 According to one interpretation, God is indeed material, even matter itself, but this does not imply that God is or has a body. Another interpretation, however, one which will be adopted here, is that what is in God is not matter per se, but extension as an essence. And extension and thought are two distinct essences or natures that have absolutely nothing in common. The modes or expressions of extension are physical bodies; the modes of thought are ideas. Because extension and thought have nothing in common, the two realms of matter and mind are causally closed systems. Everything that is extended follows from the attribute of extension alone. Every bodily event is part of an infinite causal series of bodily events and is determined only by the nature of extension and its laws, in conjunction with its relations to other extended bodies. Similarly, every idea or mental event follows only from the attribute of thought. Any idea is an integral part of an infinite series of ideas and is determined by the nature of thought and its laws, along with its relations to other ideas. There is, in other words, no causal interaction between bodies and ideas, between the physical and the mental. There is, however, a thoroughgoing correlation and parallelism between the two series. For every mode in extension that is a relatively stable collection of matter (an individual body), there is a corresponding mode in thought (an idea or mind). In fact, he insists, “a mode of extension and the idea of that mode are one and the same thing, but expressed in two ways” (IIp7s). Because of the fundamental and underlying unity of Nature or Substance and its attributes, thought and extension are just two different ways in which one and the same Nature expresses itself. Every material thing (every determinate expression of Nature through extension) thus has its own particular idea (a determinate expression of Nature through thought)—an eternal adequate idea—that corresponds to and represents it. Since that idea is just a mode of one of God’s attributes—thought—it is in God, and the infinite series of ideas constitutes God’s mind or infinite intellect. As he explains,