Above all, there is analogy. That the Law of Demand is true for purchases of ice cream and movies, which no one would want to deny, makes it more persuasive also for gasoline. Analogy gives the law its majesty. If the law applied only to trivial items for which it has been 'proven' in modernist style, no one would care. That laboratory rats view cherry soda as a luxury good, though interesting, is not much of a basis for a human science.
Analogy is essential for science, but is of course the quintessential literary device.
These are all good reasons for believing the Law of Demand, but only the first three, I repeat, are Scientific by the dichotomous definition of English modernism. The other eight are artistic and literary.