The Base Rate of Kant
People sometimes say that increasing specialization within philosophy means that there could never be another figure like Locke or Hume or Kant -- a figure with giant impact across a broad range of philosophical subdisciplines. The massive growth of the research university has created armies of specialists in each subfield whose copious volumes one must master to become a major player in the subfield; and no one person could master the work of a broad range of subfields.
Let's consider the merits of this theory.
First: Is there any need for a theory to explain the recent lack of Kants? Well, what's the base rate of Kant? We could calculate rate per century or we could calculate rate per professional philosopher.
Consider by century: It seems plausible that no philosopher of at least the past 60 years has achieved the kind of huge, broad impact of Locke, Hume, or Kant. Lewis, Quine, Rawls, and Foucault had huge impacts in clusters of areas but not across as broad a range of areas. Others like McDowell and Rorty have had substantial impact in a broad range of areas but not impact of near-Kantian magnitude. Going back another several decades we get perhaps some near misses, including Wittgenstein, Russell, Heidegger, and Nietzsche, who worked ambitiously in a wide range of areas but whose impact across that range was uneven. Going back two centuries brings in Hegel, Mill, Marx, and Comte about whom historical judgment seems to be highly spatiotemporally variable. In contrast, Locke, Hume, and Kant span a bit over a century between them. But still, three within about hundred years followed by a 200 year break with some near misses isn't really anomalous if we're comparing a peak against an ordinary run.
(I don't mention Descartes despite his huge importance because he didn't have the same kind of impact in ethics/political as did the other three. Also, here is evidence that my judgments about importance aren't too idiosyncratic.)
If we consider the rate of Kants per thousand working professional philosophers, it does seem to be vastly higher in the early modern era than recently. But field-changing ideas can only occur so fast -- probably not much more than once per generation per subfield, since few philosophers are going to be ready to retool every ten years for the newest thing. Probably Kant rate per generation, in cultures with lively enough philosophical communities, is a better way to conceptualize the denominator of the expected base rate.
Thus, I don't think that the recent lack of Kants is a fact so anomalous that it cries out for explanation. There's only so much space at the top for heroes and field definers. Inevitably, Kants will be rare.
But maybe it's still true that the size of the community in each subfield makes it impossible for any one philosopher in the foreseeable future to have a huge impact across the subfields? Maybe a new Kant simply couldn't arise in a discipline as populous as 21st century philosophy? I see two reasons for skepticism about that theory.
(1.) People with huge impact are sometimes young. This was true historically (e.g., Hume was 26 when he finished the Treatise) and it seems to be still true (e.g., Lewis did much of his most influential work when he was in his 20s and 30s). If Lewis (or Kripke, or Chalmers, or...) could master enough of one subfield in 10 years to have a huge impact by age 30, then by age 60 -- and philosophers are by no means washed out by age 60 -- they ought to be able to master, well enough to potentially have a huge impact, several disparate subfields. Nor does it seem that there should be substantial barriers to this in practice. Although sociologically it would be difficult to leap from math to philosophy to physics to have huge impacts in all three fields -- so maybe there will never be another Descartes -- philosophy is not so sociologically divided. Setting aside language-driven divides, the sharpest sociological divide seems to be between "value theory" fields (ethics/aesthetics/political) and "LEMMings" fields (language/epistemology/metaphysics/mind plus logic). But even that divide is quite permeable. There are plenty of philosophers with interests and ambitions on both sides of the divide, and no one finds it odd.
(2.) Across academia as a whole, field-transforming contributions are sometimes achieved by adopting a novel angle or method and then applying it fruitfully both to traditional problems and to new previously unthought-of problems. In philosophy, this means having a new (implicit or explicit) metaphilosophy. Although none of the metaphilosophical revolutions of the past hundred years have generated a Kant, I think we can imagine how they might have, had things played out somewhat differently. Had there been a single dominant figure in the linguistic turn who managed to compellingly apply early linguistic-turnish thoughts to both metaphysics and epistemology and to ethics and political philosophy, that person might have had approximately Kant-sized influence. I see no reason to think such a scenario implausibly unlikely. Similarly, there could have been, I think, a Kant-sized ordinary language philosopher or logical positivist. More recently -- in my opinion! -- philosophy has received a charge of new ideas from the methods and results of empirical psychology, now that psychology has matured past Freudian and behaviorist strictures. People like Jesse Prinz and Shaun Nichols have been able to master enough of the literature in disparate areas of philosophy to have impact in those areas partly through applying methods and ideas from psychology. So I see no reason a great philosopher couldn't arise with a fresh angle, a new approach, applied compellingly to a broad range of the biggest issues, with a consequent Kant-like impact. We won't see it coming in advance -- but that's just us, stuck in our paradigms. Although it is impossible to have encyclopedic mastery of the ever-increasing existing literature across a wide range of subfields of philosophy, such encyclopedic mastery has never been a prerequisite of field-changing genius.
Caveats:
* This isn't to say that I'm particularly fond of Kant! I'm just making a sociological point, and he seems the best example. (See here and here for rough criticism.)
* One way in which it might be harder now to have a broad impact across subfields with a new approach is that once that approach catches on in one subfield, there will be a larger pool of people than there used to be who might quickly adopt it to other subfields, attenuating the aspiring Great Philosopher's direct impact on those subfields.
* Our perspective on the past is probably distorted by the Winnowing of Greats. Appropriately winnowed, maybe David Lewis (or whoever) will some day stand out like Kant. (And yes, I know that much of this post in is conflict with much of that earlier post. I take an appropriately Whitmanian attitude.)