Last updated: 19 may, 2010 - 04:25 GMT

Lady behind the trolley problem

Judith Jarvis Thomson doesn't seem like a woman who would push a big man in front of a train. She's now in her 80s and rather frail.

She's also thoughtful and ferociously clever.

Judith Jarvis Thomas

Little did Judith Jarvis Thomas know that she would spawn a mini-industry

For much of her career this renowned moral philosopher has taught at the Massachusetts Institute of Technology, grappling with issues of life and death.

She is also the inventor of the so-called The Trolley Problem – indeed, she it was who coined the phrase.

She was intrigued by the following thought experiment: imagine there's a runaway train (or trolley), hurtling down a railway line. It's out of control; the brakes have failed. Ahead, five people are tied to the track. All five apparently face certain death. You, however, are standing by the rails, and have a chance to save them. By turning a signal switch you can divert the train down another track. The only problem is that one person is bound to this other track. What should you do? Should you flick the switch?

Most people believe you should: that would be the right thing to do. But three decades ago Judith Jarvis Thomson then conjured up the following hypothetical case.

The runaway train is once again heading towards five people. This time you're standing on a footbridge, next to a very big man. The only way you can stop the train is to push the big man over the footbridge onto the track: his bulk will stop the train; five lives will be spared. What should you do? Should you push him?

At the time (she's now changed her mind!) Professor Thomson believed, along with most people, that it was right to turn the train in Case One, but not to push the Fat Man in Case Two. What was the moral difference between the two? After all, in both cases it's a choice of one life or five.

Little can Professor Thomson have known that her Trolley Problem would spawn a mini-academic industry. Since the publication of her original article, other moral philosophers have come up with ever more ingenious and surreal scenarios involving runaway trains. The train is always racing towards five unfortunates, and the reader is presented with various means to rescue them, although at the cost of another life.

In the past decade, psychologists and neuroscientists have also jumped on the tram-wagon – they too are trying to learn from how we instinctively respond to these cases. But this is not just a debate between scientists and armchair philosophers.

Our answer to Professor Thomson's Trolley Problem could have implications in numerous practical areas, most obviously, the conduct of warfare.

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