The Wealth of Nations by Part 3 Chapter 1 Page 9

exchange their rude for manufactured produce. It is this commerce which supplies the inhabitants of the town both with the materials of their work, and the means of their subsistence. The quantity of the finished work which they sell to the inhabitants of the country necessarily regulates the quantity of the materials and provisions which they buy. Neither their employment nor subsistence, therefore, can augment but in proportion to the augmentation of the demand from the country for finished work; and this demand can augment only in proportion to the extension of improvement and cultivation. Had human institutions, therefore, never disturbed the natural course of things, the progressive wealth and increase of the towns would, in every political society, be consequential, and in proportion to the improvement and cultivation of the territory or country.