In the seventeenth century, Newton published his famous experimentum crucis, in which he claimed that light is heterogeneous and is composed of rays with different refrangibilities. Experiments, especially the crucial experiment, were important for justifying Newton's theory of light, and eventually his theory of color. A century later, Goethe conducted a series of experiments on the nature of color, especially in contradistinction to Newton, and he defended his research with a methodological principle formulated in “Der Versuch als Vermittler.” Goethe's principle included two elements: a series of experiments and resultant higher empirical evidence, which functioned as mediator between the objective (a natural phenomenon) and the subjective (a theory or hypothesis). Although the notion of experimentum crucis became popular among scientists for reconstructing experimental research and for justifying theories, especially for rhetorical purposes, Newton's justification of his theory of light and color is best reconstructed in terms of Goethe's methodological principle. Finally, Goethe's principle has important consequences for the contemporary philosophical underdetermination thesis.