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"?

As useless as art, literature, music, or just about anything you do as an end in itself rather than a means toward some other end in itself. Most important science wasn't done for the purpose of achieving "practical" results, but to satisfy some inner compulsion, of the kind that Plato describes very well in his cave story.

That said, just about all scientific disciplines have emerged, one way or another, from philosophy (though philosophy itself, if you consider Plato as an important milestone, seems to have been inspired by mathematics), so it's certainly been of very considerable practical use in that sense.

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