ELI5: what can we know

In philosophy, asking "what can we know" means trying to figure out which things we can be one hundred percent sure are true, and which things we are only guessing about.

Imagine you are wearing sunglasses with red lenses. Everything you look at seems red, like a white cat or a blue ball. Because of your glasses, your eyes might trick you. In philosophy, this big question is called epistemology, which is just a fancy word for the study of knowledge and how our brains learn about the world.

Philosophers usually look at two main ways we discover things:

• Using your senses: This is learning by seeing, touching, hearing, smelling, or tasting. You know an apple is sweet because you bite it. But sometimes your eyes can play tricks on you, like when a straight stick looks bent under water, or when you are having a dream that feels totally real.
• Using your mind: This is learning by thinking and using pure logic. You know for sure that two plus two equals four, or that a circle has no corners, even without going outside to check. You do not need to look at every triangle in the world to know that all triangles have three sides.

So, asking "what can we know" is like being a detective for your own mind. It asks if you really know your bedroom is there when you close your eyes, or if you can only truly be sure of the thoughts inside your own head right now.