Is divine simplicity a defined doctrine that appears in Church council documents?

It does appear in council documents. Along with immutability, simplicity does appear in at least one council document. Immutability appears probably in about a half a dozen council documents that deal with the nature of God and all that. Simplicity is a good concept to understand and hold to, because basically what you're saying is that God's not made in parts, like we are. God is whatever He is, He's all one. He's not divided. Now, of course, you have to understand what that means in the sense of the Trinity. And that's where it gets a little sticky, because you have three persons. You have three personalities in one God, simplicity, one God, substance. You say, how do you understand that? It's very hard to understand. What they can tell you is what you can't say. They're very good at that. When you have three distinctions in one unity, one substance, one essence, that's tough. So the concept of simplicity, yes, that is a de fide doctrine, as far as the Church has defined what God is. I can tell you this, that all the descriptions that Thomas Aquinas has put on who God is, the Church did not take. It took some concepts like immutability, definitely, and simplicity, as I said, in one council document. But they didn't go much further than that. Because the Church knew that God is much more complicated, let's say, in understanding than the terminology that metaphysical science is going to put on God and say, God has to be this way because our metaphysics says that He has to be this way. No. He's always God. He's not made of parts. As far as anything else is concerned, we'll take it on a case-by-case basis, which they have done. So I think that would answer your question.