Regarding David Mitch's suggestion for Hayek's quote, I wonder what happened
to Larry Willmore's mail, available at eh.net, which pointed out three
previous statements of the same view, two of which are rightly hinted at by
David himself:

Hayek's quote follows in the footsteps of Mill (On Comte, CW, X, pp. 306).
Mill's passage, 'A person is not likely to be a good political economist who
is nothing else' was approvingly quoted by Marshall (appendix C of
Principles, where the word political is omitted).

Moreover, in the Marshall obituary, Keynes wrote that 'the master-economist
must possess a rare combination of gifts. ... He must be mathematician,
historian, statesman, philosopher - in some degree (CW X, p. 73, full quote
given by Larry).

Larry's conclusion that Hayek had a lot of company when he stated that
nobody can be a great economist who is nothing else weighs against
attributing the concept to Hayek. Perhaps Mill or Marshall deserve priority
(by the way, in my opinion David Colander's list of suggestions for
Marshall's quotes is excellent),
best,

Tiziano Raffaelli