Read the following passage and answer the question that follows.
In Hume's eyes productive labour was the greatest asset of a country, and foreign trade was valuable because it enabled a nation to use more and more varied labour than would otherwise be possible. But commerce was of mutual advantage to the nations involved, not a benefit to one and injury to the other. 'The increase of riches and commerce in any one nation,' added Hume, 'instead of hurting, commonly, promotes the riches and commerce of all its neighbours.' 'The emulation in rival nations serves ... to keep industry alive in all of them.'
The importance of foreign trade, in the eyes of Hume, was due to the fact that: