John McDonnell's speech was hardly the slickest the Labour party conference has ever seen and at some points he even looked a little overwhelmed by the occasion. There will be others who will see in Mr McDonnell a man who wants to re-start the class wars of the 1970s, principally by increasing taxes. But whether you see him as ineffectual or frightening, or both, as Shadow Chancellor he should be judged on what his plan is for the economy – on what he says rather than how he says it.

The fundamentals of the plan are radical – he is the most radical shadow chancellor in decades - but there was also a promise from Mr McDonnell that the party would take a workable approach; he also used the familiar strategy of announcing a review to avoid or postpone the inevitable conflict in his party. The fundamental idea at the heart of his plan is that the deficit can be cut without also cutting spending – in other words, the new Labour team are not deficit deniers, they just deny that spending cuts are the only way to tackle the deficit.

So how will it be done? How will Mr McDonnell, as chancellor, cut the deficit if not through spending cuts? In his speech, he singled out tax as one of his main weapons – he would tackle tax avoidance, he said, and ensure big companies pay their fair share; and he indicated that the door is open to a so-called Robin Hood tax - in other words a levy on financial transactions. Mr McDonnell also promised to spend his way out towards surplus – the economic recovery, he said, was based on insecure, unsustainable foundations and there would be increased investment rather than further spending cuts.

Mr McDonnell says all of this is possible - and it is true that the UK Government is not doing enough to control tax avoidance by the wealthiest individuals and corporations. Fairer tax is also a critical part of reducing the gap between the rich and the poor. But the limits to what can be achieved through tax avoidance measures should be acknowledged. As every loophole is closed, another will be opened and Mr McDonnell will also have to make it clear how precisely he intends to crack down on tax avoidance effectively and how much he believes it would raise. And however effectively he does it, reducing tax avoidance alone will never be enough to tackle the deficit.

Mr McDonnell's other big idea, increasing Government investment as a route to prosperity, has more potential. The man Mr McDonnell is seeking to replace, George Osbourne, can boast of some growth and some jobs from austerity, but with the appalling human cost so obvious, there is room for Labour to propose an alternative of increasing public spending as part of continuing programme to reduce and pay off the deficit. That is the trick Mr McDonnell will have to pull off in the coming years: suggesting alternative anti-austerity economics that do not threaten the fitful recovery. He must convince us that he wants to reach a sensible destination – paying off the deficit – but also convince us that there is an alternative, more radical way to reach it.