Final answer:
Augustine sets out three things within a human soul that reflect the Trinity: mind, knowledge, and will. In Book 9 of "On the Trinity" by Augustine, the third element within a human soul that reflects the Trinity is 3) will.
Step-by-step explanation:
In book 9 of On the Trinity, Augustine sets out three things within a human soul that reflect the Trinity. They are mind, knowledge, and will. Augustine believed that the will, or the faculty of choice, reflects the Trinity because just as the Father, Son, and Holy Spirit are distinct yet united in one God, the human will can make choices that align with or deviate from the divine will. "On the Trinity" by Augustine, he expounds on the three elements within the human soul that mirror the divine Trinity: mind, knowledge, and will. Augustine posits that the mind represents the Father, the source of understanding and awareness. Knowledge, akin to the Son, embodies the awareness of the mind—generated, yet distinct. Completing the trifold reflection, the will corresponds to the Holy Spirit, symbolizing the dynamic, unifying force binding mind and knowledge. Augustine emphasizes the interconnectedness of these elements, illustrating a profound theological understanding through the analogy of the human soul, reflecting the complex and unified nature of the divine Trinity.