The freedom which they enjoyed did not extend to commerce and manufactures. In those matters the policy of Great Britain was founded on the "mercantile theory," then universally accepted, and was directed towards securing a monopoly of trade. Other countries pursued a like policy towards their colonies, though they treated them with far less liberality. The restrictions placed by Great Britain on colonial trade were based on the Navigation acts of 1660 and later years, which were originally aimed at the maritime power of the Dutch. Briefly, they confined trade with the colonies to English or colonial ships; the Americans were debarred from exporting a number of the most important products of their country, their tobacco, grain, sugar, hides, and timber for masts, except to Great Britain; no foreign ship might enter their harbours, nor, with certain exceptions, could they import foreign merchandise, except in ships sailing directly from England. Various acts debarred them from manufactures which would have entered into competition with English goods; they depended on the mother-country for the commonest and most necessary things, for their cloth, hardware, and a host of manufactured articles. Port duties were imposed by England, and were collected by officers of the customs, whose business it was to prevent contraband trading. These duties were not imposed for the sake of revenue, but for the regulation of trade; and the whole system of restrictions was founded on the idea that colonies should be made to serve the interest of the mother-country by giving its merchants and manufacturers the monopoly of the colonial market.