It's the elephant in the living room of British environmental politics: the question of whether it is possible to push through green policies at a time of economic slowdown. Commendably, David Cameron became the first senior political leader to answer this yesterday. Even better, he got the answer broadly right.
The Conservative leader told an audience at Westminster's Royal Horticultural Halls that, although the cost of living is "the number one concern for Britain's families", the plain fact is that we "can't afford not to go green". Protecting the environment is a "necessity", not a "luxury".
Those who argue that because the economy is slowing, the environment ceases to be a front-line issue, simply betray the fact that they never understood the green case. The future malign effects of climate change will make a recession today look mild in comparison. The very future inhabitability of the planet is at stake here.
There were also some welcome specifics from Mr Cameron, coming out against the expansion of Heathrow and the building of a new coal-fired plant at Kingsnorth, Kent. Furthermore, there was a pledge to reward domestic energy producers if they feed into the grid and to step up tidal-power technology research.
Mr Cameron has done British politics a favour. All three major political parties have committed themselves to taking radical action to cut carbon emissions. None risks being undercut by a populist anti-green campaign from one of their rivals.
But only one of those parties has the levers of government in its hand. Over to you, Mr Brown.