An employee and a boss may think they are good friends after all the late nights, trips abroad and hours spent together: but when the day arrives for the appraisal or pay rise, and both turn out to be modest, the friendship stumbles and falls.

Honesty about any relationship is likely to improve it, even if the honest thing to do is not put too much hope in it! The mistakes that people can make in friendship are also exemplified in some of the things people commonly say about it. For example, many would say that the test of good friendship is being able to pick up immediately where you left off even if you haven’t seen the friend for some time. Aristotle, though, thought that good friendship depends on shared living and spending substantial, regular, quality time together. ‘Cut off the talk, and many a time you cut off the friendship,’ he said. The question is how much time, how much talk is needed?

And yet, if it is really quite easy to make mistakes by thinking the relationship is something other that what it is, the best kinds of friendship (however that is judged) are essential for a happy life: human beings need people they can call friends and not just people who are relatives, partners, acquaintances, colleagues or associates. In other words, the corollary of friendship’s ambiguity is that it is packed with promise and strewn with perils.

Philosophy is frequently overlooked as a resource for thinking through friendship in this way. This has much to do with the fact that only a relatively small number of philosophers have written on the subject at any length. What is more, those who have, although generally agreeing that friendship is essential for a happy life, also say that it provides no automatic satisfaction of human desires for deeper relationships or society’s need for connection. Friendship is ‘a problem worthy of a solution’, as Nietzsche gnomically put it. Or as Aristotle wrote: ‘The desire for friendship comes quickly. Friendship does not.’ The implication is that the best kinds of friendships are only possible between people who properly value it and who understand how many things from the personal to the political can compromise, undermine and destroy it. There is an art to friendship. Philosophy can teach us something about it.

This is an edited extract from the Introduction of The Meaning of Friendship by Mark Vernon (Palgrave Macmillan).