This letter was written two years before "The Wealth of Nations", in 1774.
My favorite paragraph:
"A degree can pretend to give security for nothing but the science
of the graduate; and even for that it can give but a very slender
security. For his good sense and discretion, qualities not discoverable
by an academical examination, it can give no security at all. But
without these, the presumption which commonly attends science must
render it, in the practice of physic, ten times more dangerous than the
grossest ignorance, when accompanied, as it sometime is, with some
degree of modesty and diffidence."