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Shall we propose, as a third branch of our education, astronomy?

'Very good,' replied Glaucon; 'the knowledge of the heavens is

necessary at once for husbandry, navigation, military tactics.' I

like your way of giving useful reasons for everything in order to

make friends of the world. And there is a difficulty in proving

to mankind that education is not only useful information but a

purification of the eye of the soul, which is better than the

bodily eye, for by this alone is truth seen. Now, will you appeal

to mankind in general or to the philosopher? or would you prefer

to look to yourself only? 'Every man is his own best friend.'

Then take a step backward, for we are out of order, and insert

the third dimension which is of solids, after the second which is

of planes, and then you may proceed to solids in motion.