"Look, you've got a choice," said the mayor of London, England. "When people have concerns, you either address them or play on them."

The mayor, Sadiq Khan, was sitting beside the prime minister of Canada on a stage in Montreal on Thursday, the two paired together for a conversation entitled "Diversity, Inclusion & Future of Progressive Politics," part of the Global Progress 2016 summit hosted this year by the Liberal-friendly (and Liberal-connected) Canada 2020 organization.

At issue was the spectre of popular sentiment turning against immigration and diversity, a concern that has grown in the past 12 months with the rise of Donald Trump in the U.S. and the victory of Brexit in the U.K.

"What you do as a leader when you are faced with anxiety and concerns really defines, I think, the kind of leader you are," Trudeau said, perhaps a bit hopefully.

He's one of at least two Canadian politicians angling to be so defined by the moment.

Leitch and 'anti-Canadian values'

Unmentioned in the course of Trudeau and Khan's conversation was Kellie Leitch, she of the proposal that we screen newcomers for anti-Canadian values. But Khan's dichotomy presents an opportunity to judge her decision to do so.

Even while allowing for the fact that some of us might be worried that some number of immigrants arrive here holding "anti-Canadian values," it's entirely unclear whether a significant problem actually exists or how the Conservative MP's proposal would address any such problem.

For all her enthusiasm about this conversation, Leitch has so far declined to get into the details. What questions would she ask? Would we attempt to confirm the responses? If so, how? What good would this do? What great problem is it meant to address?

"It looks to me like she hasn't really thought it through," Jason Kenney, the former Conservative immigration minister, said last week.

niqab during the citizenship oath, Kenney knows a thing or two about flimsy proposals. Leadership candidate Kellie Leitch talks with reporters at the national Conservative summer caucus retreat in Halifax on Wednesday. (Andrew Vaughan/Canadian Press) And as the immigration minister who proposed banning theduring the citizenship oath,knows a thing or two about flimsy proposals.

Asking philosophical questions about the existence of Canadian values or the very concept of shared values is to give Leitch's proposal too much credence. If she means to address something, she might start addressing the practical questions. For now, she seems to be playing with a concern.

How much insecurity exists is difficult to say, though Leitch seems to have picked up on some amount of concern. And whether Trudeau and his government are doing enough to address it remains to be seen.

Opposite Leitch's enthusiasm for protecting Canadian values, Trudeau enthuses about diversity and the strength and prosperity that comes with it.

And he soon might test his own hypothesis.

'Mood for more immigrants'

John McCallum is due to present a new three-year plan for immigration in November and the minister has been talking about "substantially increasing" the flow of newcomers.

He's been consulting with stakeholders and told CBC radio's The House this weekend he has detected a "general mood for more immigrants." How many more is the question.

There is a case to be made for increasing immigration to counter the burden and economic drag of an aging population. The Conference Board of Canada, for instance, has suggested "Canada will need to bump its immigration levels up to one per cent of its population within the next two decades ... in order to sustain a healthy level of economic growth."

That would be equivalent to 350,000 immigrants per year, approximately 45,000 more than Canada is expected to accept in 2016.

Immigration Minister John McCallum has been talking about 'substantially increasing' the number of newcomers to Canada. (Chris Wattie/Reuters)

In his interview with The House, McCallum linked a final decision to cost and the government's ability to convince Canadians.

"If you have more immigrants, you spend more money and there's always an opportunity cost: if you spend more money bringing in more immigrants, you have less money for other things," he said.

"Also, we want to be sure that we can communicate properly to Canadians why it is that it's good for Canada and good for jobs in Canada to have more immigrants."

The extent of Canadian openness

Canadians, he said, have generally been disposed to accept immigration, but that can't be taken for granted.

"I think Canadians are certainly open to immigration," he said, "but they want the immigrants to be well settled, they want to ensure there are jobs for Canadians."

So ambition must be matched with execution: whether screening for values or increasing immigration, the idea needs to be matched with policy.

If people are losing their jobs, if cultural conflict is felt, if integration fails to occur, the argument for welcoming newcomers becomes more difficult to make.

If immigration is increased, but the economy struggles or public services are strained, the newcomers might become easy scapegoats. Even if those newcomers might also be the best chance of ensuring future prosperity.

"Stronger diversity" is something the prime minister has embraced rhetorically, but is also apparently committed to delivering.

Doing so would be no small thing, because greater diversity, continued prosperity and general harmony would be the most effective response to the insecurities Kellie Leitch seems willing to engage.