David Lewis

David Kellogg Lewis (1941-2001) was one of the most influential philosophers of the 20th century within the field of analytic philosophy; he made significant contributions to metaphysics, philosophy of logic, and philosophy of language. He was best known for his book On the Plurality of Worlds, in which he defended modal realism, the theory that every possible world exists in just the same way that the actual world exists. He was recently voted the 13th most important philosopher of the past 200 years at Leiter Reports.

This image below is from a photo of Princeton University's philosophy department in 1979, which can be seen in full here. Sadly, Lewis abandoned the neckbeard in his later years, and is more frequently pictured with a beard growing over his chin.