Think of your best friend and what really makes a person a person. Of course, you know that your best friend exists, but how do you really, actually know that?
“That’s easy,” you say, “I know that my best friend exists because I see her walking towards me.”
I agree that you see a body that looks exactly like your best friend, and that you identify that body with her, and that’s not a bad reason. But does seeing a body fully support the conclusion that your best friend exists? I think not. A body alone doesn’t make a person. For example, what if that person walking towards you was actually a robot made to look like your best friend. I don’t think the robot would fool you just based on looks. For example, if the robot didn’t have the same non-verbal communication as your best friend, or the same thoughts, feelings, mannerism, actions, or beliefs, then you might begin to wonder. That is because your best friend is not a body; rather, she is a person with thoughts, feelings, intentions, and patterns of action. In a word, she is an invisible center of thought, a center that produces all sorts of visible effects (e.g. speech, action, writing, building).
So…back to the original question: how do you know that your best friend exists? If the answer is not that you see her, then it must be that we experience intelligible effects that are characteristic of her center of thought. We do not see the center of thought, but we do see the effects, the output, the sigs. Again, the body is certainly part of your friend, but not necessary. When you are chatting on the internet with her, you don’t need to see her body to know that you are communicating with your best friend.
Now, by analogy, look at the natural world. You don’t have to look far to see that nature has its own storehouse of visible, intelligible effects. For example, everything happens in an orderly fashion according to the laws of nature. Who is responsible for these effects? Should we not make the same inference that we did in the case of your best friend? Why not? Many people naturally do, and say that God is the cause behind all the effects that we see in nature, the invisible person or thought.
If it troubles you that you cannot see God, it shouldn’t: strictly speaking, you cannot see your best friend either.