4.17.30   And what did it profit me, that all the books I could procure of the so-called liberal arts, I, the vile slave of vile affections, read by myself, and understood? And I delighted in them, but knew not whence came all, that therein was true or certain. For I had my back to the light, and my face to the things enlightened; whence my face, with which I discerned the things enlightened, itself was not enlightened. Whatever was written, either on rhetoric, or logic, geometry, music, and arithmetic, by myself without much difficulty or any instructor, I understood, Thou knowest, O Lord my God; because both quickness of understanding, and acuteness in discerning, is Thy gift: yet did I not thence sacrifice to Thee. So then it served not to my use, but rather to my perdition, since I went about to get so good a portion of my substance into my own keeping; and I kept not my strength for Thee, but wandered from Thee into a far country, to spend it upon harlotries. For what profited me good abilities, not employed to good uses? For I felt not that those arts were attained with great difficulty, even by the studious and talented, until I attempted to explain them to such; when he most excelled in them who followed me not altogether slowly.