Descartes was bothered by the idea that we know that we know things (cogito ergo sum), but then what we know might not be real, or worth knowing. Berkeley,

Hume, Locke, and Plato also had great ideas about how we know things, but all of them come up short in some way. How is it that we know things? Which theories resonate most with you? Which ones make you think you might not know what you thought you know?