Mechanists, in contrast, insist explanation is a matter of elucidating the causal structures that produce, underlie, or maintain the phenomenon of interest. For mechanists, the philosophical problem is largely about characterizing or describing the worldly or ontic structures to which explanatory models (including arguments) must refer if they are to count as genuinely explanatory. A rainbow, for the mechanist, is explained by situating that phenomenon in the causal structure of the world; the explanation is an account of how the phenomenon was produced by entities (like rain drops and eyeballs) with particular properties (like shapes and refractive indices) that causally interact with light propagating from the sun. Mechanists typically distinguish several ways of situating a phenomenon within the causal structure of the world.