  The uniqueness of a being by substance is, William argues, a consequence of its absolute simplicity, since nothing could differentiate two or more absolutely simple beings. The absolute simplicity of a being by substance, in turn, is a consequence of the fact that it cannot be caused (not, at least by an external causation; William argues that the being by substance is the Christian Trinity of persons and he admits a kind of internal causation in the Trinity). For everything that is caused is other than its being and thus is a being by participation, whereas a being by essence or substance is its being. But a composite or non-simple being must have an external cause, and must therefore be a being by participation. Therefore, the being by substance, that is to say, God, cannot be composite, but must be absolutely simple.