Then I am not sure why people can't know things are true, according to one system with stark definitions.

I can know the sky is dark out right now. I just looked. I am looking, out the corner of my eye, so it didn't just change back to daylight when I wasn't paying attention for that one second.

Furthermore, I think I'm pretty safe to say that I also know that the sun will come up the next day and will set again in the evening. I can't prove it, but I think there's enough evidence to provide reasonable doubt.

My point is, we can know a lot. That's what our brains do -- notice patterns, and make predictions for the future. Saying we can't actually know anything ever, just because there's room for any amount of doubt at all, no matter how insubstantial, is way too Descartes for me. It gets in the way of building on previous knowledge and learning new things, and it provides an easy out for those who'd really prefer to stop encountering things that challenge their worldview. "Scientists can't REALLY know anything" is what keeps Creationism in schools. And it's dishonest.