'Sonder' is an invented word with a real resonance. It's defined in John Koenig's Dictionary of Obscure Sorrows as 'the realization that each random passerby is living a life as vivid and complex as your own.' Transient strangers comprise the background of your own life – they're your cast of extras, the people you bump into but don't think about, part of the colour of the changing scenery. You rarely truly notice them, and likewise, you are rarely fully noticed by them. The exception is in these moments of sonder. These are moments in which you extrapolate your own despairs, your joys, your heartaches, your petty frustrations, your comforts, to those around you. Everyone else feels these things too.

It can be an overwhelming feeling, one that Tom (Peter Dewhurst) deftly cultivates in the audience in the part-play, part-conversation How We Think We Think, written by Melanie Ball and directed by Joe Ball. He asks us to turn the gaze at the person next to us and truly notice them. Look at what they're wearing. Look at their expressions, their lines. Who are they? Imagine the kind of life they might have led so far. How did they get here? Why? It's startling to try to reflect on others in so much detail. And it's uncomfortable also to be considered like this. As though we are real.

The fact that we lose sight of the reality of strangers' lives is important context for How We Think We Think, which tells Tom's story of witnessing a man's suicide at a tube stop one early morning. The man had struck up a conversation with him minutes earlier, but would otherwise have been the kind of person to go unnoticed. Despite his real pain, he was the kind of person who'd just be part of the scenery under the fluorescent lights and the roaring rhythm of the London Underground.

Tom grapples with his responsibilities as a witness, and indeed as the last person to have ever spoken to the man. His darkly funny discussion traverses a range of fascinating topics: the psychology of decision making; the future-oriented nature of the brain's frontal lobe; and the ethics of taking on another's tragedy as your own. At times it gets a little convoluted and difficult to trace the play's internal logic, and the feeling of chaos is mirrored as Tom throws folders of research onto the floor. But it's constantly thought-provoking, and captures you in that moving place of sonder. It's an intense play that demands compassionate attention. And it provokes you to bring that attention into ordinary life as well.