Duns Scotus’ rational route to God begins, oddly enough, with the Bible. God revealed himself to Moses as “I am who I am,” and for most of Christian theological history theologians have thought this self-disclosure means that God is, somehow or other, Existence itself, or the sole ultimate explanation of the whole of existence. “You revealed yourself to Moses as I am who I am. You are true existence. You are total existence. This I believe and this I seek to know, if it possible for me to know it.” So begins Scotus’ Treatise on the First Principle; on the topics it treats it is the most powerful work of natural theology I have encountered.
Scotus’ strategy is to reflect on the very nature of existence, because in doing so he thinks he can show that God exists. If it is even just possible for something to exist, it follows that God mustexist. Suppose that nothing whatsoever exists. In such a circumstance, it would not be possible for something to begin to exist. This is because nothing can cause itself to exist, and, in our imaginary scenario, there is nothing else around to cause something to exist. So if there is nothing at all, then necessarily there will always be nothing at all. Now, in fact we know that it is possible for something to begin to exist. We know this because some things did in fact begin to exist. The actual existence of these things is then the source of our knowledge that it is possible for something to begin to exist. But only our knowledge of that possibility, not the possibility itself, depends on there being any actual things which began to exist. That possibility is instead a necessary condition for anything beginning to exist—after all, if it were not possible for something to begin to exist, then there would be nothing which began to exist. So the possibility that something begins to exist is part of the explanation of why there are things which have begun to exist, while conversely, the fact that things have begun to exist does not in any way explain why it is possible for something to begin to exist. Thus, while we know that it is possible for something to come into existence only because there are in fact some things which began to exist, we can also know that these existing things cannot explain why it is possible for something to come into existence.
So we know that something can come into existence. Moreover, we can think about something which can come into existence, while disregarding the fact that it exists, or even if it does not exist at all. We can think about it, and go on to consider what must be true about it, if we attend just to the fact that it is the sort of thing which can exist, even if it does not exist. If this all sounds very abstract, here is a familiar example to show that this is not all as abstruse as it first seems. Think of Tolkien’s myths and legends preserved in books like The Lord of the Rings and The Silmarillion. In particular, think of an orc, such as Grishnákh. In Tolkien’s mythology, the orcs are a type of personal animal which has some sort of genealogical relationship to the elves. The Dark Lord Morgoth tortured elves and corrupted them, and over many years in unknown ways produced from them the race of orcs. If we consider Grishnákh, we can think not only about his personality and personal appearance, but also about his provenance. That is, given that orcs are a kind of animal, we know that Grishnákh is descended in some way from orc parents. We also know that he depends for his ongoing life on things like the air of Middle Earth, water, food, the goodwill of the Valar, and ultimately on the goodwill of Illúvatar, who, in Tolkien’s mythology is the original source of everything besides himself. And we know Grishnákh can be destroyed, as he is destroyed when the Ent, Treebeard, stomps on him. We know that Grishnákh bears some sort of genealogical relationship to elves, even if we don’t know how the Dark Lord brought about orcs from elves—by selective breeding or by genetic manipulation or in some other way, we just don’t know. But we do know that orcs wouldn’t have been if elves hadn’t been. And elves wouldn’t have been if the Valar hadn’t made a world for them to be in, and if Illúvatar hadn’t made the elves to be in that world. Thus, in thinking about a single orc we can come to see something of the wider web of dependencies that obtain in Middle Earth. And all these dependencies obtain even though, as we are sometimes unfortunate to recognize, Middle Earth and its creatures are not real. It’s a world that has a high level of integrity and depth and so feels to so many readers that it could or should be real. But it’s not real, or at least it’s not real like our world is real. Thus, in thinking about an orc in Middle Earth, we are thinking about a merely possible being, but even as a merely possible being the orc stands in so many relationships of dependence, and so we can also think about at least some of these other merely possible things the orc depends on.
The point of the example is to show that we really do have some acquaintance with the sort of thinking that Scotus is asking us to do. He wants us to set aside for the moment that we know that so many of the things we think about are in fact real: tables, chairs, horses, cows, and so on. Ignore the fact that they’re real, he’s asking us. Think of all the things in the real world as though they made up one big fantasy world, like Tolkien’s Middle Earth. Attend just to their natures, and in doing so you will see that there is nothing about their natures which demand that they exist, just as in considering the nature of Grishnákh we find nothing about him that demands that he exists. So if there really are tables and chairs and so on, these exist because something else has caused them to exist. More generally, we can see that if something can come into existence then, should it begin to exist, it will do so as an effect of some cause. This is because it could not cause itself to exist. So something can come into existence only if something else can cause it to exist. Now think about this something else. If it too is the sort of thing which, if it exists, had a beginning, then it too exists only if something else causes it to exist. We can go through the same rational procedure with this new something else, with the same result: if it is yet a third thing which, if it exists, began to exist, then it too exists only if something else causes it to exist.
And now we’re drawing close to our target. For what it would take for something to be able to begin to exist, is for something that in fact exists to be able to cause it to exist. However many links there are in the chain of causes, none of them will actually exist unless there is something at the very back which already exists. The characterIllúvatar is obviously like God in many ways, but he is unlike God in a very profound way: he doesn’t make all the things he originates to be really real, real like tables and chairs and you and I are real. In the world of Tolkien’s stories Illúvatar really is the first thing, but he is just as much a part of that merely imaginary world as Grishnákh.So to explain the actual existence of things, we need a first thing with a feature Illúvatar clearly does not have, the feature of really existing.But this one who Already Exists cannot itself have been caused to exist. After all, if it had been caused, then it began to exist. But we already know that this great Already did not begin to exist. It always was. And so it always was uncaused. It is in fact that most Actual Thing which is the necessary condition for there being any actual thing whatsoever that begins to exist.
Of course, being the First Efficient Cause isn't enough to be God. But Scotus has more to say about that.