Lesson four of a course on holding the model while an AI does the work. The pencil is Dijkstra’s (A note on two problems in connexion with graphs, 1959): given the map, the shortest route is a solved problem, and a good executor finds it every time. What it cannot find is what is not on the map. Wiener’s moral (1960): be sure the purpose put into the machine is the one you really want.