The Wealth of Nations by Part 4 Chapter 5 Page 106

The price at which the exportation of corn is prohibited, if it is ever to be prohibited, ought always to be a very high price.

The laws concerning corn may everywhere be compared to the laws concerning religion. The people feel themselves so much interested in what relates either of their subsistence in this life, or to their happiness in a life to come, that government must yield to their prejudices, and, in order to preserve the public tranquillity, establish that system which they approve of. It is upon this account, perhaps, that we so seldom find a reasonable system established with regard to either of those two capital objects.

IV. The trade of the merchant carrier, or of the importer of foreign corn in order to export it again, contributes to the plentiful supply of the home market.