A district office argues that putting up a board at the harbour gate showing the day's fish prices will get fishermen a better price for their catch, since they will know what buyers are paying before they agree to a sale. On which of the following assumptions does the argument depend — that is, which one, if it turned out to be false, would leave the argument with nothing to stand on?

Knowledge helps a seller only if he can act on it by taking his catch elsewhere. Deny that freedom and the board becomes information he can do nothing with, and the argument falls apart — the mark of a genuine assumption. The board need not cover every kind of fish to help with some; the buyers' feelings are irrelevant; and the argument is about improving on what buyers offer today, not about beating last year's prices.