for having invited the man who formerly busied himself about the subject of the nature of heaven, and whom some call a mathematician, to a participation in virtue, he made him wise and called him so. For having given an appropriate name to his transformed disposition, he named him, as the Hebrews would call it, "Abraham," but in the language of the Greeks, "the elect father of sound;" for says he, On what account dost thou investigate the motions and periods of the stars? and why hast thou bounded up so high from the earth to the heavens? Is it merely that you may indulge your curiosity with respect to those matters? And what advantage could accrue to you from all this curiosity? What destruction of pleasure would it cause? What defeat of appetite? What dissolution of pain or fear? What eradication of the passions which disturb and agitate the soul? For as there is no advantage in trees unless they are productive of fruit, so in the same way there is no use in the study of natural philosophy unless it is likely to confer upon a man the acquisition of virtue, for that is its proper fruit.
 On which account some of the ancients have compared the discussion and consideration of philosophy to a field, and have likened the physical portion of it to the plants, the logical part to the hedges and fences, the moral part to the fruit, thinking that the walls which are built around for the sake of protecting the fruit have been erected by the possessors of the land, and that the plants have been created for the sake of the production of fruit; thus, therefore, they said that in philosophy it is requisite for the consideration of the physical and the logical part of philosophy to be referred to the moral part, by which the moral character is improved, which has a desire at the same time for both the acquisition and the use of virtue.