[quote:3a9ae94fda="Electric_Man"][quote:3a9ae94fda="drunkymonkey"]but it's better to get someone before they commit a crime rather than after, surely?[/quote:3a9ae94fda]

I can't remember which book it's in, but this was discussed in Discworld. You can arrest someone before they do a crime on a suspicion, but that would hardly hold up in court,

"We caught him before he did it."
"Did what?"
"Well, he hasn't done it yet..."[/quote:3a9ae94fda]How about arresting someone when they decide to meet up with a child without parent's consent?