Welcome to the realm of the thought experiment

What do a hurtling trolley, a shallow pond and a famous violinist all have in common?

Illustration by Marie Montocchio / Ikon Images

Imagine you’re walking through a maze. As you attempt to navigate your way through the corridors, occasionally running into dead ends, you pass through various spaces in which unusual, sometimes surreal, events are unfolding.

“Welcome to the Chinese Room,” booms a disembodied voice as you enter the first room you come across. Inside, a woman is picking up a note that somebody has pushed under the door on the far side of the room. Scribbled on the paper are a couple of lines of Chinese characters. “I have no idea what they mean,” the woman says. But she plugs the symbols into a huge contraption, follows a few instructions, and a few seconds later, after some whirring and humming, another note splutters out, with a printed line of response, also in Chinese, which the woman slips back under the door. “Ha,” she smirks, “I’ve fooled the people on the other side into believing that I can really understand the language!”

Down a passageway, you pass a shallow pond, in which a child is struggling to stay above water. Just as you are about to wade in to save her, you notice that you’re wearing some expensive brogues, and your new Gucci trousers. There’s no time to remove them. For a moment, before getting your feet wet, you wonder whether a child’s life is worth ruining your stylish outfit.

Sodden, but warmed by self-satisfaction, you continue on your way. A locked chamber contains a small spyglass. Peering through, you see a woman reading a book. Everything in the room – the desk, the bed, the walls, the book, the woman’s clothes – is either black or white. The TV is on, showing a black and white film. It is a room entirely devoid of colour.

Later, you spot a man who suddenly transmogrifies into a vampire. Another man is opening a small matchbox, in which, scuttling inside, you spy a beetle. There’s a woman with tubes sticking out of her, attached to an elderly man who’s playing a beautiful rendition of Vivaldi’s Four Seasons on the violin.

This labyrinth, of freaks and oddity, is philosophical heaven, bursting with thought experiments. A thought experiment is an imaginary set-up conjured up by a philosopher to test or provide some insight into an idea or an intuition. The Chinese Room, the Shallow Pond, Mary’s Room, the Famous Violinist: these are all well-known scenarios in philosophy.

On these pages, every few weeks, I’ll be bringing you a particular philosophical thought experiment. Who thought it up, and why? Why does it matter?

The philosophical thought experiment is as old as philosophy. In the 5th century BCE, Zeno imagined that Achilles and a tortoise were in a race, with the tortoise given a head-start. Achilles is much the faster competitor. Still, suggested Zeno, he can never overtake the tortoise, since by the time he reaches the point where the tortoise was, it has moved to a second point, and when Achilles reaches this new point, the reptile has moved forward again… and so on.

Plato, Descartes, John Locke, David Hume, Jean-Jacques Rousseau – almost every philosopher in the canon has made use of thought experiments. I’ll be focusing on thought experiments from the past 100 years. The column will feature the Experience Machine, the Trolley Problem, Twin Earth and the Veil of Ignorance. They’re great fun, of course, but they also address serious issues. The Famous Violinist, for example, was created by the philosopher Judith Jarvis Thomson (1929-2020), and was composed as a defence of abortion. The Trolley Problem, in which a runaway train will kill five people tied to a track unless redirected down a spur, where it will only kill one, was also devised in an article about the ethics of abortion.

My love of thought experiments is not universally shared. They’re ludicrously unrealistic, critics bemoan, and realism matters, particularly in the moral realm. Our intuitions are primed to respond to ordinary cases and are totally unreliable in contrived settings. In any case, our moral instincts only make sense within the complex web of circumstances in which typical actions occur.

But it’s precisely to avoid all the real-world noise, the messiness of life, that the philosopher invents the thought experiment. One way philosophers have tested whether a feature of the moral world is relevant (for example, is killing worse than letting someone die?) is by imagining two artificially constructed situations that are identical except for the presence in only one of the factors in question.

Be that as it may, the cry of implausibility tends not to be levelled at thought experiments that have nothing to do with value. And many of the thought experiments I’ll be describing fall within this category. So strap in. The out-of-control thought-experiment trolley has begun to roll.

David Edmonds will be writing a regular column on philosophy in the New Statesman