Predicting the future is as impossible as remembering the past. When asked what he feared most, former British Prime Minister Harold MacMillan said "events, dear boy, events". Or maybe he didn't. The internet can't even agree on what former politicians actually said, yet current ones insist on pretending that they can predict the future.

To be crystal clear, no economist in Treasury or Standard and Poor's has any idea what the price of iron ore will be in 3 years' time. Nor do they know what the growth rate, inflation rate or unemployment rate will be. The "projections" in the budget papers are not even "forecasts" of what Treasury think will happen, they are simply an estimate of what would happen to key macroeconomic variables if the economy moved back towards its long run trends.

Did you get that? When the budget papers "project" that the unemployment rate will fall from 5.75 per cent to 5.5 per cent between now and 2019, they aren't predicting that outcome or even saying it is likely. They are simply saying that IF the economy were to move rapidly back towards its trend performance then the unemployment rate would fall. That's a very big IF.

Wayne Swan famously got into trouble when he conflated Treasury projections with Treasury's possession of a crystal ball. The Labor Treasurer who managed the Australian economy so well during the GFC that we avoided a recession came unstuck on his promise that the economy would be back in surplus by 2012-13 "come hell or high water". Not surprisingly, Treasury failed to predict "high water" in the form of the massive Queensland floods that hit in 2010-11, which both cost the Federal Government billions in emergency assistance and knocked economic growth in Queensland for six.

The budget papers provide pretty good information about what happened to government spending and revenue collection in the previous year. They also do a pretty good job of explaining how changes in government policy announced in the budget will effect spending and revenue in the coming twelve months. But after that it gets pretty dodgy pretty fast.

Peter Costello introduced the "forward estimates" to the Commonwealth budget in 1996-97. The "forwards", as hardcore "econobabblers" like to call them, provide a guess about likely revenue and expenditures, to the decimal place, for all major areas of government activity in the three years beyond the upcoming budgeted year. The decimal places play an important role as most people confuse precision with accuracy. You shouldn't.

When combined with the budget for the coming year, the three years of forward estimates provide a four-year horizon over which government policy can be evaluated and dissected. Not surprisingly, Peter Costello's 2004 budget made no mention of the upcoming GFC, nor did his budgets of 2005, 2006 or 2007. And because the Treasurer saw no clouds on the horizon he had no problem convincing himself, or the electorate, that his massive and permanent tax cuts were "affordable".

Despite Costello's stated interest in long-term planning, including his wildly inaccurate intergenerational reports, Costello's budgets also failed to predict the end of the mining boom, the collapse of the price of iron ore or the impact of the rise of ISIS on national defence spending.

But even though "events" have made a mockery of Costello, Swan and Hockey's forward estimates, Scott Morison and Malcolm Turnbull insist not only on taking them seriously, but asking the public to pay attention to their "10-year plan". Indeed, Malcolm Turnbull's inability to detail the cost of his long-term plan to cut the company tax rate is now dominating analysis of both the economics and politics of his first budget.

Of course the simplest thing for the Prime Minister to say would be that he has no more idea how much cutting the company tax rate will cost future budgets than Peter Costello had about the future cost of his tax cuts. But such honesty would both trash the Liberal's claim to be "great economic managers" and blunt their petty attack on the accuracy of ALP costings of increasing tobacco excise.

So instead of making the simple point that the Liberal Party simply believes that cutting the company tax rate is the right thing to do, Malcolm Turnbull is instead going to try and rely on Treasury modelling to "prove" that his ideological preference for cutting taxes will "trickle down" into benefits for all. It's a risky strategy.

Conservative politicians around the world love to argue that cutting taxes for business owners and high income earners is a great way to help non-business owners and non-high income earners. The problem they face is the lack of evidence to support their argument. But, as Peter Costello showed, a lack of evidence need not lead to a lack of success. The trick is to get the timing right.

Costello cut taxes at the peak of the mining boom. The economy was doing well, wages were rising as unemployment was falling. While economists know that mining booms don't last, politicians like to pretend the opposite. Poor Colin Barnett is still feigning surprise that a "once in a century" boom didn't last for a century. But Peter Costello made it out before the boom turned to bust which, of course, allowed the Coalition to pretend the bust was all Labor's fault.

Morrison and Turnbull, on the other hand, are trying to sell big tax cuts at a time when unemployment is rising, wages are flat and, most significantly, at the same time they are trying to sell $80 billion worth of cuts to health and education.

Of course, the Coalition are arguing that by the time the company tax cuts kick in the economy will be booming and the budget will be back in the black. The problem with that argument is that they said that two years ago. Indeed, the reason they called the 2 per cent increase in the top tax rate a "temporary deficit repair levy" was that they thought the deficit would be temporary. It's not.

Managing the economy is a lot harder than plugging assumptions into an economic model. Indeed, can you remember Paul Keating even talking about economic modelling?

The first step towards good economic management is for our treasurers to admit, to themselves and to the public, that the world economy has a much bigger impact on the Commonwealth budget than the Commonwealth budget has on the Australian economy. Peter Costello loved taking credit for the Chinese boom and Tony Abbott loved blaming the ALP for the GFC but, in reality, our treasurers were bit players in both.

But what our treasurers can help us do is to ride the waves that the world sends our way. Wayne Swan did a great job surfing down the face of a tsunami, but he lost out by telling us more about his next trick than he did about his first one.

Australia has 723,100 unemployed people, a health and education system that is stretched to breaking point and a public transport system that is a joke by world standards. You don't need economic models or forward estimates to understand that the kids in kindergarten today only get one chance to learn how to read and that the frail folk in our aged care homes only get one chance to live out their last years in comfort and company.

Managing the economy for future generations is an important goal, but delivering essential services for those who need them today is more important still. For government who do so little to prevent climate change to feign interest in the future is obscene.

Richard Denniss is the chief economist for the Australia Institute.

Twitter: @RDNS_TAI