The Prime Minister's proposals on the future of the benefits system are certainly eye-catching. The heart of the plan is to strip housing benefits from those under 25, forcing them either to find work or to remain living with their parents. It is easy to see why David Cameron likes the idea: the move would not only save the Exchequer an annual £1.8bn, it is also a sure-fire winner with traditional Tories.

Taken in isolation, Mr Cameron's basic premise is a sound one. The benefits system should indeed encourage work, rather than stack the incentives against it. And the current arrangements are both expensive and badly designed, too often entrenching social divisions rather than relieving them. But welfare reforms do not exist in isolation, and the Prime Minister's latest batch of ideas are rather more about politics than reality.

In practical terms, the most immediate obstacle is the economy. Although the latest unemployment numbers show a slight dip, more than a million young people are still unable to find work. Mr Cameron's proposals would only be viable at a time of sufficient economic growth to offer realistic opportunities of employment. That time is not now, nor is it fast approaching.

More importantly, it is simply not politically – or, arguably, morally – reasonable to squeeze the benefits of those at the lower end of the social scale while still handing out support to pensioners at the top. While the £5bn price tag associated with universal benefits such as free bus passes for the elderly is but a fraction of the overall pension bill, it dwarfs the money to be raised from tinkering with housing benefits for the young. That is the place to start.

The Prime Minister himself admits his ideas are, at best, an indication of Tory policy beyond the next general election. Of particular interest, then, is his timing. Mr Cameron's remarks, offering red meat to the party faithful, come just days before the publication of Liberal Democrat-backed constitutional reforms that he has pledged to support but are anathema to many Conservatives. It is in this context that the Prime Minister's speech today is best understood.