In its simplest terms, sadness is often thought of as the natural reaction to a difficult situation.

You feel sad when a friend moves away or when a pet dies.

When a friend says, "I'm sad," you often respond by asking, "What happened?"

But your assumption that sadness has an external cause outside the self is a relatively new idea.

Ancient Greek doctors didn't view sadness that way.

They believed it was a dark fluid inside the body.

According to their humoral system, the human body and soul were controlled by four fluids, known as humors,

and their balance directly influenced a person's health and temperament.

Melancholia comes from melaina kole, the word for black bile, the humor believed to cause sadness.

By changing your diet and through medical practices, you could bring your humors into balance.

Even though we now know much more about the systems that govern the human body,

these Greek ideas about sadness resonate with current views, not on the sadness we all occasionally feel, but on clinical depression.

Doctors believe that certain kinds of long-term, unexplained emotional states are at least partially related to brain chemistry,

the balance of various chemicals present inside the brain.

Like the Greek system,

changing the balance of these chemicals can deeply alter how we respond to even extremely difficult circumstances.

There's also a long tradition of attempting to discern the value of sadness,

and in that discussion, you'll find a strong argument that sadness is not only an inevitable part of life but an essential one.

If you've never felt melancholy, you've missed out on part of what it means to be human.

Many thinkers contend that melancholy is necessary in gaining wisdom.

Robert Burton, born in 1577, spent his life studying the causes and experience of sadness.

In his masterpiece "The Anatomy of Melancholy," Burton wrote, "He that increaseth wisdom increaseth sorrow."

The Romantic poets of the early 19th century believed melancholy allows us to more deeply understand other profound emotions, like beauty and joy.

To understand the sadness of the trees losing their leaves in the fall is to more fully understand the cycle of life that brings flowers in the spring.

But wisdom and emotional intelligence seem pretty high on the hierarchy of needs.

Does sadness have value on a more basic, tangible, maybe even evolutionary level?

Scientists think that crying and feeling withdrawn is what originally helped our ancestors secure social bonds and helped them get the support they needed.

Sadness, as opposed to anger or violence, was an expression of suffering that could immediately bring people closer to the suffering person,

and this helped both the person and the larger community to thrive.

Perhaps sadness helped generate the unity we needed to survive,

but many have wondered whether the suffering felt by others is anything like the suffering we experience ourselves.

The poet Emily Dickinson wrote,

"I measure every Grief I meet With narrow, probing Eyes - I wonder if it weighs like MIne - Or has an Easier size."

And in the 20th century,

medical anthropologists, like Arthur Kleinman,

gathered evidence from the way people talk about pain to suggest that emotions aren't universal at all,

and that culture, particularly the way we use language, can influence how we feel.

When we talk about heartbreak, the feeling of brokenness becomes part of our experience,

where as in a culture that talks about a bruised heart, there actually seems to be a different subjective experience.

Some contemporary thinkers aren't interested in sadness' subjectivity versus universality,

and would rather use technology to eliminate suffering in all its forms.

David Pearce has suggested that genetic engineering and other contemporary processes cannot only alter the way humans experience emotional and physical pain,

but that world ecosystems ought to be redesigned so that animals don't suffer in the wild.

He calls his project "paradise engineering."

But is there something sad about a world without sadness?

Our cavemen ancestors and favorite poets might not want any part of such a paradise.

In fact, the only things about sadness that seem universally agreed upon are that it has been felt by most people throughout time,

and that for thousands of years,

one of the best ways we have to deal with this difficult emotion is to articulate it, to try to express what feels inexpressable.

In the words of Emily Dickinson,

"'Hope' is the thing with feathers - That perches in the soul - And sings the tune without the words - And never stops - at all -"