Here's a subtle distinction, and one that is immediately recognisable. It's the difference between being useful to your friend, and feeling used by your friend.

All friends like to be useful. You're only too happy to feed your friend's cat while they're on holiday, or to help them when they move into a new flat. You offer the help as a gift. It's elicited by the quality of the friendship. They, too, will offer you gifts in return.

But help with the cat or flat is very different from offering you a service. Services are not elicited, they are advertised, ordered and bought. If a friend treats you like that, you rapidly cease to feel useful, for you've already started to feel used. Feeling useful is a means by which friendships flourish. Feeling used is a means by which friendships end.

So, I suspect that the new Rent a "Friend" website – through which you can hire someone to be a companion or partner (strictly platonic: yeah, right) – is misnamed, misleading and, should you use it to find friendship, a mistake.

It's true that friends exchange goods. Common definitions of friendship say as much. A friend provides a shoulder to cry on, or a word of truth, or is someone who will just be there. It's also true that you can hire people to perform services for you and, with luck, they will do so in a friendly manner. Think, say, of the money you might pay a counsellor or psychoanalyst. And think again. For it's the very artificiality of the relationship, signalled by the money, that means you can use the person to listen to you. They will absorb anything you care to utter, and they'll tell you hardly a thing about themselves. While friendly, it's not friendship. Hence, you call them your shrink, not your friend.

It's obvious that friendship is throttled by cash. Ask anyone who's worked for a friend. Or speak to someone else whose friend suddenly became wealthy. The problem is money's very genius. It is too brilliant a way of setting the price. Once caught in its nexus, it inevitably makes you hypersensitive to whether or not you're getting your money's worth. That's essential when buying a car. It's poison when seeking friendship, which is why Rent a Friend is insidious.

Or put it this way. Price is a perfectly sensible factor to consider when buying strawberry punnets at the supermarket. But friends are different. They are I-thou relationships, as Martin Buber put it, not I-it, as the financial assessment sees them. Turn an I-thou into an I-it, and you kill what you had stone dead. It's companionship by cost-benefit analysis. My "friends" are the service providers in my optimised life.

There are surely commentators who will tell you the opposite. We've learnt that money can, in fact, buy us love, they might argue. After all, does not your socio-economic background tell you far more about your likely marriage-partner than pretty much any other characteristic you care to mention? So, too, with friendship, they might continue. It's never been free anyway. Rent a Friend is just making that fact explicit.

But it isn't. Rather, it repeats the confusion between being useful and being used, between offering a gift and buying a service.

What's worrying about the new website is that it reflects an instrumental way of thinking about personal relationships that is already pervasive. You see it in the explanations of evolutionary psychology, say, such as the idea that we're wired for 150 friends, or that altruism is really selfishness in disguise. As a piece of science, debate it. As a prescription for life, dump it.

For the true nature of friendship is different. It's caught well by Ralph Waldo Emerson: "The only way to have a friend is to be one." So here's a radical idea. Just give, and don't count the cost. It's an indictment of our times that the astonishing act is one which is offered as a gift, and for no other reason than because it's good.