A cooperative store argues that printing the weight of each packet in large figures on the front of the pack will raise its sales, because shoppers who now judge packets by price alone would see that its packets hold more. Suppose it emerged that shoppers already know the weight of every packet, because the weight is announced on the shelf label beneath it. What would this show about the store's argument?

The argument silently supposes that shoppers judge by price because the weight is unknown to them. Once the shelf label is shown to carry the weight already, that supposition is false, the printing adds no new information, and the predicted rise in sales loses its basis. The finding says nothing about how much the packets actually hold, a second statement of a known figure is not a reason to buy, and the argument's whole force depended on what shoppers know.