In Chap. 14 I spoke at length about crucial experiments. I argued that crucial experiments are crucial only against the background of a relatively stable set of assumptions, assumptions we are not prepared to abandon lightly. Crucial experiments, therefore, are tests not only of isolated hypotheses but of an hypothesis plus all those assumptions that underlie the enunciation of that hypothesis. The Fresnel, Young, and Foucault experiments were crucial in the controversy over the nature of light only when viewed against the assumption that light must be wavelike or particulate, but at least one or the other and in no case both. So these famous optical experiments were crucial to the question of the nature of light and crucial also to underlying hypotheses, including this one. What are called “crucial experiments” are only the surface ripples manifested by deep and complex crosscurrents in scientific thinking. They certainly constitute much more than the superficially simple decisions they are sometimes represented as exhibiting.