3 inches is no more reasonable than 70 mph as a speed limit. But if you accept that some limit should exist, it has to have some value, even if all the choices are equally arbitrary
If any speed limit and any knife length is equally arbitrary, then a nation-wide speed limit of 1 mph and a legal knife length of 100 inches should be perfectly acceptable to everyone. Yet it's quite likely many people would object to both.
I don't see that cabaalis said that any limit is equally arbitrary, and I doubt s/he thinks that.
Some limits are more reasonable than others. Saying "you can have knives but the blade must be no more than 5mm long" would be stupid; so would saying "you can drive a car but no faster than 1mph". (Because in either case you might as well, and should in preference, just say "you may not"). Likewise for a limit of 10m or 500mph (because then you might as well, and should in preference, just not bother with the limits).
If you're going to have a limit on the length of a knife blade, presumably for the sake of a small reduction in knife crime, you want a limit long enough that some (non-criminally) useful knives are shorter and short enough that the restriction, if it reduces the number of long knives in circulation, would actually do something to impede crime.
It seems plausible that a 3-inch limit would do that. Maybe a 4-inch limit too. The exact choice of limit isn't completely arbitrary: some choices are better than others. But really, the only answer to "why 3 inches rather than 4?" is "that's where we happened to make the tradeoff".
I'm guessing (not least because the above all seems kinda obvious) that your actual objection is to having any limit at all. (Perhaps on the grounds of some more general libertarian principle?) That's a reasonable objection, but I don't think "why 3 inches rather than 4?" is a good way to make it.
I wasn't trying to make an objection. I was just trying to understand what the argument was for making the limit 3".
As for the "criminally useful" argument, I struggle to imagine what crimes could be committed with a 4" knife that couldn't be committed with a 3" knife.
You say it's plausible to you that a 3" or 4" limit would reduce knife crime. What makes you think that?
It seems plausible to me -- I don't claim any more than that, and I am no expert on this stuff -- that:
1. If possession (in public places) of knives above (say) 3" is illegal, fewer people will go about carrying knives above (say) 3". It probably won't make any difference to someone planning to get into a knife fight, of course.
2. If fewer people are casually carrying big knives, then there will be fewer opportunities for conflicts to escalate to fights involving big knives. For instance, a bar fight is less likely to end up fatal if fewer people in it are carrying serious knives. A burglary is less likely to end up fatal if the burglar isn't. Some muggers may choose to make do with musclepower rather than knives (so they're less likely to get into serious trouble if a policeman thinks they look suspicious). Etc.
3. Fights involving big knives are less dangerous than fights involving smaller knives, e.g. because a bigger knife is more likely to end up doing serious damage to internal organs. (If an expert fighter is specifically trying to do you serious harm, I'm sure they can do it with a small knife. Or a toothpick. But in cases where the goal is "establish dominance" or "get away safely from the house I just burgled" or "make my victim sorry he didn't just hand over his wallet" and there isn't serious intent to kill, I expect smaller knives to do less harm on average.)
Astute readers will notice that I'm now talking about knife fights rather than knife crime. My guess is that premeditated crime-with-knives probably wouldn't be much affected by this sort of ban. So, though I'm sure all the things I'm hoping would be reduced would technically be knife crimes (i.e., crimes committed using a knife), I think my use of that term was unhelpful. Sorry about that.
Well, same arguments as with guns, really. Smartly, UK police has figured out that a policeman can't shoot anyone by mistake if they don't have a gun - so UK policemen don't carry them at all. If you reduce a number of people carrying long knives, you reduce the number of people stabbed with long knives. Obviously, just like with guns - people who want to have a knife and use it to commit crime, are still going to.
So, the law is actually more specific [1] than just three inches - it's obviously been written with the intent of permitting traditional swiss army knives, while banning essentially everything else. (Including leatherman tools due to their locking blades - and most kitchen knives, which don't fold)
I suspect someone measured their boyhood swiss army knife blade, determined that it was about 2.5 inches, and rounded up to the next whole number.
