But there's an awful lot of mad people in the world, so that probably ought to be adapted slightly to read "being in a minority of one among the nearest 1,000 people".
It doesn't take much effort these days, no matter how unusual your thoughts, to find someone in the world who will either agree or co-operate with you, and the very fact another joins in makes whatever crazy idea you've had more acceptable.
So if you live in America and want to go and shoot an elk at the weekend, that's normal. It's more unusual elsewhere and if you wanted to walk around the east end of London dressed all in orange carrying a hunting rifle, this is not the month to do it.
And if you're a highly-intelligent neuroscience student who wants to buy two semi-automatic Glock pistols, a 12-gauge pump-action shotgun and a semi-automatic version of a military assault rifle with 100 bullets in its magazine in the space of just four months, someone will sell them to you along with 6,350 rounds of ammunition.
You can buy the workings for dozens of booby traps and flammable devices and fill your apartment with them. You can research such things, you can try to join a gun club and cause such concern they decide they don't want you, you can order gas masks and body armour and spend thousands of dollars on this stuff.
That's fine. That's not unusual enough to cause concern or questions or flag up your name on a watch list or anything sensible like that.
It's only when you wander into a cinema and start shooting people at random, killing 12 people including a six-year-old girl and wounding 59 with guns that fire up to 60 rounds a minute, that you become unhinged enough for people to notice.
It's only when you dye your hair red and look spaced-out in court that people start to ask if you're mentally competent to stand trial, although a quick look at the local laws shows if you can talk to your lawyer it doesn't matter how bonkers you are, you're facing the death sentence.
And when those things have all happened, the problem will not be that you were allowed to buy guns, or that you may have a mental illness, or any other factor which might in the cold light of day be something which ought to have caused concern a long time before you opened fire.
Nope, the problem will be that so many people died because they didn't have guns too:
Ah yes. The reason 12 people died is because they foolishly did not go to the cinema tooled up with a series of automatic weapons to protect themselves. Silly dead people!
Lunatics of the US gun lobby aside, let's look at the facts.
First off, the right to bear arms might be 200 years out of date but that's up to the Americans to keep or discard, and either way it doesn't give anyone a right to bear arms at 60 rounds a minute. There isn't really a need for that unless you want to kill a lot of people very quickly, which is why in 1994 Bill Clinton made assault weapons illegal.
That law expired in 2004 and partly because it had absolutely zero impact on overall crime rates, it was quietly dropped. Efforts to reinstate it have stalled not least because every time there's a mass shooting like the one in Aurora the firearms industry which is worth $3.5billion a year ups its spending.
After the Columbine high school shootings in 1999 the gun lobby doubled its political spending. When the Clinton law expired it increased by 17 per cent, and after the 2007 killings at Virginia Tech it went up 40 per cent on the previous election cycle.
Lastly, the figures on spree killings are eye-watering. Since 1976 the US has averaged at 20 a year, every year. Between 1976 and 2010 there were 645 incidents involving 937 killers and 2949 fatalities, never mind the injured. That's 86 or more people dying every year, for 34 years.
Meanwhile people buy guns because they're scared, they bought 25 per cent more of them in the US after the financial crisis in 2008, and the politicians don't walk to talk about how to stop people using guns for bad things because they're scared too.
Different people have different rules for what constitutes 'insane'. But in general terms it means you're not thinking clearly, that your thoughts are unsound, and as a result you've become detached from reality and the consequences of your actions.
I really cannot think of anything more insane than selling someone an automatic rifle and expecting them not to use it on people; than sending thousands of bullets in the post and trusting it's just for fun; or a security service which does not have a computer that flags up suspicious purchases of such things.
I have tried and failed to work out the logic of allowing assault weapons to become legal after they've been banned or of allowing letting 86 people get slain every year when even if you continue to allow standard weapons more of them would survive.
Granted, it is best to listen carefully to the kind of people who think everyone should have a machine gun with them when they go to a cinema, and treat them very carefully indeed.
But while you're doing that, you're supposed to be luring them in to the padded cell where they won't hurt themselves or anybody else.
You don't make them part of the political system with a say in what goes on - not unless all 311million citizens are totally barking mad.
In which case, James Holmes would be better off out of it.
At least they don't all have passports.