The question of philosophy's relevance to extra-academic concerns is much with us today. Plato tells us that, once the philosopher has seen the truth in the full light of the sun, she must return to the cave, there to put knowledge to work in making a better world, even though, being temporarily unaccustomed to the dark, she risks ridicule from those still in thrall to illusion. This paper reflects upon the life and career of Otto Neurath as a modern exemplification of this ideal of philosophical engagement. In spite of, or, perhaps, because of his never having held an academic appointment, Neurath made a difference for the good in human affairs. The key components of what I term Neurath's "philosophy of science in action" are explicated in order to understand how that could be. Foregrounded are Neurath's socialism, his own version of the thesis of the empirical underdetermination of theory by evidence, his anti-metaphysical stance, and his commitment to physicalism and the unity of science. The paper concludes with a discussion of the contemporary relevance of Neurath's model of engaged philosophy of science.