If common sense, which is increasingly uncommon prevails, Coun. Doug Ford will apologize for making an allegation about Toronto Police Chief Bill Blair he can’t prove.

How do we know he can’t?

Because when Blair called him on it, Ford couldn’t back up his allegation Blair leaked word about the police service allegedly planning to subpoena Mayor Rob Ford in the extortion trial of Sandro Lisi, as “payback” for Blair not being reappointed by the police services board.

After Blair said Ford was lying and threatened to sue him for defamation, Ford said his allegation was just his opinion, suggesting the Etobicoke councillor, who is managing his brother’s mayoral campaign, has a poor understanding of libel and slander law.

Basically, you can’t make a defamatory allegation without any proof, and then claim it’s just your opinion.

On Tuesday, after Blair served Ford with a defamation notice for not retracting his remarks, Ford said dismissively he’s “shakin in (his) boots” and will consult with his lawyers before deciding what to do.

We hope his lawyers give him good advice.

That said, was it necessary for Blair to threaten to launch a defamation suit over this tawdry affair?

Blair is a public figure with the ability to get his message out to the public through the media any time he desires, in a job where criticism goes with the territory.

Everyone knows Doug and Rob Ford have been launching verbal broadsides against Blair for months over the police investigation of the mayor, following media reports the mayor had been caught on videotape smoking crack.

When Blair told the media police had found a video consistent with these media reports, Doug Ford called him “the most political police chief” he’d ever seen and accused him of a conflict of interest because he’d gone on a fishing trip with police services board member Andy Pringle, former chief of staff for mayoral candidate John Tory.

All of which ignored the fact Rob Ford had indeed smoked crack while in a “drunken stupor” — as he eventually admitted.

In other words, the Fords have a history of blaming the chief for problems Rob Ford brought on himself.

Blair should realize most people understand that and calm down.