 Bertrand Arthur William Russell (1872–1970) was a British philosopher, logician, essayist and social critic best known for his work in mathematical logic and analytic philosophy. His most influential contributions include his championing of logicism (the view that mathematics is in some important sense reducible to logic), his refining of  Gottlob Frege’s  predicate calculus (which still forms the basis of most contemporary systems of logic), his defense of  neutral monism  (the view that the world consists of just one type of substance which is neither exclusively mental nor exclusively physical), and his theories of  definite descriptions,  logical atomism and  logical types.  