The Wealth of Nations by Part 4 Chapter 9 Page 21

The unproductive class, however, is not only useful, but greatly useful to the other two classes. By means of the industry of merchants, artificers, and manufacturers, the proprietors and cultivators can purchase both the foreign goods and the manufactured produce of their own country which they have occasion for with the produce of a much smaller quantity of their own labour than what they would be obliged to employ if they were to attempt, in an awkward and unskilful manner, either to import the one or to make the other for their own use.

By means of the unproductive class, the cultivators are delivered from many cares which would otherwise distract their attention from the cultivation of land. The superiority of produce, which, in consequence of this undivided attention, they are enabled to raise, is fully sufficient