Augustine "graduate[d]" from his studies in Carthage, and was qualified to be a teacher "of those arts called the liberal." He went back to Thagaste to be a teacher of rhetoric and oratory, sometimes to law students. His field of expertise was speaking and writing in a polished, educated, and persuasive manner. He brought back his "concubine" to live with him in Thagaste. Though he was faithful to her, he acknowledges his wrong in living with a woman outside of marriage for so long. He mentions that they were together for mutual pleasure, and didn't desire children.
During this time, though he was not as spiritually bereft as he was as a student, Augustine began peddling rhetorical tricks for a living. He was practicing the Manichaean religion, including the peculiar beliefs that the Manichees held about astrology. Nebridius, a cultured Carthaginian friend of Augustine's, showed him that astrology was "completely bogus." This was another step, like his reading of Hortensius and his fascination with philosophy and religion, on his path to conversion.
At this time, Augustine renewed a friendship with a man he had known as a boy. They became very close, and since this man had no allegiance to any particular creed or religion, Augustine instructed him in Manichaeism. Augustine speaks of this friendship (though not a true friendship in his estimation since they were not both Christians at the time and therefore not "of the Spirit") as "sweet to me above all the sweetness of life I had experienced." Soon this friend became very ill with a fever, and was baptized a Christian while unconscious. He recovered, and Augustine told him of his baptism and joked about it in the Manichaean manner. Manichees did not believe in Christian baptism, and considered it silly and superfluous. This angered his friend greatly, and Augustine kept quiet thereafter. Augustine left for some time, during which the friend quickly sickened and died. Augustine laments that he was in such error about Christianity at the time, and says that his friend was "snatched away from his lunacy" so that he might go to God as a Christian.
Augustine felt this loss very deeply. He entered a depression, feeling that everything was evil and "look[ed] like death." Augustine explains how he was attached to the transient things of life, and would therefore always be miserable. Attachment to anything of this earth will always result in misery, he argues, because it will always pass away. Things are brought into being and immediately begin their progress into non-being. This attachment to things not of God will always bring the human soul despair - only attachment to God and his eternal things can make a human being happy.
Augustine feels compelled to explain the temporal nature of the Earth, and how that temporality relates to God. If God is eternal and the Earth is temporal, then we must, as Christians, understand the Earth as God's creation and as part of the eternity of God only in its wholeness (not in individual people or institutions). Augustine explains that since the creations of God are part of God, though of a lower order, the creation of God as a whole is eternal as well. The individual parts of the creation, however, are made to pass away from the Earth, and even such sweet things as a close human friendship, though they seem spiritual, are still only things of the Earth. Augustine would have been better served, he implies, to put his faith in and derive his pleasure from the friendship of God, rather than the friendship of a mortal man.
Augustine now excuses himself for The Beautiful and the Fitting, a work he began in Carthage. He calls it "miserable folly," and chides himself for the pretentious act of dedicating it to an orator in Rome whom he had never met (Hierius). He also criticizes human attempts to write and speak about eternal, spiritual things, which he says cannot adequately be described by this temporal method. Augustine goes on to criticize his assertion in The Beautiful and the Fitting that evil is a substance that causes division and conflict. Now, as a Neoplatonist Christian, Augustine sees that evil cannot be a substance, but is only a degree of separation from God. He also explains that he didn't then understand that the soul and mind must be "enlightened by light from outside itself, so that it can participate in the truth."
At this time Augustine was reading Aristotle's Ten Categories (usually called Categories in today's parlance). Again he criticizes the limitations of this work, dismissing it as a scientific rather than philosophical text. He says that it concerns only things of the material world, and thus cannot apply to God. He ends Book IV, as usual, with praise and supplication to God to support and to guide him.
Book IV finds Augustine set up with a career and a companion back in his hometown of Thagaste. He had begun to transform into the "success" that his parents had hoped he would be, and he pursued public office as his rhetorical and oratorical education had prepared him to do. He was a young man with high hopes and aspirations, and was likely to do well by the standards of his day.
He was consumed with the Manichaean religion, but was slowly beginning to see the absurdities and inadequacies of it when compared to Christianity. The astrological component of Manichaeism, which was fantastical even for that time, was quickly discarded by Augustine. Nebridius, who was both wealthy and cultured, became a good friend of his, and it is possible to assume that the humbleness of Augustine's origins caused him to be a bit dazzled by the worldly Nebridius. However, Nebridius had a completely philosophical and inquiring mind, and proved to be a good influence on Augustine on his path to Christianity.
The Manichaean religion was outlawed in Carthage at various times during Augustine's lifetime, and the number of followers was never particularly large (especially when compared to Christianity). Nevertheless, it appealed to many intellectual and spiritual people of the time. Much of Confessions is concerned with not only explaining Augustine's own errors in his experiments with the religion, but with refuting the religion itself. As stated in Book III, this activity is useful for later Christian readers because it reinforces the Christian belief system's more philosophical precepts.
Augustine's first book, The Beautiful and the Fitting, was an aesthetic and philosophical work meant to impress his fellow students and teachers. It was dedicated to a man Augustine had never met, and he hoped to gain some notoriety for it. He soon came to be ashamed of this early work. The book, (De Pulchro et Apto in Latin), is now lost.
Augustine's problems with Aristotle's book Ten Categories are to be expected. By this time Augustine was firmly entrenched in his Neoplatonist beliefs, even if he didn't always own up to them. The philosophy of Aristotle is in many ways diametrically opposed to both Platonism and Neoplatonism, and Augustine's refutation of Aristotle's materialism shows how concerned he was with spiritual philosophy.