Vanity is one of the most obvious modern vices.

We sneer at people who think of themselves too highly, people who care too much about how they appear and what other people think of them.

But most of us do care about what other people think of us, not just in terms of our physical appearance.

Vanity isn't just an aesthetic concern but about how we seem to other people. What does the world think about me?

Like Jean-Jacques Rousseau, the 18th-century French philosopher, we only really became modern humans when we became vain.

As societies developed, at some point between hunter-gatherers and the Enlightenment, Rousseau says that humans became capable of living only in the opinion of others.

And so we get our self-worth solely from the value that others place in us.

Rousseau distinguished between two types of self-love.

L'amour de soi, the natural desire to worry about your own survival. And l'amour propre, the desperate need to shine in the eyes of other people. Or, in other words, vanity.

On this view, vanity was a product of living in society.

Humans had become increasingly vain and incapable of independent self-assessment.

The more they see one another, the less they can do without seeing one another more, Rousseau said.

And you can see this when kids become adults and they go through their teenage years and they suddenly realise that they have a particular position or status within society.

And that's true for humans in general.

As we became more and more socialised, we started to care more and more about how other people perceived us, until eventually, the mask replaced the face beneath.

Rousseau despaired that everyone began to look at everyone else and to wish to be looked at himself.

The one who sang or danced best, the handsomest, the strongest, the most skilful or the most eloquent, came to be the most highly regarded.

We now value what people think of us, not the skill or the virtue itself.

It's not your eloquence or beauty or strength that you care about; it's that others see you as eloquent, beautiful and strong.

You create an image of yourself that you want others to see.

And your self-worth is determined by how other people respond to your mask.

But Rousseau's occasional friend, the Scottish philosopher Adam Smith, saw things a little bit differently.

He thought that Rousseau was probably right, that vanity was the major characteristic of modern humans, but also that it was necessary and the source of our redemption.

Smith asks, "What purpose is all the toil and bustle of this world? "

And answers that it's to be observed, to be attended to, to be taken notice of with sympathy, complacency and approbation.

It is the vanity, not the ease or the pleasure, which interests us. For Rousseau, amour-propre is an inescapable feature of modern humans. It's also the source of social inequality.

In societies where appearance is more important than reality, seeming to be virtuous replaces being virtuous as our primary motivation.

And so we pursue wealth and power in order to seem important.

But Smith thought that this supposed vice was the source of our sociability and our morality.

We are vain, and society makes us so, but it's our vanity that allows society to actually function.

We do care about what other people think of us, and so we adjust our behaviour accordingly.

We want to be seen as responsible, so we behave responsibly.

We want to be seen as generous, and so we give generously.

We want to be seen as kind, and so we act kindly.

Rousseau thought that vanity leads us to care about status more than morality.

It makes us worry about the mask, and not the face beneath.

His vanity leaves us all living in a hall of mirrors.

But by contrast, Smith's story shows that you can't have any society without the mask, because the mask is what makes moral-social interaction possible.

Seeing yourself through the eyes of others is to subject yourself to the judgement of humanity. It's our vanity that makes us accountable to other people.

But vanity for Smith isn't merely a mask. We want, he says, "not only praise, but praiseworthiness."

And so our vanity forces us to earn the praise of others. We need to deserve our social status.

Unlike Rousseau, Smith didn't think that vanity is the result or the cause of moral corruption, because there can be no morality and no society without vanity.

So are you vain?

Well, hopefully you are, because to be social and therefore to be human is to be vain.

Or as the novelist Kurt Vonnegut put it, "We are what we pretend to be, and so we must be careful about what we pretend to be."