Locke had the penetration to see that the laws existing in his time against the exportation of gold and silver coin must necessarily be futile, and, while it was permitted to export bullion, could answer no conceivable purpose. These laws, which date from the time of Edward the Third, were, curiously enough, not repealed till the year 1819, though as early as the time of the Restoration they had been pronounced by so competent a judge as Sir William Petty to be "nugatory" and "impracticable." Nothing, as Locke says towards the conclusion of his answer to Lowndes, could prevent the exportation of silver and gold in payment of debts contracted beyond the seas, and it could "be no odds to England whether it was carried out in specie or when melted down into bullion." But the principle on which the prohibition of exporting gold and silver coin ultimately rested seems to have been accepted by him as unhesitatingly as it was by almost all the other economists of the time. That principle was that the wealth of a nation is to be measured by the amount of gold and silver in its possession, this amount depending on the ratio of the value of the exports to that of the imports. When the value of the exports exceeded that of the imports, the Balance of Trade, as it was called, was said to be in favour of a country; when, on the other hand, the value of the imports exceeded that of the exports, the Balance of Trade was said to be against it. A favourable balance, it was assumed, must necessarily increase the amount of gold and silver in the country, while an unfavourable balance must necessarily diminish it. And, lastly, the amount of gold and silver in its possession was the measure of a nation's wealth. These views form part of what political economists call the Mercantile Theory, which it was the peculiar glory of Adam Smith to demolish.