Keynes, in praising Alfred Marshall, wrote the qualifications for an economist:

"The study of eocnomics does not seem to require any specialized gifts of an unusually high order. Is it not, intellctually regarded, a very easy subject compared with the higher branches of philosophy or pure science? An easy subject, at which every few excel! The paradox finds its explanation, perhaps, in that the master-economist must possess a rare combination of gifts. He must be a mathematician, historian, statesman, philosopher - in some degree. He must understand symbols and speak in words. He must contemplate the particular in terms of the general, and touch abstract and concrete in the same flight of thought. He must study the present in the light of the past for the purposes of the future. No part of man's nature or his insitutions must lie entirely outside his regard. He must be purposeful and disinterested in a simulataneous mood; as aloof and incorruptible as an artist, yet sometimes as near the earth as a politician."

The above was culled from "The Worldly Philosophers", a secondary source (my apologies.) I intend to plow through primary Keynes later this year, will cite source if I see it.