De libero arbitrio voluntatis (On Free Choice of the Will), often shortened to De libero arbitrio, is a book by Augustine of Hippo which seeks to resolve the problem of evil in Christianity by asserting that free will is the cause of all suffering. The first of its three volumes was completed in 388; the second and third were written between 391 and 395. The work is structured as a dialogue between Augustine and his companion Evodius; it ranges over several topics, and includes an attempted proof of the existence of God.
Intended also as a refutation of Manichaeism, De libero arbitrio denied God's responsibility for sin and emphasised human freedom and accountability. As a result, it became associated with Pelagianism, another doctrine which Augustine considered heretical; he subsequently defended the work by softening its libertarian message. In the 13th century, Thomas Aquinas expanded on the political ideas expressed in this book, and is credited with popularising Augustine's claim that an unjust law is not a law.