Pleasure, according to the ancient Greek philosopher and party animal, Epicurus, is the beginning and the end of living happily.
But what if you can't feel pleasure? For those with a condition called anhedonia, the joy of a first kiss, the deep satisfaction of solving a puzzle, or the warmth of hearing a favourite song are sensations only other people can experience.
In strict terms, anhedonia can be described as a sense of emotional flatness that often leads to a loss of a desire for physical intimacy, difficulty adjusting to social situations, or complete withdrawal.
Considered more of a symptom than a disease, the effects can be debilitating, with implications that reach beyond an ability to live happily.
What is pleasure?
Boiled down to its most basic principles, pleasure is the feeling we get that drives us to seek out a stimulus.
The ability for our brain to feel pleasure serves as a significant control mechanism to bond us together into social groups, and to help us reinforce certain advantageous behaviours.
As Joshua Garfield from the Australian drug and alcohol research centre Turning Point, explains, we can break pleasure into two kinds: anticipatory and consummatory.
"So if you've been invited to dinner with a friend, and they tell you they're cooking one of your favourite foods," he told ScienceAlert.
"You experience anticipatory pleasure upon hearing this news, and – assuming they actually cook it well – consummatory pleasure during and immediately after eating it."
Both factors, Garfield says, play a role in our ability to learn:
"If you feel a great deal of anticipatory pleasure, but then the thing you were anticipating turns out to be underwhelming, this may register as a 'prediction error' that helps you reassess the value of the goal."
While Garfield makes it clear his research into pleasure as a psychologist focuses largely on the individual, he also points out pleasure is a strong social force.
Again, Epicurus would agree, claiming pleasure is the beginning of every choice and every aversion.
So an inability to feel pleasure is more than just a glum or empty existence. It is an impediment to our ability to learn and interact socially.
Just what exactly is anhedonia, though?
What causes anhedonia?
Our experience of pleasure is the product of neurochemistry inside a circuit of brain bits called the cortico-ventral basal ganglia (BG).
A part of the brain called the medial prefrontal cortex (mPFC) acts as a traffic cop for pleasurable acts, allowing oh-so-good dopamine levels to rise in a part of the BG called the striatum and eliciting a pleasant sensation.
Research published by Stanford University neuroscientists in 2016 supported previous suspicions that lower activity in the mPFC could be primarily responsible for anhedonia.
The fundamental cause of this low activity could be genetic, although stress or trauma early in life in some cases substance abuse can also impact on our brain's ability to mediate rewards.
Disease? Or symptom?
But is anhedonia a disease in its own right, or a feature of other conditions?
"It's generally considered a symptom, not a primary condition," Garfield explains.
In other words, anhedonia is what psychologists might look out for to contribute to a diagnosis of other conditions, such as depression, schizophrenia, some personality disorders, post-traumatic stress disorder (PTSD), and substance use disorders.
It's also usually treated according to the disorder rather than as a disease in its own right; if depression is considered to be the cause, a physician might recommend personal therapy, medication, or in extreme cases electroconvulsive therapy.
Nonetheless, it could be complicated. According to Garfield, genetics or early life stress could make some people generally more vulnerable to conditions linked with anhedonia.
"There's reasonably good evidence that frequent use of addictive substances causes anhedonia," Garfield says.
While it's hypothesised that this could lead to a feedback loop, Garfield says the evidence for this is mixed and inconsistent.
"There's also a hypothesis that anhedonia may precede addiction and make people more vulnerable to developing addiction," says Garfield.
"But I've seen no convincing evidence that anhedonia in people who've never used substances makes them more likely to develop later substance use problems, and I have seen some evidence suggesting the opposite."
Clearly, as usual, biology is complicated and doesn't like to be put into neat boxes. But with 300 million people across the globe with depression, and the condition considered to be on the rise among adolescents, it's time we got a better understanding of why some people can't feel pleasure.
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