Why is murder wrong?

Philip is being deliberately annoying and Phoebe, his twin sister, is falling for it.
They are discussing whether murder is wrong, and Phoebe is having trouble
persuading Philip that it is ...

* * *

'Don't be ridiculous! Everyone knows murder is wrong! It's obvious!' she says.

'Why?' asks Philip, looking mischievous.

'Well, for a start, every civilised society in the world makes it against the law,' says
Phoebe triumphantly. 'You can't believe they're all wrong, surely?'

'Not long ago all intelligent people thought the world was flat,' counters Philip.
'Does that mean it is?'

'But, but it's violent and barbaric. It's painful!' stutters Phoebe in disbelief. 'It can't
be right to hurt other people, surely?'

'Why not? Why shouldn't I hurt other people if they annoy me?' asks Philip,
pretending to be genuinely puzzled. 'Perhaps I'd enjoy hurting them.

'Well, you wouldn't like it, would you? And you shouldn't do things to other people
you wouldn't want done to yourself!' Phoebe produces her trump card. How many
times has she heard her parents and teachers saying exactly this to other children?

'Why?' asks Philip.

Phoebe is speechless.

* * *

Who is right? Is it clear that murder is wrong, as Phoebe claims? Or is Philip
correct that you really can't show that it is?

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