We said last time that we could know that God exists if we were able to identify some effects the cause of which could only be God. We use this sort of reasoning all the time. If I see smoke, I conclude there is fire. If I see a maple leaf, I conclude there is somewhere a maple tree. Only a maple tree can cause a maple leaf.
This is an important point, because it's double-edged: I know that in order for there to be a maple leaf, there needs to be something which can bring about the maple leaf and adequately explain its existence--that is, something to cause the maple leaf; likewise, I know that a maple leaf cannot be the cause of itself, that maple leaves "just happen," any more than people or planets or platypuses do. Each needs a cause sufficient to explain its existence. I know that everything needs a cause, and I know that nothing is the cause of itself.
So, what's the cause of the maple leaf? A maple tree. But what's the cause of the maple tree? Another maple tree, I suppose; and sunlight, and water, and nutrients from the soil, and so forth. But what is the cause of the sunlight and water and nutrients and soil? Well, they're all made from elementary particles, if that's what you're asking. But what is the cause of those particles? If we keep going backward with these questions and addressing them in a scientific way, drilling down into increasingly smaller components of matter, we will eventually hit a question that is not scientific, but philosophical: why do they exist at all?
This is a philosophical question rather than a scientific one because it deals not with empirical observation but with logical necessity--we float along in the boat of our empirical observations until we come upon the hard ground of logical questions, and the boat can't carry us anymore. Logically, there is nothing about the nature of anything we observe in reality that says the existence of that thing is necessary. The maple leaf does not necessarily need to exist, evidenced by the fact that at one time it didn't exist. If its existence were logically necessary, it would always have existed. What we find in reality, then, is a series of beings the existence of which is not necessary, might-not-have-been, always lingering on the brink of nothingness; such beings are called contingent beings.
When we try to explain the existence of one contingent being by the causal power of another contingent being--that is, when we say one thing exists because of another thing's bringing it about--we find that we still must explain the existence of that other contingent being by the existence of yet another contingent being that brought it about, and so on, and so forth... until we realize that trying to explain the existence of the universe by contingent beings traps us on a train going backward infinitely, forever, with no stopping... and, in this case, no beginning. No starting point. No source. No explanation.
What is required, then, to stop this infinite regress, this never-ending recession from one insufficient cause to another, is a being that is not contingent, but is necessary. What is needed is a being that absolutely must exist, by its nature, and therefore always has existed. A being whose essence is to exist. Only such a being would be able to bring anything else into being out of nothing, to start from scratch, to make the first link in the chain. An eternal, necessary being that brings everything else into existence--what does that sound like? Who does that sound like?
This is a much less eloquent and less pithy version of the argument from contingency that St. Thomas Aquinas makes in question 2, article three of the first part of the Summa when he asks the question, "Does God exist?" (You can find it here about 5 paragraphs down, the one that starts with "The third way....") It is one of five ways that St. Thomas proposes to prove the existence of God, and all of them more or less cover this same sort of ground: we see motion, motion is caused by another, so for any motion to start, there must be an mover who is himself unmoved; we see causes, every cause is the effect of another cause, so for any cause-and-effect relation to start, there must be an uncaused cause; and so forth. And each proof ends with, "this we call God."
Notice there is no reference to or reliance on Scripture, or any special revelation, or any article of faith, or anything special or particular to a Christian or a person of faith. You can argue with the arguments if you like, but they present themselves on the bases of empirical observation and logical analysis--that is, on science and reason. These are arguments accessible to any person, no matter what their background.
Notice, too, that these arguments only get you so far. Such arguments will tell you that there is an eternal creator of the universe, and they'll tell you one other important thing about this creator, the rebuttal to a mistake commonly made by those who profess not to believe in God: God is not the biggest being in a universe of other beings more or less like him, different only in degree rather than in kind; no, God is the very ground of being itself. God is the cause of nature, and thus beyond nature. The usual objection to these sorts of arguments is, "If everything needs a cause, what caused God?" But surely you can see within the argument itself that this objection has missed the point. Every contingent, that is, non-necessary thing, needs a cause for its existence. But the point of the exercise is precisely that there must be something that exists that exists necessarily, that is the source of existence for everything else. God is not a thing among other things; rather, God is the root of all things, their source, their foundation. God is not the biggest object in the painting of reality, or even the whole painting itself; God is the artist, painting the picture.
But this won't tell you much else about God, that He's loving or wise or kind or has created you in His image and likeness with the intention that you should live forever with Him in perfect happiness and glory. But this sort of argument does at least have the potential to open up the possibility to the mind of the atheist or agnostic or skeptic that the existence of God as Christians understand Him is not irrational or silly or a fairy-tale, but rather, that it's quite reasonable to assent to the proposition that God exists. There are other arguments, though. On these, more next time.