Justin Trudeau, who chats easily through an interview that runs twice as long as promised, is prepared, open, knowledgeable and yet, on some subjects, somehow unreachable.

For instance, there is the crucial question about why he is in the race to be leader of the federal Liberal Party, since he'd said at various times he wasn't ready, or he wanted to wait until his children were older.

His answer, which is basically that Prime Minister Stephen Harper has to be stopped, doesn't address why he should be the one to lead Liberals.

"For me, I suddenly began to see, if someone doesn't stand up to him in a very different way than we tried in the past, he's just going to keep setting the terms of the battle."

Asked what "a very different way" means, Trudeau said, "The conventional approach to politics these days is all about targeting your supporters, mobilizing them, encouraging other people to stay home, getting your vote out, going negative on the other guys, because as we all know, negative advertising works, particularly politically. Nobody can beat Mr. Harper at that game."

Federal Liberal leadership This is the last in a series of profiles of the candidates for leadership of the Liberal Party of Canada. The candidates will be showcased at an event in Toronto April 6, commencing a week of voting by party members and supporters. The winner will be announced April 14 in Ottawa.

"I'm not a negative person," he continued. "I know that people right across the country are tired of negativity, are tired of people going after each other."

Yet as a twice-elected MP in the Quebec riding of Papineau, Trudeau has not shied away from taking shots at opponents. He once had to apologize for calling Environment Minister Peter Kent "a piece of shit" in the House of Commons after Kent chided an NDP MP for not participating in a conference the government had effectively blocked her from attending.

'There are no terrific scandals out there'

Asked if he's vulnerable to anything in particular the Conservatives might throw at him, Trudeau said, "There's no arrest records as a teenager that have been sealed. There are no terrific scandals out there. I've made some bad decisions in my life, sure, who hasn't? Have I learned from them? Absolutely."

The Conservatives might simply appropriate the usual jibes tossed at Trudeau: he's vacuous, has no substance and is merely a celebrity with great hair, even though he has an arts degree from Montreal's McGill University and taught school for a time. Mostly, he’s accused of capitalizing on the name of his father, former prime minister Pierre Elliott Trudeau.

"I've had people do that to me all my life," Trudeau said. "People, all my life, conclude things about me based on perception, based on prejudice, based on expectations that have nothing to do with reality.... There's a silliness around people's attacks on me that make it really easy for me to dismiss."

The only criticism that makes him bristle is the suggestion that he seemed weak when he led question period for the Liberals last month, reading his questions from notes.

"I was nervous, obviously," Trudeau said. "It was my first time up to ask a question of the prime minister. Bob Rae, just an hour before, had decided he wasn't going to be the first one asking questions in QP, and they turned to me and said, 'Justin, do you want to do it?' I had less than an hour's preparation."

He said he's not going to fret about it, and adds, "At least I don't have a lectern, like Mr. Mulcair has."

Legalizing marijuana and carbon pricing

On policy, Trudeau said he's in favour of legalizing marijuana and leans toward carbon pricing, "whether it's for cap and trade, whether it's some sort of levy, whether it's even a carbon tax." The subject of a carbon tax has been "polarized" he says, and he's hoping to be able to start an "honest discussion" about it.

Trudeau is opposed to the Northern Gateway pipeline. The proposed route through British Columbia is too environmentally significant and vulnerable, he thinks, whether it's the Great Bear Rainforest, or the Hecate Strait off Haida Gwaii, and he objects to "the complete lack of work with First Nations people."

He is championing what he calls the middle class because "the promise underlying this country is that you can work hard and you can succeed. And there's a lot of people who are working very, very hard and they're finding it more difficult to succeed than they ever did before."

He is against the kind of electoral co-operation that would see "progressive" candidates stand down in certain ridings to prevent vote-splitting, as Green Party Leader Elizabeth May and Liberal leadership rival Joyce Murray are advocating. But, he said, "I will more than gladly send Liberal Party memberships to any Green Party member who wants to join us."

Tries to meet 1,000 people a day

When he’s campaigning, Trudeau said he meets as many people as possible, maybe up to 1,000 a day. Before the cutoff for leadership voting registration, he says he would go to six or seven events a day, and draw in about 300 people at three or four of those events. "I'd speak to them, I'd shake hands with as many as I can ... I sometimes get to stay until everybody's left the room."

Most of them feel as if they already know him, and can recite the pivotal events of his life: his Christmas Day birth in 1971, his parents' turbulent celebrity-drenched marriage followed by a bitter divorce, his eulogy at his father's funeral watched by thousands and his brother Michel's tragic early death.

"If I'm in a room full of seniors, five of them will say, oh my god, you're the spitting image of your father, and another five will say you're the spitting image of your mother," Trudeau said.

Younger people, beyond his Twitter and Facebook postings, may have heard of his party trick of falling down flights of stairs, his variations of hairstyles and facial hair arrangements, his athleticism and the fact that he once taught snowboarding.

Often nostalgic about his father, he brightens when asked about a picture of his dad performing an advanced yoga pose called the Peacock, balancing his body horizontally, supported by just his wrists.

He names the place the photo was taken in 1970, and the photographer. And, can he do it? Yes, but, "Not on the floor, but I do it on railings and on the edge of tables, and stuff. I used to do it an awful lot when I was younger. Now that I've hit 40, I'm not quite as able to do it as I used to be."

He acknowledges he hasn't led a boring life, and accepts that he is an unknown quantity in many ways, even to himself. "If I wasn't going to learn or evolve through this process, what would be the point of having someone young running for leader?"

He added, "I think the Conservatives really have a challenge on their hands, not because I'm a known factor, because there's still an awful lot to learn about me, because I'm very much a real person, and I'm not trying to pretend to be something I'm not."