The central figure of American pragmatism after William James and the most influential American philosopher of the first half of the 20th century. Dewey fused a naturalistic, experimental theory of knowledge with a deep commitment to democracy — not as a set of institutions merely but as a way of life. His 1916 Democracy and Education is the fullest statement: democracy is the political form of a society in which all its members have genuine opportunity to develop their capacities through cooperative inquiry, and education is the practice through which it is continuously renewed.
Epistemologically, Dewey treated inquiry as a continuous, experimental adjustment of beliefs to solve real problems. There is no Platonic realm of fixed essences behind experience; the tools of knowledge — concepts, theories, categories — are instruments, to be judged by their success in guiding action. Applied to ethics and politics, this makes normative questions empirical in an expanded sense: we test proposals by their consequences for human flourishing, revise them in light of those consequences, and iterate.
Politically Dewey was a reformist liberal and New Dealer who spent decades defending democratic socialism's practical reforms against their Marxist and conservative critics alike. The Public and Its Problems (1927), his answer to Walter Lippmann's elitist critique of democracy, argued that publics form through the need to manage the indirect consequences of social action — and that democratic politics is the work of keeping those publics awake, informed, and able to act. Chastened but not defeated by the 20th century, Dewey kept writing into his nineties.