The covering-law model of explanation was a centerpiece of the logical empiricist conception of science. According to that model, explanations are arguments showing that the event to be explained (the explanandum event) was to have been expected on the basis of laws of nature and the antecedent and boundary conditions (the explanans). For advocates of the covering-law model, the philosophical problem of explanation is thus largely a matter of analyzing the formal structure of explanatory arguments (Hempel and Oppenheim 1948; Hempel 1965). A rainbow, for example, is explained under the covering-law model by reference to laws of reflection and refraction alongside conditions concerning the position of the sun and the nature of light, the position of the raindrops, and the position of the person seeing the rainbow. The description of the rainbow is the conclusion of a deductive argument with law statements and descriptions of conditions as premises, and so the rainbow was to be expected in light of knowledge of the laws and conditions.