The analogy speaks to the number of paths that need be considered so that, later, we can find the overall shortest path (there are likely other intermediate pla

The analogy speaks to the number of paths that need be considered so that, later, we can find the overall shortest path (there are likely other intermediate places that we're also evaluating, along with Toronto). Once we know the shortest path from Waterloo to Toronto, that's the only one we need to consider (on that segment) from then on.