This paper explores a PSR-connected trail leading from theistic idealism to a form of substance monism. In particular, I argue that the same style of argument available for a Leibnizian form of metaphysical idealism actually leads beyond idealism to something closer to Spinozistic monism. This path begins with a set of theological commitments about the nature and perfection of God that were widely shared among leading early modern philosophers. From these commitments, there arises an interesting case for metaphysical idealism, roughly (...) the thesis that only minds and mind-dependent states actually exist. However, I contend, that same theistic reasoning also leads to an idealist form of substance monism, the view that God is the only actual substance and that almost everything else is merely an intentional object in God’s mind. (shrink)