We can't possibly do that because...
Women can’t read the news because they don’t sound authoritative and no one would believe them. (50s)
Women can’t become priests because their voices are not commanding enough. (mid-90s)
We can't employ women in offices because they'll start growing beards, 1900s. (Or was that "women shouldn't cut their hair"? A generation of bearded ladies was not forthcoming.)
If wolf-whistling is made a crime, even fewer women will become mothers because it will “drive the sexes even further apart”. (Andrew Cadman on Conservative Woman)
We can’t have escalators in the New York subway because they might enable terrorism.
We can’t prosecute all these MPs and ministers for sexual harassment because it would undermine the government and destabilise the country.
Famous judge Lord Denning (1899-1999) said you can’t criticise the police even when they’re wrong because they'd lose the people's respect. He also said: “We must not allow this cult of homosexuality, making it equal with heterosexuality, to develop in our land. We must preserve our moral and spiritual values.”
And in 1990 Denning suggested homosexuals’ vulnerability to blackmail should stop them from being appointed as judges. Sir Terence Etherton, now Master of the Rolls as Denning once was, is openly gay.
We can’t protect minorities (or ourselves if we’re members of a minority) because we might attract the wrong kind of attention.
You can’t do as you would be done by because the other guy might not like what you like.
We can’t try to make the world a better place because first we’d have to convince everyone in the world that our version of “better” was the right one.
We can’t just sack this director of Oxfam who paid Haiti victims for sex because it would undermine the work of the organisation.
But the wise ones said: A woman shall not read from the Torah in order to respect the public. (Tractate Megillah, before 200CE. Presumably "because some people wouldn’t like it" is meant.)
We can’t send shy people to charm school! (Common in the 70s. I always wanted to know why we couldn’t, but at least it was a step forward from “shy people are just selfish”.)
You can’t be against abortion because you’d be standing with right-wing fundamentalists.
You can't be against pornography because you'd be standing with Mary Whitehouse. (The Clean Up TV campaigner died in 2001.)
If we no-platform the neo-Nazis, they will go underground and become more dangerous.
We can’t arrest/imprison/kill their leader, or he will become a martyr.
We can’t protect children because we’d be playing into the hands of the people who want to make society more authoritarian. (George Orwell nailed this argument here.)
We can't set up a national DNA database because you might be convicted of a crime when your fag end was found in the vicinity.
We can’t have the Tridentine Mass back because we’d be siding with the Lefebvrian schismatics. (Pope Benedict reintroduced it, 2012.)
Women don’t need the vote because they can ask nicely for what they want/tell their husband how to vote.
We can’t have equal pay or women’s rights because men would stop holding the door open for women, and women would lose their femininity. (Bizarrely revived in 2011 along with the word “chivalry” which I thought had died circa 1972.)
We can’t make divorce easier because the result would be anarchy. (Can we have no-fault divorce soon, please?)
You can’t stop animal experiments/the tobacco trade/pylons across national parks/motorways through tranquil villages because jobs would be lost.
There's no point having a law against it because you can’t change people’s minds by legislation.
Legal wouldn't let us.
The CEO prefers to do these things his way.
It's too expensive.
We don't have the time.
We'd have to work with organization X. (Michael Schrage)
Will never fly.
Tried it before.
Not in our budget.
Out of scope.
(Idea Voodoo by Tom Fishburne)
It can’t be done.
It’s just science fiction.
They tried it in [year] and it didn’t work.
They tried it in [place] and it didn’t work.
[Prominent individual] says it won’t work.
It’s just [failed ideology]ism.
It will never happen so why bother.
And, the most effective of all… It will never work because you can’t change human nature.
We will nevah, nevah do XYZ!
Yes, we’ve found the Titanic. It’s a grave site and will never be disturbed. (10 years later objects from the Titanic are being sold on eBay.)
We will never clone human beings.
My college, Magdalene, was all-male until the late 1980s. I was one of the first female postgrads and a lot of the boys wore black armbands in mourning. 30 years later and it's a very diverse college: its exam results have gone through the roof now that the rather dense rugger buggers are no longer such a feature. (LW)
Magna Carta: “Not only only shameful and demeaning but also illegal and unjust,” said the Pope of the day, before annulling it. It became law in 1225.
More examples here.