"In the work of the active scientist there are not merely occasions for using metaphor, but necessities for doing so, as when trying to remove an unbearable gap or monstrous fault. I now turn... to... the necessity built into the process of scientific rationality itself, an epistemological necessity... It is simply the limitation of induction. Where logic fails, analogic continues. The bridge is now made no longer of steel but of gossamer. It breaks often, but sometimes it carries us across the gulf... there is nothing else that will."