In like manner he was of opinion that a man should employ some time in astronomy, that he might know by the stars the hour of the night, what day of the month it is, and what season of the year we are in, in order that we might know when to relieve a sentinel in the night, and when it is best to venture out to sea, or undertake a journey, and, in short, that we might know how to do everything in its proper season. He said that all this was easily learnt by conversing with seamen, or with such as go a-hunting by night, or others who profess to know these things; but he dissuaded very much from penetrating farther into this science, as even to know what planets are not in the same declination, to explain all their different motions, to know how far distant they are from the earth, in how long time they make their revolutions, and what are their several influences, for he thought these sciences wholly useless, not that he was ignorant of them himself, but because they take up all our time, and divert us from better employments. In fine, he could not allow of a too curious inquiry into the wonderful workmanship of the Deity in the disposition of the universe, that being a secret which the mind cannot comprehend, and because it is not an action acceptable to God to endeavour to discover what He would hide from us. He held, likewise, that it was dangerous to perplex the mind with these sublime speculations, as Anaxagoras had done, who pretended to be very knowing in them, for in teaching that the sun was the same thing as fire, he does not consider that fire does not dazzle the eyes, but that it is impossible to support the splendour of the sun. He did not reflect, neither, that the sun blackens the sky, which fire does not; nor lastly, that the heat of the sun is necessary to the earth, in order to the production of trees and fruits, but that the heat of fire burns and kills them. When he said, too, that the sun was only a stone set on fire, he did not consider that a stone glitters not in the fire, and cannot last long in it without consuming, whereas the sun lasts always, and is an inexhaustible source of light. Socrates advised, likewise, to learn arithmetic, but not to amuse ourselves with the vain curiosities of that science, having established this rule in all his studies and in all his conferences, never to go beyond what is useful. He exhorted his friends to take care of their health, and to that purpose to consult with the learned; and to observe, besides, each in his own particular, what meat, what drink, and what exercise is best for him, and how to use them to preserve himself in health. For when a man has thus studied his own constitution, he cannot have a better physician than himself.