I'm a strong believer in a persons intentions being the only valid factor in deciding how they should be punished. If you intended to kill but were stopped from doing so by forces outside of your control, you're no better than the murderer who got a bit luckier on the day, and should be punished as such. (Why attempted murder serves a lesser sentence than murder is totally beyond me, 'We know you tried to kill him, but simply because you weren't very good at it we'll halve your sentence. Give it another try when you get out.'
The converse of this is, say a friend promised to help you move. He got half-way to your apartment before his car broke down, he had every intention of helping you but forces outside of his control stopped him. Would you feel any less gracious to him for making a solid effort?
I dunno, I've always felt the most a person can do is have good intentions and hope that things work out in the positive way you want them to. This person deserves nothing but the best, regardless of the effect outside forces have on the outcome of their work. Inversely, someone wishing evil designs on the world should be punished for what they try to do, regardless of effect.
As my dad once said when someone offered him a glass of wine and he declined, 'It's as good as if you gave it to me.' That kinda stuck with me.
Back to the original post, I feel like this is more a moral dilemma than us questioning whether it could work in real life. Lack of proof will always and forever prevent someone from being tried for something they haven't done. And right too, it would be nothing less than a travesty for an innocent person to be sent to jail for something they weren't even going to do. (Tom Cruise in Minority Report.
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)
But if it COULD be proven, I believe the criminal-to-be should be punished.
Ethics FTW!