Twenty years ago popular mass movements brought down the Stalinist regimes in Eastern Europe. Such movements are the crucial vehicles of social transformation and the sole means through which capitalism can be surpassed.
There is a key reason for their importance. Only through their actual experiences within mass movements can great numbers of working people develop the potential to challenge existing society.
Through the processes of mass movements, people can radically change and develop their personal and social capacities, their social relationships, their visions and understandings of the present and the future. This quality - of self-transformation through popular struggle - gives mass movements their central importance in socialist thinking.
Mass movements challenge "normality". In capitalist society "normality" means a few people run society, while the majority of us have little say in what happens. Mass movements have the potential to upset this balance, empowering the powerless and cutting the powerful down to size.
Popular mass movements can begin with quite small local acts of resistance. Most small-scale struggles are "contained", but a few - always unpredictably - prove to be sparks which, suitably fanned, can set a whole society alight.
Significant movements are not homogeneous. They involve people who are already "organised", but also those who were previously "unorganised". Mass movements activate the imaginations of many who previously saw no opportunity for such collective action.
When popular resistance takes the form of relatively isolated struggles, the scale and nature of the demands people feel able to put forward tend to be limited. However, a change in the scale of a movement, a shared sense that all of a sudden great majorities are swinging into action, can also expand people's horizons very fast. What previously seemed impossible can now appear within reach.
A mass movement also changes participants' views of themselves and political life. It becomes possible not only to demand big changes in government but also to challenge local tyrannies that previously seemed inevitable: bullying managers, low wages, sexist or racist ill treatment.
A movement that only focuses on large political questions is weaker than one that also nourishes a multitude of local acts of resistance and demands.
In the 1960s the US civil rights movement spawned campus movements among students, the rebirth of the women's movement, a huge anti-war movement, new-found militancy on the shop floor, movements among Native Americans and demands for gay and lesbian liberation. Each nourished the others.
Writing about mass strikes in Russia a century ago, Rosa Luxemburg noted that the "most precious thing" was the changed culture, or psychology, of the millions of people involved.
The formerly silent can look their rulers in the eye and utter truths they previously dared not speak. Such developments are most likely to occur in movements that provide the widest possible opportunities for debate and decision-making.
One criterion for judging movements is always vital. Are new organisational forms emerging, with new kinds of democratic control from below? One thing that seriously limited the 1989 movements across Eastern Europe was that, while they mobilised widely, there was no real development of new democratic institutions.
By comparison with the movement in Poland in 1980, where delegate-based inter-factory strike committees were central to the struggle, 1989 involved a falling back. The challenge to managerial power was immensely less.
These issues matter. The more movements encroach on the day to day management of everyday life, and the more workers get a taste of managing society themselves in new democratic ways, the more powerful the movement and the closer it comes to the vision of a socialist revolution.
Mass movements do not always realise their potential. Whether they succeed will depend on the arguments within them. Movements are, after all, arenas of argument and debate. Socialists need to learn the lessons of the past for the movements of the future.