Very insightful:

This division of labour, from which so many advantages are derived, is not
originally the effect of any human wisdom, which foresees and intends that
general opulence to which it gives occasion. It is the necessary, though
very slow and gradual, consequence of a certain propensity in human nature,
which has in view no such extensive utility ; the propensity to truck,
barter, and exchange one thing for another.

http://www.online-literature.com/adam_smith/wealth_nations/2/