Seymour Hersh, the grizzly veteran American journalist, is talking "tradecraft". I ask about the Irish Times.

"Your first obligation is to protect the source, and you destroy the document to protect the source."

And if the Supreme Court orders you to reveal your source?

"You go to jail."

Surely refusing to reveal your source is flaunting the constitutional system you're supposedly defending?

"You have to go to jail. You would flaunt the system, I think, if you fled, if you left the country. You pay the price for defiance."

His view on his vocation is simple.

"It's basically the government's job to keep [classified information] secret and my job to find it out.

"I always hated that word, 'investigative' journalist. You're either a stenographer or you're a journalist. If you're a journalist, presumably you're pursuing.

"In America, 75% of the stories that are so-called 'investigative' are really just tips in which they write about an issue and they don't take the time to explore. If the president's accused of not telling the truth about something, a story about people who are attacking the president seems to be fine for the newspapers. But I always think your job is to go and find out what the truth is and resolve the issue."

Hersh talks in rapid, rambling monologues. He sounds (and looks a little) like Woody Allen, and tells a story like Billy Connolly: there are tangents off tangents off tangents, but it eventually comes back to the original anecdote.

He is utterly unpretentious; interested in his company and, judging by the hole in his chinos and his lowflying zip, uninterested in his appearance. Hersh broke the My Lai massacre story in 1969, won the Pulitzer prize and then broke stories for the next 30 plus years, including Abu Ghraib. But I want to know how he started, and what he learned.

"I flunked out of law school and pranced around and worked in a bar. I'd done all of those things." Then he found a job with a Chicago police reporting news service. It was the early '60s and he was in his early 20s.

"I covered central police and on boring nightsf we'd smoke the dope they'd confiscated, with some cops, and we'd watch the dirty movies they'd confiscated, with some cops.

"You got a sense of what really goes on. You got a sense that police lie, almost as people breathe, that's just the way it is. They protect each other."

One night, he heard a radio report that two "white Irish cops" had shot a suspect. "Instead of waiting for them to come into the police station headquarters and talk to us, I went down to the parking lot, because I was already sort of edgy. I wanted to be the first to talk to the guys as they drove in. I got there just as they got out of their car, and nobody knew I was there. Another cop asked them what happened. The first cop said, 'Well, I told him to beat it and I shot him in the back.' I heard that.

At least, I thought that's what I heard.

"I did nothing about it. I talked to my editors. They said, 'Are you kidding?

Are you kidding? You think you heard something?'

"I was pretty sure what I heard, I wasn't 100% positive. Even so, I would have had every cop in the city saying, 'come on'. I would have got nowhere with it.

"Any time anybody talks about courage, I always think about, there's nothing courageous in not doing a story."

And what has he learned?

"I've been a reporter for 45 years and from police reporting and writing about murders and crimes, to Congress and to the White House and to what I do now, I've never met anyone in 45 years that thought he was doing something wrong. Not one.

"Very good people end up doing very bad things."