Plato

Plato stands behind Russell's discussion of universals, the realm of abstract entities that mathematics and language seem to require. His vision of an open world of free possibilities—where much remains unknown because there is so much to know—echoes through the book's closing reflections on why philosophy is worth studying at all. He matters because he supplies the alternative to pure empiricism: the idea that some things are known not by encounter but by reason, and that this knowledge is what gives unity to the sciences.