Descartes did the thought experiment for us. Almost everything we claim to be true can be doubted.
1. Certainly anything having to do with my senses. We may say, "Seeing is believing." Sure. But that only means we find it most convincing. Our senses can be fooled. We could be dreaming. We could be in a matrix, a holodeck, or a computer simulation.
Is it likely? Probably not. I'm going to assume not -- reasonable people do.
But I can't prove it. In fact, I can't disprove the claim that God created the world ten seconds ago with all our memories intact and the universe having apparent age.
Do I think so? No. But I can't disprove it. And neither can you.
2. What about the things of my mind? Certainly my reasoning can be faulty without me realizing it. The human brain overwrites old memories, making even my memories of the past fallible. Drugs will do the trick to mutate my thoughts. So many, many people are mentally unstable. Maybe there's a little bit of insanity in all of us.
What about logic? Since I know of no instances of it ever being wrong, let's say it is reasonable to assume it is certain and reliable. Of course, we don't always have the right premises or follow valid logic. But if the premises are true and the logic is valid, then the proper inference must follow.
Let's throw a good deal of mathematics in here. It would seem to be certain beyond reasonable doubt. If done properly, most math seems reliable without exception.
3. What about the existence of God or God's dependability? Absolutely certain?
Faith comes in here, so the epistemological answer is no. Faith by its very nature implies epistemological uncertainty. This is as far as you and I am concerned. God no doubt has no doubts about his own existence.
I have extreme confidence that a vast number of things are ontologically certain. Their existence is quite likely certain. The problem is not with their certainty, but with mine. I believe them by faith, not by certainty on my part.
I am stuck in my head, apparently. I am finite. I am extremely convinced that I see only the smallest infinitesimal of reality. Further, the way my mind puts together the pieces of the small portion I see is often riddled with fallacy.
Yes, likely countless things are certain. But very few things are certain to me, whether I am convinced or not. They require some degree of faith on my part -- some more, some less. God may wield the ontological argument for his own existence with perfection. Epistemologically, it is a matter of faith -- and thus uncertainty -- for me and you.
4. "I think; therefore, I am." cogito ergo sum. Descartes came close, but failed because he assumed that the "I" exists, which is a matter of faith. The proper formation is cogito ergo esse. "I think; therefore, something exists."
Everything else requires faith on some level. Logic requires very little faith, for it never seems to fail. The fundamentals of math are apparently related and never seem to fail. The notion of cause and effect seems consistently reliable, although scientists continue to seek the most useful ways to express it. The idea of space and time is incredibly useful, although our perceptions of it turn out to be rather complicated.
Suffice it to say, almost everything we believe, we believe on the basis of faith to some degree or another. Things of which we are incredibly sure, are usually still ultimately a matter of faith. Much of which we are certain is also incorrect, although we usually do not know it.
God knows all the data. God knows all the data in its proper relation to all the other data. God knows it all certainly, for God either created it or its possibility.
But for us, these are matters of faith.
