Douglas Den Uyl
Spinoza's Modern Humanism
Substance is where everything comes from and to which everything is ultimately referred or dependent upon. There's nothing outside of it, and there's nothing else to explain what it is other than itself. Of course, once one begins thinking about substance in this way, substance starts looking a lot like what at least many people would ordinarily say about God!
That is precisely what Spinoza does in the opening propositions of the first book of the Ethics—he links substance and God together so that they are both names for the same thing. One notices also that the definition of God has a lot to do with infinity. Spinoza shows that substance is infinite (E1P8). We tend additionally to think that there can only be one of the kind of God Spinoza defines, so Spinoza proves that there cannot be more than one substance and that it couldn't have come from anything other than itself (E1P5-6). In addition, Spinoza formally links God and substance together (E1P11). Other concepts, such as existence and self-causation, have helped along the way, but the important idea for our purposes is that everything whatsoever, including God or substance itself, is encompassed by this concept expressed by the two terms 'substance' and 'God.' But those terms do not quite give us the picture we need. For that picture, the terms 'attribute' and 'mode' are also required.
As we see in its definition, an attribute describes the essence of a through the attributes of the substance. Since God or substance is infinite, whatever the attributes of God or substance are, they must also be infinite. Moreover, it turns out that God or substance would not only have attributes that are infinite, but also infinitely many infinite attributes (E1P10-11). Thus, if we could give a description of one of these attributes—say 'thought'—that description would mean, when applied to God or substance, that thought was infinite in God or substance. The idea that God's thoughts are applicable to everything that exists is probably not too far off from what most people would intuitively say about God. What Spinoza adds, however, is the idea that there are infinitely many different attributes of the same God or substance. It's like saying God or substance is infinite in both 'length' and 'breadth,' although since we mentioned 'thought,' those terms used in this context are of course metaphors, not actual descriptions. Still, we can imagine on the one hand a line of thought, and we can imagine on the other a range of thoughts. God or substance would be infinite in both ways (not to mention infinitely many others).
Within 'thought' we might be more comfortable talking about 'thoughts.' A 'line of thought' is generally another name for a string of discrete thoughts. These discrete thoughts might more generally be called 'modes' of thought. To be a mode of thought would be to be this or that thought. That's the idea behind the definition of 'mode' Spinoza gives, noted above. The 'affections of substance' used in Definition 5 are the particular manifestations of substance. Of course, those manifestations must be understood in a certain way. In this case we are considering them as kinds of thoughts. However, given what Spinoza has said about substance or God and attributes, there are an infinite numbers of ways in which God or substance may manifest itself (E1P16). Thus, there are also an infinite number of things that can be said about any given manifestation (E1P25Cor.). In this case, we're just talking about one such way—the manifestation as a thought of some sort. Although there are an infinite number of possible ways to consider any particular manifestation, our limited nature only allows us to really speak of the manifestations of two attributes, thought and extension (matter) (E2P1-2). So when we talk about things in the world around us, we do so in terms of something being either a mode of thought, or of matter, or both.
Now that we have a rough idea of substance, attribute, and mode, we can look with particular interest to the 18th proposition of the first book of the Ethics: 'God is the immanent, not the transitive, cause of all things.' Though the meaning of this proposition is worked out in many successive propositions (as well as beyond book one), we can move to the point immediately. Remembering that there is nothing outside of God (or substance) and that everything that is must be understood in terms of God, this proposition is telling us that God is not simply something upon which everything depends and who makes it possible for things to do anything that they do or be anything that they are. Spinoza's point is actually more radical: everything actually is God (or substance). Indeed, how could it be otherwise? If God is truly infinite in infinite ways, and there's nothing outside of God or independent of God, then what makes the most sense is to see the things around us, including ourselves, as simply particular manifestations of God (or substance). To say otherwise would be to give something some independence from God. But that itself would mean that God faces some limit, namely, something that is not God, and that thing would throw up a boundary—or limit—to what God is. God's all-encompassing infiniteness would not be so all-encompassing or infinite in that case (E1P14). It is more plausible to think in terms of the one substance or God expressing itself in particular or modal ways than it is to suppose there is something that God or substance is not. But, of course, in saying all this, proposition 18 makes immanence an essential feature of all that is, not to mention an essential feature of Spinoza's philosophical system.
An idea often associated with Spinoza is the idea of 'deus siae natura' or 'God or nature' (E1App.; E4Pref.). The idea is quite simple: God and nature are interchangeable terms for Spinoza. Consequently, we actually have three terms that turn out to be identical in meaning and reference: substance, God, and nature. The significance of this equation of God and substance with nature is, at least, a reinforcement of immanence. Since God and nature are the same, there is no separation of God from nature. God didn't create the natural order; God is the natural order (and it is God).