[masukomi]

You don't "know". You just do your best. How do you "know" you're not just in the middle of a long running hallucination? You don't. You take what you believe to be true and move forward as best you can. Yes, you'll screw up, and yes you'll inadvertently harm people along the way, but you'll harm a lot fewer if you try and choose actions that won't be harmful.