For the next several posts, I will roughly be following the lead of a rather clever fellow who's covered this ground before. He was Italian, of considerable physical stature, and had absolute chicken scratch for handwriting, but we'll let that pass. It's often a good idea to walk in the footsteps of St. Thomas Aquinas, one of the greatest minds of the history of thinking things, and this case is no exception.
First a brief explanation of St. Thomas' style. Academic texts from his time took the shape of the public debates, the disputatio, that the students and teachers would have on various topics. A question would be posed, objections to the position would be raised, and the master (teacher) would give his own view of the question and answer the objections. The questions were arranged within the texts in a logical, systematic scheme of organization, with general questions and sub-articles within each question. Now, you might expect the first question addressed in St. Thomas' Summa Theologica (Summary of Theology) to be "Does God exist?" But you'd be, as the French say, le wrong. The first question St. Thomas asks is on the nature of sacred doctrine--whether it's knowable, rational, etc. Only after establishing that theology is truly a science, an object of human knowing, does St. Thomas come to the most fundamental issue for theology: the existence of God.
And yet, the question "Does God exist?" is not even the first question asked here! It is preceded by two other questions, and once you've read through Question 1, articles 1 and 2 of Question 2 shouldn't surprise you: "Is the proposition 'God exists' self-evident?" and "Is it demonstrable?"
So, first, is it self-evident that God exists? Is it so obviously true that it is impossible to deny? This is a relevant question; if the answer is "yes," then we need not ask "Does God exist?" for the question would be as trivial as "Are circles round?" "A circle is round" is a self-evident proposition, because the predicate is contained in the subject; that is, it's part of the definition of a circle to be round, and if you know what a circle is, then you know it has to be round, and anyone who would say a circle isn't round clearly doesn't know either what a circle or roundness is (or maybe both).
If "God exists" were a self-evident proposition--that is, if it were obviously true--then the predicate would have to be contained in the subject--in this case, existence would have to be part of the definition of God, and anyone who would deny it either wouldn't know what existence is or wouldn't know what God is, that is, God's essence.
Now, we would say that in fact it is God's essence to exist, that existence is part of what it means to be God, just as roundness is part of what it means to be a circle. But it wouldn't be obvious to you that a circle is round if you don't know what a circle is. Likewise, it wouldn't be obvious to you that God exists if you don't know who or what God is. It's possible to deny that God exists if you don't know God's essence; and, as St. Thomas says, we do not know God's essence directly, as we do the essences of circles and trees and squirrels and such, and thus we must come to know God by things that are more known to us. (We'll cover such ways of knowing in later posts.)
The second question St. Thomas asks is: "Is God's existence demonstrable?" That is, can we know through reason that God exists? If we can't, then there's not much point in continuing the intellectual exercise, is there? In answer to this, St. Thomas reminds us that there are two ways of demonstrating: you can either reason from the cause of something to its effect, or from the effect to its cause. If I see a fire, and I know that fires produce heat, I can reason from the presence of a fire that heat will result. Likewise, if I see a handprint in cement, and I know that handprints in cement are caused by hands, I can reason from the presence of the handprint that a hand was the cause of it. So, St. Thomas says, if God's existence is not self-evident to us, we can still reason to God's existence if we see effects for which only God could be the cause. That is, if we can observe effects that could only be caused by God, then we can know that God exists. More on this next time.