 Since it is difficult and often impossible to carry out experiments and since the uncontrolled phenomena economists observe are difficult to interpret (owing to the same causal complexity that bothered Mill), it is hard to judge whether a particular theory is a good basis for predictions or not. Tendencies are not universal laws. A claim such as “firms attempt to maximize profits” will be “unrealistic” in the sense that it is not a true universal generalization. Although not in these terms, Friedman objects to criticisms of tendencies that in effect complain that they are merely tendencies, rather than universal laws. If his criticism stopped there, it would be sensible, although it would avoid the problems of understanding and appraising claims about tendencies.