Perform an experiment. Choose one for which you expect a single outcome, any will do.
For example, hold a tennis ball in your hand and then let go. Expected outcome: the ball falls. Repeat as often as you think necessary. Now, try to prove that the next time you let go of the ball, it must fall.
Now, you could say, “Of course you can’t, quantum mechanics proves that there’s always a non-zero chance that the ball might instead jump to the right.” – but that would be wrong. The reason there is no proof: all of science depends on the assumption that if you were to perform an experiment identically to one already performed, you will get the same results. Quantum mechanics just muddies the waters by assigning probabilities to all possible results.
Now, according to our experience, that assumption is a very safe one. It’s still an assumption. Applying mathematics to the situation doesn’t help either, because everything derived mathematically can be boiled down to “if I make certain assumptions, these various properties must hold true”. I suspect most people at this point would decide that it’s all immaterial, that it’s silly to allow that the ball might not fall.
When I first thought about these details, I was very tempted to say I had faith that science and experience said that the ball should fall, so of course it will. In other words, to have faith that identical actions produce identical results. But then I asked myself, is there an alternative?
Yes, but it’s probably not a comforting alternative. The alternative is to allow that all our past experience may be useless. But does anyone actually think “my car has stopped every time I’ve used my breaks, but maybe it won’t next time” or “if I let go of this ball, it could stay in my hand”? All of our life would become uncertain, risky.
Plenty of room for faith here; faith that our experiences have meaning, faith that the risk is negligible, faith that there is a god ensuring our future, faith that the logic of the situation doesn’t matter.
For myself, I’ll hold that doubt and uncertainty are all that is certain.