If anything makes us human it's our minds, thoughts and emotions.
And yet a controversial new concept is emerging that claims gut bacteria are an invisible hand altering our brains.
Science is piecing together how the trillions of microbes that live on and in all of us - our microbiome - affect our physical health.
But even conditions including depression, autism and neurodegenerative disease are now being linked to these tiny creatures.
We've known for centuries that how we feel affects our gut - just think what happens before an exam or a job interview - but now it is being seen as a two-way street.
Groups of researchers believe they are on the cusp of a revolution that uses "mood microbes" or "psychobiotics" to improve mental health.
The study that ignited the whole concept took place at Kyushu University in Japan.
The researchers showed that "germ-free" mice - those that never came into contact with microbes - pumped out twice the amount of stress hormone when distressed than normal mice.
The animals were identical except for their microbes. It was a strong hint that the difference was a result of their micro-organisms.
"We all go back to that first paper for the first wave of neuroscientists considering microbes," says Dr Jane Foster, a neuropsychiatrist at McMaster University in Canada. "That really was very powerful for those of us who were studying depression and anxiety." It was the first hint of microbial medicine in mental health.
How could bacteria be altering the brain?The brain is the most complex object in the known universe so how could it be reacting to bacteria in the gut?
But their completely sterile upbringing is nothing like the real world. We're constantly coming into contact with microbes in our environment, none of us are germ-free.
At Cork University Hospital, Prof Ted Dinan is trying to uncover what happens to the microbiome in his depressed patients.
A good rule of thumb is a healthy microbiome is a diverse microbiome, containing a wide variety of different species living all over our bodies.
Prof Dinan says: "If you compare somebody who is clinically depressed with someone who is healthy, there is a narrowing in the diversity of the microbiota.
"I'm not suggesting it is the sole cause of depression, but I do believe for many individuals it does play a role in the genesis of depression."
And he argues some lifestyles that weaken our gut bacteria, such as a diet low in fibre, can make us more vulnerable.
It's an intriguing concept - that an imbalance in the gut microbiome could be involved in depression.
So scientists at the APC Microbiome centre, at University College Cork, started transplanting the microbiome from depressed patients to animals. It's known in the biz as a trans-poo-sion.
It showed that if you transfer the bacteria, you transfer the behaviour too.
Prof John Cryan told the BBC: "We were very surprised that you could, by just taking microbiome samples, reproduce many of the features of a depressed individual in a rat."
This included anhedonia - the way depression can lead to people losing interest in what they normally find pleasurable.
For the rats, that was sugary water they could not get enough of, yet "when they were given the microbiome from a depressed individual, they no longer cared", says Prof Cryan.