A third effect of tariff-monopoly is to prevent the sale abroad of domestic goods to the same extent and amount as foreign wares are kept out by these monopoly-taxes. This vital and fundamental result is almost always overlooked. If a man or a nation refuse to _buy_ of a proffered customer, they cannot by any possibility _sell_ to him; because buying and selling are reciprocal and synchronous; because it takes two to make a bargain; because material commodities, for the most part, ultimately, exchange against each other; and because the only motive a foreigner ever has to bring his goods _hither_, is to take in exchange for them our domestic goods at a profit, and carry these _hence_. To forbid entrance to foreign goods is to forbid exit to domestic goods. Monopoly-tariff-taxes, therefore, so far forth, destroy the market for home products, without creating or tending to create, any other market for them. Such taxes, accordingly, cause a dead loss all around,--to the foreign producer who wants to buy our products with his own, to the home producer who wants to sell his own products against those, and even to the government also as a tax-collector, which can get no revenue on foreign goods excluded by monopoly-taxes. There is a final and deeper point of view, from which all such monopolies are wholly condemnable. _They lessen of necessity,--from their own nature and inexorable operation_,--THE DIVERSITY OF RELATIVE ADVANTAGE AS BETWEEN EXCHANGERS, on which diversity, as we have now seen, the whole fact and gain of exchanges depend. Taxes on raw materials, for example, whether actually paid on them or used to enhance the price of other corresponding materials as in the tariff-taxes, increase the costs of all products into which such taxed materials enter, and so restrict the market of the home-producer by lessening his relative advantage as compared with the relative advantage of the foreigner over him. He cannot sell so well, perhaps cannot sell at all, his cost-enhanced products. Monopoly-taxes on industrial processes of any kind, on the means of transportation, have similar effects on the cost of products; and of course, similar effects in lessening Diversity, in restricting markets, and in destroying the life of Trade. Before quitting this subject, it may be well for us briefly to classify Monopolies.