Before about 1950 or so, it seems, academic giants straddled whole fields. They were few and far between. Now, it seems, there are no Greats though a number of Very Goods. Consider 1850-1950: In philosophy: Mill, Marx, Nietzsche, Heidegger, Russell, Wittgenstein (and others). In psychology: Freud, Piaget, James (and others). So also in other academic fields. Where are the Einsteins and Darwins of the last 60 years? Have we run dry? (Let's not pretend Hawking and Crick are Einstein and Darwin, please!)
A few obvious factors:
* Fields are much more specialized. Given the number of researchers and articles, it's going to be very difficult to have an expert command of as broad an area as all of psychology or all of physics. Part of being Great might be having command of a wide area.
* Also because there are many more people in each field, one would expect the very best thinkers to have other nearly as good thinkers nearby. Greatness might be in part a comparative measure.
* With improved communication and travel, it's easier to keep atop of the best and latest thinking from relatively remote places. This creates a more competitive, egalitarian atmosphere. It also may make it more difficult for the odd genius to incubate
away from mainstream opinion.
But there's also, I think, the following effect, little remarked upon (and somewhat in tension with my Hawking/Watson remark above), which I'll call the winnowing of greats with distance: The farther away your perspective on any body of people varying in eminence, the more isolated and comparatively great will the most eminent among them seem.
Consider the matter abstractly, first. Let's say that a field has 100 eminent practioners, with levels of eminence varying from 1 to 100 -- with most clustered near the bottom of this distribution and tailing off toward the top. For specificity, suppose the 10 most eminent are A (with an eminence of 91), B (80), C (75), D (64), E (58), F (57), G (50), H (46), I (45), and J (40). Suppose you're in the field, and you know of all 100 people. A is the most eminent, but B isn't far off, and all of A-J are among the very eminent -- in the 90th percentile among the 100 most eminent practioners, after all! To the extent greatness involves being head-and-shoulders above everyone else, A, though the most eminent, is only one of a group.
Someone who knows much less of the field might only have heard of A-J. Everyone else, from their perspective, will be a non-entity. Someone who knows still less may know only A-E, or A-C, or A. In textbooks and summaries, where only one or a few people can be mentioned, A will be mentioned almost every single time, and B and C will sometimes be mentioned; D rarely so. Suddenly A, or A-C, are no longer the best among peers but peerless.
Consider early introspective psychology: All academics know James. All psychologists have heard of Wundt. Many psychologists know about Titchener. But only specialists like me know Kuelpe and Mueller and Stumpf and Calkins and Sanford. Though I don't deny that James was the best of the lot and indeed a rare genius, from my perspective he is not a peerlessly great, solitary figure. Similarly, we all know Chaplin, but when I started learning more about silent comedy, I learned about Keaton and Lloyd, and Chaplin no longer seemed quite as peerless.
As we back away from late 20th-century philosophy and our lists get shorter, will Rawls and Kripke (and a few others) look more and more like they stand alone?
One other effect as one backs away from a field: One's judgment becomes homogenized with that of others. An early film enthusiast may not actually think Chaplin is the best. I might think Kuelpe has the edge over Titchener. The implicit uniformitarianism of ignorant distance can produce a unanimous chorus that artificially gives the impression of greatness (though over time this may become the irresistable, self-fulfilling "judgment of history").