"We are not disturbed by things themselves, but by our opinions about them" - the sentence is almost two thousand years old, spoken by the Stoic philosopher Epictetus, born a slave. And today neurology and psychotherapy prove it right in a way he couldn't even have imagined.

The idea is simple and uncomfortably accurate: between the event and our emotional reaction there's a space, and in that space our interpretation decides everything. Two people can get the same remark from a boss - one will experience it as an attack, the other as advice. The event is the same. The story we tell ourselves about it is different.

This isn't just philosophising. Donald Robertson, one of the leading contemporary experts on Stoicism, points out that it was precisely the ancient Stoics who inspired cognitive behavioural therapy (CBT) - one of the most scientifically supported psychotherapies today. Its founders, Albert Ellis and Aaron Beck, admitted that many of the tools they use were invented by philosophers long before them.

Why does the brain apply this so poorly? Because, as neurologist Rick Hanson puts it, it's "velcro for the negative and teflon for the positive" - it holds on to threats and worries far more easily than to good things. It's an evolutionary habit: our ancestors survived precisely because they stayed alert to dangers. The problem is that the same brain doesn't distinguish a real threat from an imagined one - just the thought of a difficult conversation raises cortisol as if the enemy were already at the door.

This is where freedom comes in. "Between the thought and the reaction there's a space. And in that space freedom is born", says therapist Toni Espigares. The goal isn't to erase negative thoughts - that doesn't work - but to observe them without identifying with them. Put simply: you don't have to believe everything that passes through your head. Neither did Epictetus.