[from a Burns poem apparently criticizing Alexander Pope's ideas (the proper study of mankind is man)]
....What pity, in rearing so beauteous a system,
One trifling particular, truth, should have miss'd
For, spite of his fine theoretic positions,
Mankind is a science defies definitions.
In the make of that wonderful creature, call'd Man,
No two virtues, whatever relation they claim,
Nor even two different shades of the same,
Though like as was ever twin brother to brother,
Possessing the one shall imply you've the other.
Burns here is critiquing Alexander Pope (author of "The Proper Study of Mankind is Man")
I am not sure of the title of the poem this is excerpted from, so here is the citation:
The works of Robert Burns: with Dr. Currie's memoir of the poet, and an essay on his genius and character, Volume 1, 1843
page 101. (free at books.google.com)