It turns out – almost nothing! In fact, there is only one thing that is absolutely certain. You can be certain of your experiences.
For example, if you experience you are seeing a tree, then you can be certain that you are having the experience of seeing a tree. And only that. You cannot be certain that there is a tree there (it could be an illusion). You can’t even be certain you are seeing (in the normal sense), because you could be dreaming.
And it gets worse. You can’t even be certain of “you”. It is possible for there to be thoughts with no thinker. The philosophical question of “self” is one that western philosophy continues to struggle with.
In other words, we cannot even be certain there is an objective reality! And what is the value or meaning of truth in that case? Where does that leave us?
Like all other beliefs, ultimately we need to make a choice. The only practical choice, the one I have chosen, is to trust (at least to some extent) my own experience and assume there is a reality. I believe this, which is why I will jump out of the way of a car. I believe that I, the car and the consequence of a collision between us, are real. And I don’t worry about being wrong about that.
I once witnessed a teacher being asked “what is real”? Are our everyday experiences real? He replied that every level of consciousness has its own reality. If you are being attacked by an animal in a dream, you need a dream weapon to defend yourself. A “real” weapon would be of no value.
If we concede that there is a reality, then words like “true” and “truth” have meaning and value.
Experience has shown that truth has one property that we all know and rely on, at least intuitively. Unfortunately, it is the only property, and it has limited application.
Truth is consistent with other truths. If stories or statements are inconsistent with each other or with previously known truths, then something is not true. This even works with our senses. It would be hard to doubt the experience of eating, as we can see, feel, smell, taste and sometimes even hear (e.g., sizzling) our food.
Regrettably, the opposite is not true. Just because stories are consistent does not mean they are true. In other words, it is easier to determine if something is false, than true.
I will give an example.
“I am typing this in my home office and my pet dog is in the office with me.”
Either, or both parts of this sentence could be true, as there is nothing in either that is inconsistent with what you already know to be true.
However, if I said, “I am typing this on the surface of Jupiter”, there are many inconsistencies (with previously known truths) that mean it is (very likely) false.
If instead of my pet dog, I said my pet dinosaur, that is inconsistent with our knowledge that dinosaurs are extinct. Or, instead of my pet dog, my pet whale, that is inconsistent with our knowledge about the size of whales and their need to be in water.
I am typing this in my home office, but I don’t have a pet dog. So even the original statement was not completely true, but you couldn’t know that. This demonstrates that consistency does not mean a story is true either.
We all use this property of consistency to constantly assess what we are told. If it doesn’t line up with what we already believe, we spontaneously doubt and question.
Lawyers use this fact in trials. They look for inconsistences in the story provided by the other side in order to show the judge something is untrue. This doesn’t necessarily show the other side is lying, but it does show they are at least partially wrong.
Science is also based on this property of truth. Science demands consistency between the facts and the explanations. It also refuses to consider explanations that cannot be supported by facts. Explanations without facts may be true, but there is no way to demonstrate it, so science doesn’t have a role to play in those situations.
It turns out that science is an incredibly powerful tool for finding and demonstrating new truths, and even more powerfully, that some stories are false. The wealth, comfort and convenience we experience today is a testament to the power of science to determine truth.