Thursday saw a high quality seminar take place, jointly hosted by Demos's Open Left project and Soundings. Its purported aim was to bring together thinkers from across the spectrum of the centre left, with John Cruddas and David Milliband turning up to represent this politically.

Defeat, and economic crisis, certainly broaden the scope of such seminars and the intelligence of the discussion. It becomes possible for someone like David Milliband to talk about capitalism as something which varies, not least in its injustices, rather than as some autonomous global cash-cow that won't ever be questioned. Meanwhile, those beyond the reaches of Blair's Big Tent become empowered to talk in less aggrieved terms, now that they feel listened to.

What continues to frustrate me on such occasions is the difficulty posed by that old Humean dilemma, how to move from 'is' to 'ought'. There are various ways that those on the left currently tend to combine empirical and normative claims.

1. Moral tautology: People accuse positivists of speaking in tautologies, but surely moralists do as well. Perhaps an analytical philosopher can help me out here, but isn't the sentence "we ought to pursue a more just society" actually empty of any meaning? Don't the words 'ought' and 'just' cancel each other out? This is a charge that is levelled at Kantians and Rawlsians, when they argue that the just action or society is the one that the reasonable and just person would select. The question is how moral categories can achieve some bite upon the world.

2. Appeal to 'our history': This is never to be under-estimated, but lets not forget that Tony Blair was appealing to 'Labour values' when he distanced himself from Labour policies. Weberian 'traditional authority' ('you should obey me because that's how we do things') is important, but still doesn't get you much closer to working out priorities and policies than the moral tautology. The New Labour neglect of its 'core vote' won't be reversed, simply by quoting Nye Bevan; it will require working out who that core vote is, who they consider themselves to be, and how they differ and resemble previous 'core votes' from the past.

3. Utilitarianism: Values often get smuggled into technical economic arguments. At the moment, this is taking place in the refreshingly grand, politically charged sphere of fiscal argument. The right and the left argue about whether the deficit really is too high, whether the bond markets really do have the power the media claims, whether deficit reduction really will impede the chances of growth. Clearly these are latently normative perspectives in conflict, but they purport to be technocratic. And then we're on the slippery, de-politicising slope towards 'what matters is what works'.

4. Ideology: If there's one thing ideologies are good for, it's moving from 'is' to 'ought'. But they leave me feeling a bit how Adorno felt towards religious believers - just because it would be nice and helpful if God did exist, is no reason to ignore the awkward existential possibility of nihilism. As Marx himself said "I am not a Marxist".

The reason ideology stifles is that it takes the other three - moralism, history and technocracy - and combines them neatly into a package, which it would otherwise be the job of politics to produce. What is missing, I suggested at the seminar, is a different account of history, as provided by sociology.

Anthony Giddens took a lot of flack for The Third Way, but that was because (in Hume's terms again) everyone, perhaps even Giddens himself, mistook an 'is' for an 'ought'. To draw the Labour Party's attention to globalisation, risk cultures, post-industrial production and individualism never had to be a call for even more globalisation, even more auditting, an even bigger retail sector, or even more individualism. He wasn't even a Lord back then!

But it did supply something that (1) moral tautologies, (2) reading Tawney, (3) designing tax credits and (4) reading Marx couldn't do on their own, namely re-ground a political movement in the present and for the future. Marx wrote that "men make their own history, just not under circumstances of their own choosing". Giddens played an important role in reminding people of the second half of that sentence; unfortunately he also somehow persuaded people to forget the first half, although tragically not when it came to foreign policy.

What is there between a moral instinct and a technocratic problem? How's this for an answer: a publicly expressed and measurable sociological development that demands a collective response.

This means not defining a political movement in terms of this year's policy agenda (welfare, immigration), nor in terms of new clever policy innovations (tax credits), nor in terms of media myths (crime). Instead, social history - or rather, capitalist society - will throw up ever new collective threats that capitalism itself will struggle to solve. In Marxist terms, it produces contradictions that the state's task is to alleviate; in neo-classical terms, each epoch has its own defining 'negative externalities'.

19th century 'Manchester liberalism' produced the extreme negative externalities of degrading factory conditions, corrosion of family life and disease. This was publicly expressed, measurable and it demanded a collective response, which it received in the birth of the labour movement. 20th century Fordism collectivised and rationalised the economy, but left individual risks - unemployment, ill-health, old age - as negative externalities. With the birth of national statistics and national media, these had become publicly expressed, measurable and they demanded a collective response, which they received with the post-45 political settlement.

So the question is, what might be the equivalents today, that are not only technical problems in need of a technocratic solution, but defining short-comings of contemporary capitalism? Here are four, and note that not one of them would have shaped Tony Blair's thinking fifteen years ago:

Mental ill-health and loneliness: I've been looking at some policy documents on mental health in the UK, and the figures on its reported rise are quite startling. Admittedly some stress might need to be on 'reported'. But in some respects, depression and anxiety are the supreme negative externalities of consumer-oriented capitalism, generating capitalism's latest contradiction - that we can drag ourselves outside to shop, but not to produce or co-operate with one another. Incidentally, recognising this is not in itself a call for more policies to promote, teach or maximise happiness.

Energy scarcity: We recognise that climate change will be the definining policy challenge of the next century. Maybe it will eventually lead us towards a more thoroughly planned economy. But in the medium term, energy prices will rise sharply and Britain will need to produce more energy from renewable (or else face even higher prices). Current financial structures or corporate structures weren't designed to make this happen. Energy poverty will get worse. Yet there are potential co-operative solutions, and no doubt plenty of as-yet-unthought-of ones.

Zero growth: This isn't really a negative externality, although it is a collective action problem which markets won't solve. What if Britain has no growth for the next decade? What if Britain can't pursue its environmental goals at 3% growth a year? The knee-jerk Keynesianism of some on the left amazes me. Historical sensibility, and the possibility of not being in power for another decade, would require a broader view that did not cling to the policy tools and goals of the latter half of the twentieth century. Of course national accounts will continue into the future; but they will not be so prominent for ever. This is a major challenge for the centre left, that could cripple it.

Housing: Our present financial structures, and culture of ownership, strategically under-deliver housing. This is hugely socially divisive.

It's crucial not to view any of these as policy challenges that the state can simply solve, not even if it had all the money it needed, especially the first. But, strange as it may sound, seemingly intractable collective needs and problems may be more effective bases for political mobilisation and identity than technical faults that require an engineer to come and fix. Experts will play a role in advising on each of the above, but they cannot promise results. This too is beneficial for political mobilisation. I can identify with CND in its endlessly frustrated ambition to erradicate nuclear weapons; I can't identify with a government department, perhaps least of all the most effective ones.

Far be it from me to tell the Labour Party what it is about. Maybe none of the above four is relevant or concerning to the left. What I think the emphasis on "publicly expressed and measurable sociological developments that demand a collective response" can do is to break people out of the previous four traps, in which we swing wildly between moralistic slogans and technical intricacies. Max Weber argued that to have a vocation for politics was to have a "matter of fact devotion to a cause". Take out either the facts or the devotion to a cause, and you end up with something politically inert.