Water purification is relevant to the policy debate
for two reasons. Water purification is a very excellent policy and it is about to become a very clumsy metaphor.

Just below I ask why the policy debated does not include proposals to make the tax system more progressive. Upon reflection, I honestly think that this is because it would be costly to all opinion leaders.
Now I don't imagine that people are so selfish that they consider only their personal interest when deciding which policy to support. However, I suspect that in a discussion among people who all have the same personal interest that interest powerfully affects the conclusion. If an opinion leader proposes increasing the top marginal tax rate to another opinion leader, the second opinion leader will wince, perhaps supporting the proposal or perhaps opposing it but knowing that it would cost him or her more than a bright new shiny car. This bias against progressive taxes is weak in each singly conversation but it always presses in the same direction.

This makes me think of Cesium ions disolved in water in an ultracentrifuge. Maybe I could remember back to junior high when we visited the Great Falls water purification plant. This was amazing. It took filthy water from the Potomac and you could see the filth falling out of the water which went in brown and came out blue.

How did they do it ? It is simple just gravity pulled the filth to the bottom. But why didn't that work in the Potomac (then still an open sewer) ? Well gravity is very weak compared to the buffeting of water molecules so sedimentation is very very slow. They added a gell which stuck the bits of filth together so that the pull of gravity (same for all) beat the Brownian buffeting (different for each). I think discussion is like this. It sticks opinions together so that the pull of economic interest (same for all opinion leaders) beats temprement, reason and original thought (different for all opinion leaders).

The only problem is that instead of drinking the pure water which doesn't stick together, we eat the filth at the bottom of the tank.