Evil is the mystery within Christian apologetics and systematic theology which just will not go away. A failure to truly understand its nature can act as an obstacle to most other areas of theological enquiry and can distort our notions of the character of God. This article explains and evaluates St Thomas Aquinas’s view that the existence of evil implies the basic goodness of the world and therefore indirectly assumes God’s existence too. It analyses Aquinas’s privation theory of evil and refusal to see this as the best possible material world. It concludes that, while his counterargument to evil is a powerful, thought-provoking one, it could be strengthened if he acknowledged this as the best possible material world.