It's one of the most famous lines in movie history. "Just follow the money," said Deep Throat to Bob Woodward, or rather Robert Redford playing the legendary Washington Post reporter, in All The President's Men, the iconic dramatisation of the Watergate scandal which toppled the presidency of Richard Nixon.
In truth, Deep Throat, whom we now know was Mark Felt, the FBI's then associate director, never actually uttered those words. They were the invention of an imaginative Hollywood screenwriter.
But, even so, they captured the essential truth of most political scandals. Few improprieties are possible without money and, if you want to get to the bottom of the story, the surest way is to trace where the money comes from, where it goes to and where it ends up.
The recent rash of US diplomatic cables dealing with the IRA and Sinn Fein, released courtesy of WikiLeaks, has focussed media attention on the question of how much foreknowledge Gerry Adams and Martin McGuinness had of the £26.5 million Northern Bank robbery.
Quoting an unnamed Irish official, US ambassador James Kenny told his colleagues in Washington that the Dublin government had "rock solid" evidence the two men sat on the IRA's ruling Army Council and, therefore, would have "known in advance" that the raid was going to take place.
But for those of us who spent years studying the IRA and who know full well just how much clout Messrs Adams and McGuinness carried in the Provisionals' upper echelons, a bigger story would have been that the heist had happened without them knowing.
The cable was sent in July 2005 - some six or seven months after the robbery - by Jonathan Benton, the deputy chief of mission at the American embassy in Dublin.
The excerpt that jumps out at the reader is this one: "XXXX [an Irish official whose name has been redacted by WikiLeaks] commented that IRA money was constantly moving, flowing from diversified sources into wide-ranging investments.
"While the IRA had been proficient in smuggling, robbery and racketeering since the 1970s, the Celtic Tiger economic boom of the 1990s had prompted the IRA to diversify into more sophisticated business enterprises.
"IRA investments now included real estate ventures in Dublin, London and Spanish resort areas, handled by apparently respectable businessmen. XXXX also expressed concern about the co-mingling of ill-gotten IRA funds with Sinn Fein's political coffers in the Republic of Ireland."
In other words, the IRA took advantage of Ireland's extraordinary property bubble to make an awful lot of money and used that cash, so the Irish government believed, to help finance Sinn Fein's political growth.
Just how much money the IRA has, or had, in that investment portfolio is not disclosed in the WikiLeaks cable, but the last time I spoke to Irish officials about this, in 2007, they estimated the fund probably amounted to some €300m (£255m) - although they conceded this might be a gross understatement of the true figure.
Now this not only explains how and why some well-known Provisional activists in Northern Ireland suddenly acquired smart villas on the Costa del Sol, cottages in Donegal or large homes and businesses in Belfast but, given the scale of the IRA investment, it raises the real possibility that not only was the IRA a beneficiary of the Irish property bubble, but was also a contributor; in other words, the IRA helped to create the bubble by pumping it up.
This is what seems to come out of all this: Sinn Fein, according to WikiLeaks, benefited electorally from a property bubble which its partners in the IRA helped to create and from which it gained financially, and will now take to the hustings in the Republic next spring on a platform of indignant, principled opposition to the spending cuts brought about by the very same bubble.
That will strike a great many people as a piece of breathtaking hypocrisy, but it's unlikely the Provos will lose any sleep over it. There seems a very good chance that the number of Sinn Fein seats in the Dail will grow in the coming general election and there are even predictions that it will join Irish Labour to form the next coalition government.
That just leaves one loose thread. What has happened to the IRA's investment portfolio? Who owns and controls it now, given the powers that be continually assure us the IRA has gone out of business?
Well, if the IRA had truly folded up its tents, that portfolio would have been passed on to, and distributed among, a number of individuals and, if that had really happened, we would know all about it by now.
The internal squabbling over who got what and who didn't get what would have been so intense and violent that bodies would have started appearing in border ditches. There's no way the Provisional leadership would have allowed that to happen, so the odds are that the portfolio is still controlled by the same people who always signed the cheques.
We are still talking about an awful lot of money. Let's assume the €300m estimate of 2007 is correct. Given that Irish property prices have fallen by a third since then, and those in Spain and London by a good deal less, the portfolio is now worth at least €200m (£170m). That's more than enough to finance Sinn Fein's electoral rebirth in the spring.
The question that flows from the WikiLeaks cable is this: whoever said that crime doesn't pay?