It's often said that we cannot predict which scientific discoveries will turn out to have practical value, and so we should encourage scientific curiosity and investigation even in cases where the subject matter seems frivolous or esoteric. To take one famous example, G.H. Hardy thought that number theory was perfectly useless, but it is now indispensable to cryptography. Could the same be said of philosophy? Are there philosophical theories that have had unforeseen benefits? Or is it safe to conclude that at least some philosophical pursuits really are just "useless"?