Video blogger Michael Stevens of VSauce has an awesome, ultra geeky channel where he addresses all sorts of philosophical/scientific questions from understanding illusions, Is Earth Actually Flat, to What If Everyone Jumped at Once.
In this video he explains how we really don’t know anything for 100% sure. The technical name for the idea is “solipsism”. We are really just living in our own brains. When we see a strawberry, we see red but do we experience red in the same way other people do? Of course we call that color red because we grew up associating the name “red” with the color we see, but if I could really see what you see, and if I filtered it through my world view, would I think that color is green?
Everything we know and experience about the outside world came in through our senses which may or may not actually exist. Ultimately it is all created by neurons firing in our brains – and that is our reality. Could that explain why some others see the world so differently than you do? Do those people even exist or did your brain just invent them? That of course assumes that you really have a brain and it’s not all a Matrix like computer simulation. How can you really know?