Thomas Reid was a theist and a philosopher; yet the exact relationship between philosophy and theology in his works is unclear and disputed. The aim of this book is to clarify this relationship along three lines by exploring the status, function, and detachability of theism with respect to Reid's philosophy. Regarding the first I argue that belief in the existence of God is, for Reid, a non-inferential first principle. Regarding the second I argue that theism plays at least six different roles in Reid's philosophy. And, regarding the third, I argue that, despite this, theism is largely detachable from Reid's concept of human rationality and philosophy. What emerges is a picture of the relationship between philosophy and theology in which both inquiries are motivated by natural human curiosity, and both are founded on principles of common sense.