Like scientists, philosophers use experiments to test their theories. Unlike scientists, their experiments do not require sophisticated laboratories, white-robed technicians or even rodents. They occur in the mind, and start with 'What if...'.
These "thought experiments" help philosophers clarify their understanding of certain concepts and intuitions. In the field of ethics, thought experimenters typically present a dilemma, examine the most popular "intuitive" response and then show the implications for real-world issues.
But such experiments are rarely tested on large numbers of people. So to reach a larger group, here are four typical experiments. Readers are invited to vote on how they think they would act in each case.
Here is a well-known example:
One day, you wake up in hospital. In the nearby bed lies a world famous violinist who is connected to you with various tubes and machines.
To your horror, you discover that you have been kidnapped by the Music Appreciation Society. Aware of the maestro's impending death, they hooked you up to the violinist.
If you stay in the hospital bed, connected to the violinist, he will be totally cured in nine months. You are unlikely to suffer harm. No one else can save him. Do you have an obligation to stay connected? 
The creator of the experiment, Judith Thomson, thinks the answer is "no". It would be generous if you did, she claims, but there is no obligation to stay, even if that means the violinist will die.
So how is this bizarre scenario related to the real world? Thomson used the experiment to show that a pregnant woman need not go to full term with her baby, as long as she had taken reasonable steps to avoid getting pregnant. It is thus a "pro-choice" argument.
The violinist represents the baby, and you - in the hospital bed - play the role of the mother. If you think unhooking yourself from the violinist is acceptable, but aborting an unwanted foetus is not, what are the moral differences between the two cases? In both situations, you could save a person by bearing a great burden for nine months.
One major flaw with thought experiments, especially in ethics, is that they are rarely tested on people. The sample size is minuscule. The philosopher will simply assume that most people think that one option is right (or wrong).
Philippa Foot, a renowned British philosopher, believed that if a doctor, about to save a patient's life with a large dose of a scarce drug, was suddenly interrupted by the arrival of five patients each in need of one fifth of the drug (without which death would be certain), then the doctor should give it to the five. It is, after all, better to let one person die than five.
Elizabeth Anscombe, another prominent philosopher, disagreed: "There seems to me nothing wrong with giving the single patient the massive dose and letting the others die". As these assumptions about people's intuition are central to the arguments of many philosophers, and as these assumptions can be tested, why not do so?
One of the most famous thought experiments in ethics is "the runaway trolley". It aims to clarify how we should distinguish right from wrong.
Here is the scenario with two well-known variations.
A runaway trolley car is hurtling down a track. In its path are five people who will definitely be killed unless you, a bystander, flip a switch which will divert it on to another track, where it will kill one person. Should you flip the switch?
The runaway trolley car is hurtling down a track where it will kill five people. You are standing on a bridge above the track and, aware of the imminent disaster, you decide to jump on the track to block the trolley car. Although you will die, the five people will be saved.
Just before your leap, you realise that you are too light to stop the trolley. Next to you, a fat man is standing on the very edge of the bridge. He would certainly block the trolley, although he would undoubtedly die from the impact. A small nudge and he would fall right onto the track below. No one would ever know. Should you push him?
Philippa Foot would say that everyone ("without hesitation") would choose to flip the switch in the first trolley case, but that most of us would be appalled at the idea of pushing the fat man.
The philosophical puzzle is this: Why is it acceptable to sacrifice the one person in The Runaway Trolley Car but not in The Fat Man case? Can it ever be morally acceptable to kill an innocent person if that is the only way to save many? Should some actions - such as deliberately killing innocent people against their wishes - never be done? The last thought experiment explores this idea:
An enormous rock falls and blocks the exit of a cave you and five other tourists have been exploring. Fortunately, you spot a hole elsewhere and decide to let "Big Jack" out first. But Big Jack, a man of generous proportions, gets stuck in the hole. He cannot be moved and there is no other way out.
The high tide is rising and, unless you get out soon, everyone but Big Jack (whose head is sticking out of the cave) will inevitably drown. Searching through your backpack, you find a stick of dynamite. It will not move the rock, but will certainly blast Big Jack out of the hole. Big Jack, anticipating your thoughts, pleads for his life. He does not want to die, but neither do you and your four companions. Should you blast Big Jack out?
If the roles were reversed, what would you advise your trapped companions to do?
Thought experiments, although abstract, possibly implausible and open to different interpretations, can have important repercussions on the way we think and act as individuals. They raise thorny questions about morality in medicine, war, politics and indeed in everyday life.
Is there a difference between killing someone and letting them die? Are consequences all that matter, or are there some things we should never do, whatever the outcome?
By pointing out inconsistencies in our thinking, or simply encouraging us to reflect on issues we usually ignore, they can sharpen our intellect and enrich our moral lives. They also make for great conversation topics at the dinner table or at the pub. But be warned: you may lose friends as a result. And stay away from caves and bridges.
I'm not fat, so I should be fine with the bridge scenario. I loved my ethics course in college, it really made me think. I like thinking. It's fun.
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