This passage appears to be a paraphrase of Plato, not a direct quotation. I found it in “Political Writings” by Henry St. John Bolingbroke, edited by David Armitage, published by Cambridge University Press, 1997, page 197. (See Google Books URL below.) There, Bolingbroke cites a passage from Plato’s “Protagoras” with a footnote (319b-d), and his text includes no quotation marks. The cited text from Plato is too long for me to type here, but you can find the Jowett translation of that passage at http://classics.mit.edu/Plato/protagoras.html : search for the paragraph containing “a noble art”. That’s the paragraph that Bolingbroke cites. He doesn’t use quotation marks, and while I think the conventions on quotation were less strict in Bolingbroke’s day than they are now (this was apparently written in 1736), the fact that he cites Plato and does not use quotation marks suggests to me that this is a paraphrase rather than a quotation. In addition, although Bolingbroke was reading a different translation (Jowett’s was published after Bolingbroke’s book was published), the passage Bolingbroke cites is quite long, and I don’t see anything in it that can be reasonably construed as something that might be an alternate translation of Bolingbroke’s text.
http://books.google.com/books?id=tmJ6uxHNuzoC&pg=PA197&lpg=PA197&dq=plato+%22sufficiently+qualified%22&source=bl&ots=huL-6E0THV&sig=Cv2BK1GuPIU8uAtGRCnOTT-i458&hl=en&sa=X&oi=book_result&resnum=1&ct=result)