75 For sciences from nature should be drawn, As arts from practice, never out of books, Whose rules are only left with time in pawn To show how in them use and nature looks,     Out of which light, they that arts first began     Pierced further than succeeding ages can. 76 Since how should water rise, rise above her fountain? Or spirits, rule-bound, see beyond that light? So, as if books be man's Parnassus mountain, Within them no arts can be infinite,     Nor any multiply himself to more,     But still grow less than he that went before. 77 Again, art should not, like a courtesan, Change habits, dressing graces every day; But of her terms one stable counterpane Still keep to shun ambiguous allay,     That youth in definitions once received,     As in kings' standards, might not be deceived. 78 To which true end, in every art there should One of two authors be selected out To cast the learners in a constant mold, Who if not falsely, yet else go about,     And as the babes by many nurses do,     Oft change conditions and complexions too. 79 The like surveys that spirit of government, Which molds and tempers all these serving arts, Should take in choosing out fit instruments To judge men's inclinations and their parts,     That books, arts, natures may well fitted be     To hold up this world's curious mystery.