  It it tempting, and plausible, to suppose that the implicit reasoning goes something like this: “Composite” (com + positus = “put together”) things don't just happen to be composite; something put them together. In short, composition requires an efficient cause. It follows therefore that God, as first cause, cannot be composite. Conversely, anything that is caused is in some sense composite. Hence, since everything besides God is created and therefore caused, everything other than God is composite. In short, the unique simplicity of God is important to maintain because it is required by the doctrine of creation. Nevertheless, while this line of reasoning is plausible, it is not found clearly stated in any medieval author I know  of.[4]