For over a hundred years, one of the dominant tendencies in the philosophy of science has been verificationism: that is, the doctrine that to know the meaning of a scientific proposition (or of any proposition, according to most verificationists) is to know what would be evidence for that proposition. Historically, verificationism has been closely connected with positivism: that is, at least originally, the view that all that science really does is to describe regularities in human experience. Taken together, these views seem close to idealism. However, many twentieth-century verificationists have wanted to replace the reference to experience in the older formulations of these doctrines with a reference to ‘observable things’ and ‘observable properties’. According to this more recent view, scientific statements about the color of flowers or the eating habits of bears are to be taken at face value as referring to flowers and bears; but scientific statements about such ‘unobservables’ as electrons are not to be taken as referring to electrons, but rather as referring to meter readings and the observable results of cloud chamber experiments. It is not surprising that philosophers who took this tack found themselves in a certain degree of sympathy with psychological behaviorism. Just as they wanted to ‘reduce’ statements about such unobservables as electrons to statements about ‘public observables’ such as meter readings, so they wanted to reduce statements about phenomena which, whatever their private status, were publicly unobservable, such as a person's sensations or emotions, to statements about such public observables as bodily behaviors.

At this point, they found themselves in a certain bind.