Vigorous exchange of ideas is an essential part of the business of science. What makes that interaction so much more interesting than ordinary conversation is that, in science, ideas always have to be based on documented observation of the real world.No one disputes that requirement, yet it does not automatically ensure agreement, even when all parties are observing the same bits of the same world. That is because what we see in front of our eyes is powerfully influenced by what is behind them. Observations are never free of assumptions, which in turn are never independent of previous knowledge and experience.