The death has just occurred of the philosopher Michael Dummett (1925-2011), formerly Wykeham Professor of Logic at Oxford.    His writings on the philosophy of language and the philosophy of mathematics have influenced me, particularly his thorough book on intuitionism.   Having been educated by pure mathematicians who actively disparaged intuitionist and constructivist ideas, I found it liberating to see these ideas taken seriously and considered carefully.  The precision of Dummett’s writing and thought clearly marked him out as a member of the Matherati, as also his other formal work, such as that on voting procedures.
POSTSCRIPT (2012-01-21):  The logician Graham Priest remembers Dummett as follows:

It is clear that Dummett was one of the most important — perhaps the most important — British philosopher of the last half century. His work on the philosophy of language and metaphysics, inspired by themes in intuitionist logic, was truly groundbreaking. He took intuitionism from a somewhat esoteric doctrine in the philosophy of mathematics to a mainstream philosophical position.
Perhaps his greatest achievement, as far as I am concerned, was to demonstrate beyond doubt the intellectual respectability of a fully-fledged philosophical position based on a contemporary heterodox logic. Philosophers in the United Kingdom, even if they do not subscribe to Dummett’s views, no longer doubt the possibility of this. Dummett had an influence in Australia, too. It was quieter there than in the U.K., but the relevant philosophical lesson was amplified by logicians who endorsed heterodox logics of a different stripe (for which, I think, Dummett had little sympathy). The result has been much the same.
In the United States, though, Dummett had virtually no significant impact. Indeed, I am continually surprised how conservative philosophy in the United States is with regard to heterodox logics. It is still awaiting a Dummett to awaken it from its dogmatic logical slumbers.
Graham Priest, City University of New York Graduate Center, and the University of Melbourne (Australia)

References:
His Guardian obituary is here.  An index to posts about members of the Matherati can be found here.
M. Dummett [1977/2000]: Elements of Intuitionism. (Oxford: Clarendon Press, 1st edition 1977; 2nd edition 2000).