Xenophon Memorabilia 4.7

[1]
  I think that I have said enough to show that Socrates stated his own opinion plainly to those who consorted with him: I will now show that he also took pains to make them independent in doing the work that they were fitted for. For I never knew a man who was so careful to discover what each of his companions knew. Whatever it befits a gentleman to know he taught most zealously, so far as his own knowledge extended; if he was not entirely familiar with a subject, he took them to those who knew.

[2] He also taught them how far a well-educated man should make himself familiar with any given subject.
  For instance, he said that the study of geometry should be pursued until the student was competent to measure a parcel of land accurately in case he wanted to take over, convey or divide it, or to compute the yield; and this knowledge was so easy to acquire, that anyone who gave his mind to mensuration knew the size of the piece and carried away a knowledge of the principles of land measurement.

[3] He was against carrying the study of geometry so far as to include the more complicated figures, on the ground that he could not see the use of them. Not that he was himself unfamiliar with them, but he said that they were enough to occupy a lifetime, to the complete exclusion of many other useful studies.

[4]
  Similarly he recommended them to make themselves familiar with astronomy, but only so far as to be able to find the time of night, month and year, in order to use reliable evidence when planning a journey by land or sea, or setting the watch, and in all other affairs that are done in the night or month or year, by distinguishing the times and seasons aforesaid. This knowledge, again, was easily to be had from night hunters and pilots and others who made it their business to know such things.

[5] But he strongly deprecated studying astronomy so far as to include the knowledge of bodies revolving in different courses, and of planets and comets, and wearing oneself out with the calculation of their distance from the earth, their periods of revolution and the causes of these. Of such researches, again he said that he could not see what useful purpose they served. He had indeed attended lectures on these subjects too; but these again, he said, were enough to occupy a lifetime to the complete exclusion of many useful studies.

[6]
  In general, with regard to the phenomena of the heavens, he deprecated curiosity to learn how the deity contrives them: he held that their secrets could not be discovered by man, and believed that any attempt to search out what the gods had not chosen to reveal must be displeasing to them. He said that he who meddles with these matters runs the risk of losing his sanity as completely as Anaxagoras, who took an insane pride in his explanation of the divine machinery.

[7]
  For that sage, in declaring the sun to be fire, ignored the facts than men can look at fire without inconvenience, but cannot gaze steadily at the sun; that their skin is blackened by the sun’s rays, but not by fire. Further, he ignored the fact that sunlight is essential to the health of all vegetation, whereas if anything is heated by fire it withers. Again, when he pronounced the sun to be a red-hot stone, he ignored the fact that a stone in fire neither glows nor can resist it long, whereas the sun shines with unequalled brilliance for ever.

[8]
  He also recommended the study of arithmetic. But in this case as in the others he recommended avoidance of vain application; and invariably, whether theories or ascertained facts formed the subject of his conversation, he limited it to what was useful.

[9]
  He also strongly urged his companions to take care of their health. You should find out all you can, he said, from those who know. Everyone should watch himself throughout his life, and notice what sort of meat and drink and what form of exercise suit his constitution, and how he should regulate them in order to enjoy good health. For by such attention to yourselves you can discover better than any doctor what suits your constitution.

[10]
  When anyone was in need of help that human wisdom was unable to give he advised him to resort to divination; for he who knew the means whereby the gods give guidance to men concerning their affairs never lacked divine counsel.