Duhem's idea was more subtle. He thought that for any given set of observations there is a huge and uncountable number of explanations. According to Duhem, an experiment in physics is not just an observation, but an interpretation of observations by means of a theoretical framework. Furthermore, no matter how well one constructs one's experiment, it is impossible to subject an isolated single hypothesis to an experimental test. Instead, it is a whole interlocking group of hypotheses, background assumptions, and theories that is tested. This thesis has come to be known as holism. According to Duhem, it makes crucial experiments impossible.