Do you believe in loaded questions?
Some forms of knowledge can be demonstrated. Some cannot.
Do you know that you love your __________(mother and father, wife, brother or sister, kids, etc.)? Even if you know it, and can demonstrate it sometimes, you also get angry with them (I'm sorry for the presumption, it's just for the sake of discussion) and demonstrate to the eyes of all the world that you don't love them by what you say about them. You can know it and it can be true, yet there are times where the opposite of the truth is demonstrated.
More objective truths can be demonstrated, usually. But evolution cannot be demonstrated at all. A theory can be proposed that seems to fit the evidence, but without a visual of the act in progress (which we do not now have, nor can it be found in the fossil record) it can't actually be demonstrated.